DiffMatchPatch.cs

/*
 * Copyright 2008 Google Inc. All Rights Reserved.
 * Author: fraser@google.com (Neil Fraser)
 * Author: anteru@developer.shelter13.net (Matthaeus G. Chajdas)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * Diff Match and Patch
 * http://code.google.com/p/google-diff-match-patch/
 */

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Web;

namespace DiffMatchPatch {
  internal static class CompatibilityExtensions {
    // JScript splice function
    public static List<T> Splice<T>(this List<T> input, int start, int count,
        params T[] objects) {
      List<T> deletedRange = input.GetRange(start, count);
      input.RemoveRange(start, count);
      input.InsertRange(start, objects);

      return deletedRange;
    }

    // Java substring function
    public static string JavaSubstring(this string s, int begin, int end) {
      return s.Substring(begin, end - begin);
    }
  }

  /**-
   * The data structure representing a diff is a List of Diff objects:
   * {Diff(Operation.DELETE, "Hello"), Diff(Operation.INSERT, "Goodbye"),
   *  Diff(Operation.EQUAL, " world.")}
   * which means: delete "Hello", add "Goodbye" and keep " world."
   */
  public enum Operation {
    DELETE, INSERT, EQUAL
  }


  /**
   * Class representing one diff operation.
   */
  public class Diff {
    public Operation operation;
    // One of: INSERT, DELETE or EQUAL.
    public string text;
    // The text associated with this diff operation.

    /**
     * Constructor.  Initializes the diff with the provided values.
     * @param operation One of INSERT, DELETE or EQUAL.
     * @param text The text being applied.
     */
    public Diff(Operation operation, string text) {
      // Construct a diff with the specified operation and text.
      this.operation = operation;
      this.text = text;
    }

    /**
     * Display a human-readable version of this Diff.
     * @return text version.
     */
    public override string ToString() {
      string prettyText = this.text.Replace('\n', '\u00b6');
      return "Diff(" + this.operation + ",\"" + prettyText + "\")";
    }

    /**
     * Is this Diff equivalent to another Diff?
     * @param d Another Diff to compare against.
     * @return true or false.
     */
    public override bool Equals(Object obj) {
      // If parameter is null return false.
      if (obj == null) {
        return false;
      }

      // If parameter cannot be cast to Diff return false.
      Diff p = obj as Diff;
      if ((System.Object)p == null) {
        return false;
      }

      // Return true if the fields match.
      return p.operation == this.operation && p.text == this.text;
    }

    public bool Equals(Diff obj) {
      // If parameter is null return false.
      if (obj == null) {
        return false;
      }

      // Return true if the fields match.
      return obj.operation == this.operation && obj.text == this.text;
    }

    public override int GetHashCode() {
      return text.GetHashCode() ^ operation.GetHashCode();
    }
  }


  /**
   * Class representing one patch operation.
   */
  public class Patch {
    public List<Diff> diffs = new List<Diff>();
    public int start1;
    public int start2;
    public int length1;
    public int length2;

    /**
     * Emmulate GNU diff's format.
     * Header: @@ -382,8 +481,9 @@
     * Indicies are printed as 1-based, not 0-based.
     * @return The GNU diff string.
     */
    public override string ToString() {
      string coords1, coords2;
      if (this.length1 == 0) {
        coords1 = this.start1 + ",0";
      } else if (this.length1 == 1) {
        coords1 = Convert.ToString(this.start1 + 1);
      } else {
        coords1 = (this.start1 + 1) + "," + this.length1;
      }
      if (this.length2 == 0) {
        coords2 = this.start2 + ",0";
      } else if (this.length2 == 1) {
        coords2 = Convert.ToString(this.start2 + 1);
      } else {
        coords2 = (this.start2 + 1) + "," + this.length2;
      }
      StringBuilder text = new StringBuilder();
      text.Append("@@ -").Append(coords1).Append(" +").Append(coords2)
          .Append(" @@\n");
      // Escape the body of the patch with %xx notation.
      foreach (Diff aDiff in this.diffs) {
        switch (aDiff.operation) {
          case Operation.INSERT:
            text.Append('+');
            break;
          case Operation.DELETE:
            text.Append('-');
            break;
          case Operation.EQUAL:
            text.Append(' ');
            break;
        }

        text.Append(HttpUtility.UrlEncode(aDiff.text,
            new UTF8Encoding()).Replace('+', ' ')).Append("\n");
      }

      return diff_match_patch.unescapeForEncodeUriCompatability(
          text.ToString());
    }
  }


  /**
   * Class containing the diff, match and patch methods.
   * Also Contains the behaviour settings.
   */
  public class diff_match_patch {
    // Defaults.
    // Set these on your diff_match_patch instance to override the defaults.

    // Number of seconds to map a diff before giving up (0 for infinity).
    public float Diff_Timeout = 1.0f;
    // Cost of an empty edit operation in terms of edit characters.
    public short Diff_EditCost = 4;
    // At what point is no match declared (0.0 = perfection, 1.0 = very loose).
    public float Match_Threshold = 0.5f;
    // How far to search for a match (0 = exact location, 1000+ = broad match).
    // A match this many characters away from the expected location will add
    // 1.0 to the score (0.0 is a perfect match).
    public int Match_Distance = 1000;
    // When deleting a large block of text (over ~64 characters), how close
    // do the contents have to be to match the expected contents. (0.0 =
    // perfection, 1.0 = very loose).  Note that Match_Threshold controls
    // how closely the end points of a delete need to match.
    public float Patch_DeleteThreshold = 0.5f;
    // Chunk size for context length.
    public short Patch_Margin = 4;

    // The number of bits in an int.
    private short Match_MaxBits = 32;


    //  DIFF FUNCTIONS


    /**
     * Find the differences between two texts.
     * Run a faster, slightly less optimal diff.
     * This method allows the 'checklines' of diff_main() to be optional.
     * Most of the time checklines is wanted, so default to true.
     * @param text1 Old string to be diffed.
     * @param text2 New string to be diffed.
     * @return List of Diff objects.
     */
    public List<Diff> diff_main(string text1, string text2) {
      return diff_main(text1, text2, true);
    }

    /**
     * Find the differences between two texts.
     * @param text1 Old string to be diffed.
     * @param text2 New string to be diffed.
     * @param checklines Speedup flag.  If false, then don't run a
     *     line-level diff first to identify the changed areas.
     *     If true, then run a faster slightly less optimal diff.
     * @return List of Diff objects.
     */
    public List<Diff> diff_main(string text1, string text2, bool checklines) {
      // Set a deadline by which time the diff must be complete.
      DateTime deadline;
      if (this.Diff_Timeout <= 0) {
        deadline = DateTime.MaxValue;
      } else {
        deadline = DateTime.Now +
            new TimeSpan(((long)(Diff_Timeout * 1000)) * 10000);
      }
      return diff_main(text1, text2, checklines, deadline);
    }

    /**
     * Find the differences between two texts.  Simplifies the problem by
     * stripping any common prefix or suffix off the texts before diffing.
     * @param text1 Old string to be diffed.
     * @param text2 New string to be diffed.
     * @param checklines Speedup flag.  If false, then don't run a
     *     line-level diff first to identify the changed areas.
     *     If true, then run a faster slightly less optimal diff.
     * @param deadline Time when the diff should be complete by.  Used
     *     internally for recursive calls.  Users should set DiffTimeout
     *     instead.
     * @return List of Diff objects.
     */
    private List<Diff> diff_main(string text1, string text2, bool checklines,
        DateTime deadline) {
      // Check for null inputs not needed since null can't be passed in C#.

      // Check for equality (speedup).
      List<Diff> diffs;
      if (text1 == text2) {
        diffs = new List<Diff>();
        if (text1.Length != 0) {
          diffs.Add(new Diff(Operation.EQUAL, text1));
        }
        return diffs;
      }

      // Trim off common prefix (speedup).
      int commonlength = diff_commonPrefix(text1, text2);
      string commonprefix = text1.Substring(0, commonlength);
      text1 = text1.Substring(commonlength);
      text2 = text2.Substring(commonlength);

      // Trim off common suffix (speedup).
      commonlength = diff_commonSuffix(text1, text2);
      string commonsuffix = text1.Substring(text1.Length - commonlength);
      text1 = text1.Substring(0, text1.Length - commonlength);
      text2 = text2.Substring(0, text2.Length - commonlength);

      // Compute the diff on the middle block.
      diffs = diff_compute(text1, text2, checklines, deadline);

      // Restore the prefix and suffix.
      if (commonprefix.Length != 0) {
        diffs.Insert(0, (new Diff(Operation.EQUAL, commonprefix)));
      }
      if (commonsuffix.Length != 0) {
        diffs.Add(new Diff(Operation.EQUAL, commonsuffix));
      }

      diff_cleanupMerge(diffs);
      return diffs;
    }

    /**
     * Find the differences between two texts.  Assumes that the texts do not
     * have any common prefix or suffix.
     * @param text1 Old string to be diffed.
     * @param text2 New string to be diffed.
     * @param checklines Speedup flag.  If false, then don't run a
     *     line-level diff first to identify the changed areas.
     *     If true, then run a faster slightly less optimal diff.
     * @param deadline Time when the diff should be complete by.
     * @return List of Diff objects.
     */
    private List<Diff> diff_compute(string text1, string text2,
                                    bool checklines, DateTime deadline) {
      List<Diff> diffs = new List<Diff>();

      if (text1.Length == 0) {
        // Just add some text (speedup).
        diffs.Add(new Diff(Operation.INSERT, text2));
        return diffs;
      }

      if (text2.Length == 0) {
        // Just delete some text (speedup).
        diffs.Add(new Diff(Operation.DELETE, text1));
        return diffs;
      }

      string longtext = text1.Length > text2.Length ? text1 : text2;
      string shorttext = text1.Length > text2.Length ? text2 : text1;
      int i = longtext.IndexOf(shorttext, StringComparison.Ordinal);
      if (i != -1) {
        // Shorter text is inside the longer text (speedup).
        Operation op = (text1.Length > text2.Length) ?
            Operation.DELETE : Operation.INSERT;
        diffs.Add(new Diff(op, longtext.Substring(0, i)));
        diffs.Add(new Diff(Operation.EQUAL, shorttext));
        diffs.Add(new Diff(op, longtext.Substring(i + shorttext.Length)));
        return diffs;
      }

      if (shorttext.Length == 1) {
        // Single character string.
        // After the previous speedup, the character can't be an equality.
        diffs.Add(new Diff(Operation.DELETE, text1));
        diffs.Add(new Diff(Operation.INSERT, text2));
        return diffs;
      }

      // Check to see if the problem can be split in two.
      string[] hm = diff_halfMatch(text1, text2);
      if (hm != null) {
        // A half-match was found, sort out the return data.
        string text1_a = hm[0];
        string text1_b = hm[1];
        string text2_a = hm[2];
        string text2_b = hm[3];
        string mid_common = hm[4];
        // Send both pairs off for separate processing.
        List<Diff> diffs_a = diff_main(text1_a, text2_a, checklines, deadline);
        List<Diff> diffs_b = diff_main(text1_b, text2_b, checklines, deadline);
        // Merge the results.
        diffs = diffs_a;
        diffs.Add(new Diff(Operation.EQUAL, mid_common));
        diffs.AddRange(diffs_b);
        return diffs;
      }

      if (checklines && text1.Length > 100 && text2.Length > 100) {
        return diff_lineMode(text1, text2, deadline);
      }

      return diff_bisect(text1, text2, deadline);
    }

    /**
     * Do a quick line-level diff on both strings, then rediff the parts for
     * greater accuracy.
     * This speedup can produce non-minimal diffs.
     * @param text1 Old string to be diffed.
     * @param text2 New string to be diffed.
     * @param deadline Time when the diff should be complete by.
     * @return List of Diff objects.
     */
    private List<Diff> diff_lineMode(string text1, string text2,
                                     DateTime deadline) {
      // Scan the text on a line-by-line basis first.
      Object[] b = diff_linesToChars(text1, text2);
      text1 = (string)b[0];
      text2 = (string)b[1];
      List<string> linearray = (List<string>)b[2];

      List<Diff> diffs = diff_main(text1, text2, false, deadline);

      // Convert the diff back to original text.
      diff_charsToLines(diffs, linearray);
      // Eliminate freak matches (e.g. blank lines)
      diff_cleanupSemantic(diffs);

      // Rediff any replacement blocks, this time character-by-character.
      // Add a dummy entry at the end.
      diffs.Add(new Diff(Operation.EQUAL, string.Empty));
      int pointer = 0;
      int count_delete = 0;
      int count_insert = 0;
      string text_delete = string.Empty;
      string text_insert = string.Empty;
      while (pointer < diffs.Count) {
        switch (diffs[pointer].operation) {
          case Operation.INSERT:
            count_insert++;
            text_insert += diffs[pointer].text;
            break;
          case Operation.DELETE:
            count_delete++;
            text_delete += diffs[pointer].text;
            break;
          case Operation.EQUAL:
            // Upon reaching an equality, check for prior redundancies.
            if (count_delete >= 1 && count_insert >= 1) {
              // Delete the offending records and add the merged ones.
              diffs.RemoveRange(pointer - count_delete - count_insert,
                  count_delete + count_insert);
              pointer = pointer - count_delete - count_insert;
              List<Diff> a =
                  this.diff_main(text_delete, text_insert, false, deadline);
              diffs.InsertRange(pointer, a);
              pointer = pointer + a.Count;
            }
            count_insert = 0;
            count_delete = 0;
            text_delete = string.Empty;
            text_insert = string.Empty;
            break;
        }
        pointer++;
      }
      diffs.RemoveAt(diffs.Count - 1);  // Remove the dummy entry at the end.

      return diffs;
    }

    /**
     * Find the 'middle snake' of a diff, split the problem in two
     * and return the recursively constructed diff.
     * See Myers 1986 paper: An O(ND) Difference Algorithm and Its Variations.
     * @param text1 Old string to be diffed.
     * @param text2 New string to be diffed.
     * @param deadline Time at which to bail if not yet complete.
     * @return List of Diff objects.
     */
    protected List<Diff> diff_bisect(string text1, string text2,
        DateTime deadline) {
      // Cache the text lengths to prevent multiple calls.
      int text1_length = text1.Length;
      int text2_length = text2.Length;
      int max_d = (text1_length + text2_length + 1) / 2;
      int v_offset = max_d;
      int v_length = 2 * max_d;
      int[] v1 = new int[v_length];
      int[] v2 = new int[v_length];
      for (int x = 0; x < v_length; x++) {
        v1[x] = -1;
        v2[x] = -1;
      }
      v1[v_offset + 1] = 0;
      v2[v_offset + 1] = 0;
      int delta = text1_length - text2_length;
      // If the total number of characters is odd, then the front path will
      // collide with the reverse path.
      bool front = (delta % 2 != 0);
      // Offsets for start and end of k loop.
      // Prevents mapping of space beyond the grid.
      int k1start = 0;
      int k1end = 0;
      int k2start = 0;
      int k2end = 0;
      for (int d = 0; d < max_d; d++) {
        // Bail out if deadline is reached.
        if (DateTime.Now > deadline) {
          break;
        }

        // Walk the front path one step.
        for (int k1 = -d + k1start; k1 <= d - k1end; k1 += 2) {
          int k1_offset = v_offset + k1;
          int x1;
          if (k1 == -d || k1 != d && v1[k1_offset - 1] < v1[k1_offset + 1]) {
            x1 = v1[k1_offset + 1];
          } else {
            x1 = v1[k1_offset - 1] + 1;
          }
          int y1 = x1 - k1;
          while (x1 < text1_length && y1 < text2_length
                && text1[x1] == text2[y1]) {
            x1++;
            y1++;
          }
          v1[k1_offset] = x1;
          if (x1 > text1_length) {
            // Ran off the right of the graph.
            k1end += 2;
          } else if (y1 > text2_length) {
            // Ran off the bottom of the graph.
            k1start += 2;
          } else if (front) {
            int k2_offset = v_offset + delta - k1;
            if (k2_offset >= 0 && k2_offset < v_length && v2[k2_offset] != -1) {
              // Mirror x2 onto top-left coordinate system.
              int x2 = text1_length - v2[k2_offset];
              if (x1 >= x2) {
                // Overlap detected.
                return diff_bisectSplit(text1, text2, x1, y1, deadline);
              }
            }
          }
        }

        // Walk the reverse path one step.
        for (int k2 = -d + k2start; k2 <= d - k2end; k2 += 2) {
          int k2_offset = v_offset + k2;
          int x2;
          if (k2 == -d || k2 != d && v2[k2_offset - 1] < v2[k2_offset + 1]) {
            x2 = v2[k2_offset + 1];
          } else {
            x2 = v2[k2_offset - 1] + 1;
          }
          int y2 = x2 - k2;
          while (x2 < text1_length && y2 < text2_length
              && text1[text1_length - x2 - 1]
              == text2[text2_length - y2 - 1]) {
            x2++;
            y2++;
          }
          v2[k2_offset] = x2;
          if (x2 > text1_length) {
            // Ran off the left of the graph.
            k2end += 2;
          } else if (y2 > text2_length) {
            // Ran off the top of the graph.
            k2start += 2;
          } else if (!front) {
            int k1_offset = v_offset + delta - k2;
            if (k1_offset >= 0 && k1_offset < v_length && v1[k1_offset] != -1) {
              int x1 = v1[k1_offset];
              int y1 = v_offset + x1 - k1_offset;
              // Mirror x2 onto top-left coordinate system.
              x2 = text1_length - v2[k2_offset];
              if (x1 >= x2) {
                // Overlap detected.
                return diff_bisectSplit(text1, text2, x1, y1, deadline);
              }
            }
          }
        }
      }
      // Diff took too long and hit the deadline or
      // number of diffs equals number of characters, no commonality at all.
      List<Diff> diffs = new List<Diff>();
      diffs.Add(new Diff(Operation.DELETE, text1));
      diffs.Add(new Diff(Operation.INSERT, text2));
      return diffs;
    }

    /**
     * Given the location of the 'middle snake', split the diff in two parts
     * and recurse.
     * @param text1 Old string to be diffed.
     * @param text2 New string to be diffed.
     * @param x Index of split point in text1.
     * @param y Index of split point in text2.
     * @param deadline Time at which to bail if not yet complete.
     * @return LinkedList of Diff objects.
     */
    private List<Diff> diff_bisectSplit(string text1, string text2,
        int x, int y, DateTime deadline) {
      string text1a = text1.Substring(0, x);
      string text2a = text2.Substring(0, y);
      string text1b = text1.Substring(x);
      string text2b = text2.Substring(y);

      // Compute both diffs serially.
      List<Diff> diffs = diff_main(text1a, text2a, false, deadline);
      List<Diff> diffsb = diff_main(text1b, text2b, false, deadline);

      diffs.AddRange(diffsb);
      return diffs;
    }

    /**
     * Split two texts into a list of strings.  Reduce the texts to a string of
     * hashes where each Unicode character represents one line.
     * @param text1 First string.
     * @param text2 Second string.
     * @return Three element Object array, containing the encoded text1, the
     *     encoded text2 and the List of unique strings.  The zeroth element
     *     of the List of unique strings is intentionally blank.
     */
    protected Object[] diff_linesToChars(string text1, string text2) {
      List<string> lineArray = new List<string>();
      Dictionary<string, int> lineHash = new Dictionary<string, int>();
      // e.g. linearray[4] == "Hello\n"
      // e.g. linehash.get("Hello\n") == 4

      // "\x00" is a valid character, but various debuggers don't like it.
      // So we'll insert a junk entry to avoid generating a null character.
      lineArray.Add(string.Empty);

      string chars1 = diff_linesToCharsMunge(text1, lineArray, lineHash);
      string chars2 = diff_linesToCharsMunge(text2, lineArray, lineHash);
      return new Object[] { chars1, chars2, lineArray };
    }

    /**
     * Split a text into a list of strings.  Reduce the texts to a string of
     * hashes where each Unicode character represents one line.
     * @param text String to encode.
     * @param lineArray List of unique strings.
     * @param lineHash Map of strings to indices.
     * @return Encoded string.
     */
    private string diff_linesToCharsMunge(string text, List<string> lineArray,
                                          Dictionary<string, int> lineHash) {
      int lineStart = 0;
      int lineEnd = -1;
      string line;
      StringBuilder chars = new StringBuilder();
      // Walk the text, pulling out a Substring for each line.
      // text.split('\n') would would temporarily double our memory footprint.
      // Modifying text would create many large strings to garbage collect.
      while (lineEnd < text.Length - 1) {
        lineEnd = text.IndexOf('\n', lineStart);
        if (lineEnd == -1) {
          lineEnd = text.Length - 1;
        }
        line = text.JavaSubstring(lineStart, lineEnd + 1);
        lineStart = lineEnd + 1;

        if (lineHash.ContainsKey(line)) {
          chars.Append(((char)(int)lineHash[line]));
        } else {
          lineArray.Add(line);
          lineHash.Add(line, lineArray.Count - 1);
          chars.Append(((char)(lineArray.Count - 1)));
        }
      }
      return chars.ToString();
    }

    /**
     * Rehydrate the text in a diff from a string of line hashes to real lines
     * of text.
     * @param diffs List of Diff objects.
     * @param lineArray List of unique strings.
     */
    protected void diff_charsToLines(ICollection<Diff> diffs,
                    List<string> lineArray) {
      StringBuilder text;
      foreach (Diff diff in diffs) {
        text = new StringBuilder();
        for (int y = 0; y < diff.text.Length; y++) {
          text.Append(lineArray[diff.text[y]]);
        }
        diff.text = text.ToString();
      }
    }

    /**
     * Determine the common prefix of two strings.
     * @param text1 First string.
     * @param text2 Second string.
     * @return The number of characters common to the start of each string.
     */
    public int diff_commonPrefix(string text1, string text2) {
      // Performance analysis: http://neil.fraser.name/news/2007/10/09/
      int n = Math.Min(text1.Length, text2.Length);
      for (int i = 0; i < n; i++) {
        if (text1[i] != text2[i]) {
          return i;
        }
      }
      return n;
    }

    /**
     * Determine the common suffix of two strings.
     * @param text1 First string.
     * @param text2 Second string.
     * @return The number of characters common to the end of each string.
     */
    public int diff_commonSuffix(string text1, string text2) {
      // Performance analysis: http://neil.fraser.name/news/2007/10/09/
      int text1_length = text1.Length;
      int text2_length = text2.Length;
      int n = Math.Min(text1.Length, text2.Length);
      for (int i = 1; i <= n; i++) {
        if (text1[text1_length - i] != text2[text2_length - i]) {
          return i - 1;
        }
      }
      return n;
    }

    /**
     * Determine if the suffix of one string is the prefix of another.
     * @param text1 First string.
     * @param text2 Second string.
     * @return The number of characters common to the end of the first
     *     string and the start of the second string.
     */
    protected int diff_commonOverlap(string text1, string text2) {
      // Cache the text lengths to prevent multiple calls.
      int text1_length = text1.Length;
      int text2_length = text2.Length;
      // Eliminate the null case.
      if (text1_length == 0 || text2_length == 0) {
        return 0;
      }
      // Truncate the longer string.
      if (text1_length > text2_length) {
        text1 = text1.Substring(text1_length - text2_length);
      } else if (text1_length < text2_length) {
        text2 = text2.Substring(0, text1_length);
      }
      int text_length = Math.Min(text1_length, text2_length);
      // Quick check for the worst case.
      if (text1 == text2) {
        return text_length;
      }

      // Start by looking for a single character match
      // and increase length until no match is found.
      // Performance analysis: http://neil.fraser.name/news/2010/11/04/
      int best = 0;
      int length = 1;
      while (true) {
        string pattern = text1.Substring(text_length - length);
        int found = text2.IndexOf(pattern, StringComparison.Ordinal);
        if (found == -1) {
          return best;
        }
        length += found;
        if (found == 0 || text1.Substring(text_length - length) ==
            text2.Substring(0, length)) {
          best = length;
          length++;
        }
      }
    }

    /**
     * Do the two texts share a Substring which is at least half the length of
     * the longer text?
     * This speedup can produce non-minimal diffs.
     * @param text1 First string.
     * @param text2 Second string.
     * @return Five element String array, containing the prefix of text1, the
     *     suffix of text1, the prefix of text2, the suffix of text2 and the
     *     common middle.  Or null if there was no match.
     */

    protected string[] diff_halfMatch(string text1, string text2) {
      if (this.Diff_Timeout <= 0) {
        // Don't risk returning a non-optimal diff if we have unlimited time.
        return null;
      }
      string longtext = text1.Length > text2.Length ? text1 : text2;
      string shorttext = text1.Length > text2.Length ? text2 : text1;
      if (longtext.Length < 4 || shorttext.Length * 2 < longtext.Length) {
        return null;  // Pointless.
      }

      // First check if the second quarter is the seed for a half-match.
      string[] hm1 = diff_halfMatchI(longtext, shorttext,
                                     (longtext.Length + 3) / 4);
      // Check again based on the third quarter.
      string[] hm2 = diff_halfMatchI(longtext, shorttext,
                                     (longtext.Length + 1) / 2);
      string[] hm;
      if (hm1 == null && hm2 == null) {
        return null;
      } else if (hm2 == null) {
        hm = hm1;
      } else if (hm1 == null) {
        hm = hm2;
      } else {
        // Both matched.  Select the longest.
        hm = hm1[4].Length > hm2[4].Length ? hm1 : hm2;
      }

      // A half-match was found, sort out the return data.
      if (text1.Length > text2.Length) {
        return hm;
        //return new string[]{hm[0], hm[1], hm[2], hm[3], hm[4]};
      } else {
        return new string[] { hm[2], hm[3], hm[0], hm[1], hm[4] };
      }
    }

    /**
     * Does a Substring of shorttext exist within longtext such that the
     * Substring is at least half the length of longtext?
     * @param longtext Longer string.
     * @param shorttext Shorter string.
     * @param i Start index of quarter length Substring within longtext.
     * @return Five element string array, containing the prefix of longtext, the
     *     suffix of longtext, the prefix of shorttext, the suffix of shorttext
     *     and the common middle.  Or null if there was no match.
     */
    private string[] diff_halfMatchI(string longtext, string shorttext, int i) {
      // Start with a 1/4 length Substring at position i as a seed.
      string seed = longtext.Substring(i, longtext.Length / 4);
      int j = -1;
      string best_common = string.Empty;
      string best_longtext_a = string.Empty, best_longtext_b = string.Empty;
      string best_shorttext_a = string.Empty, best_shorttext_b = string.Empty;
      while (j < shorttext.Length && (j = shorttext.IndexOf(seed, j + 1,
          StringComparison.Ordinal)) != -1) {
        int prefixLength = diff_commonPrefix(longtext.Substring(i),
                                             shorttext.Substring(j));
        int suffixLength = diff_commonSuffix(longtext.Substring(0, i),
                                             shorttext.Substring(0, j));
        if (best_common.Length < suffixLength + prefixLength) {
          best_common = shorttext.Substring(j - suffixLength, suffixLength)
              + shorttext.Substring(j, prefixLength);
          best_longtext_a = longtext.Substring(0, i - suffixLength);
          best_longtext_b = longtext.Substring(i + prefixLength);
          best_shorttext_a = shorttext.Substring(0, j - suffixLength);
          best_shorttext_b = shorttext.Substring(j + prefixLength);
        }
      }
      if (best_common.Length * 2 >= longtext.Length) {
        return new string[]{best_longtext_a, best_longtext_b,
            best_shorttext_a, best_shorttext_b, best_common};
      } else {
        return null;
      }
    }

    /**
     * Reduce the number of edits by eliminating semantically trivial
     * equalities.
     * @param diffs List of Diff objects.
     */
    public void diff_cleanupSemantic(List<Diff> diffs) {
      bool changes = false;
      // Stack of indices where equalities are found.
      Stack<int> equalities = new Stack<int>();
      // Always equal to equalities[equalitiesLength-1][1]
      string lastequality = null;
      int pointer = 0;  // Index of current position.
      // Number of characters that changed prior to the equality.
      int length_insertions1 = 0;
      int length_deletions1 = 0;
      // Number of characters that changed after the equality.
      int length_insertions2 = 0;
      int length_deletions2 = 0;
      while (pointer < diffs.Count) {
        if (diffs[pointer].operation == Operation.EQUAL) {  // Equality found.
          equalities.Push(pointer);
          length_insertions1 = length_insertions2;
          length_deletions1 = length_deletions2;
          length_insertions2 = 0;
          length_deletions2 = 0;
          lastequality = diffs[pointer].text;
        } else {  // an insertion or deletion
          if (diffs[pointer].operation == Operation.INSERT) {
            length_insertions2 += diffs[pointer].text.Length;
          } else {
            length_deletions2 += diffs[pointer].text.Length;
          }
          // Eliminate an equality that is smaller or equal to the edits on both
          // sides of it.
          if (lastequality != null && (lastequality.Length
              <= Math.Max(length_insertions1, length_deletions1))
              && (lastequality.Length
                  <= Math.Max(length_insertions2, length_deletions2))) {
            // Duplicate record.
            diffs.Insert(equalities.Peek(),
                         new Diff(Operation.DELETE, lastequality));
            // Change second copy to insert.
            diffs[equalities.Peek() + 1].operation = Operation.INSERT;
            // Throw away the equality we just deleted.
            equalities.Pop();
            if (equalities.Count > 0) {
              equalities.Pop();
            }
            pointer = equalities.Count > 0 ? equalities.Peek() : -1;
            length_insertions1 = 0;  // Reset the counters.
            length_deletions1 = 0;
            length_insertions2 = 0;
            length_deletions2 = 0;
            lastequality = null;
            changes = true;
          }
        }
        pointer++;
      }

      // Normalize the diff.
      if (changes) {
        diff_cleanupMerge(diffs);
      }
      diff_cleanupSemanticLossless(diffs);

      // Find any overlaps between deletions and insertions.
      // e.g: <del>abcxxx</del><ins>xxxdef</ins>
      //   -> <del>abc</del>xxx<ins>def</ins>
      // e.g: <del>xxxabc</del><ins>defxxx</ins>
      //   -> <ins>def</ins>xxx<del>abc</del>
      // Only extract an overlap if it is as big as the edit ahead or behind it.
      pointer = 1;
      while (pointer < diffs.Count) {
        if (diffs[pointer - 1].operation == Operation.DELETE &&
            diffs[pointer].operation == Operation.INSERT) {
          string deletion = diffs[pointer - 1].text;
          string insertion = diffs[pointer].text;
          int overlap_length1 = diff_commonOverlap(deletion, insertion);
          int overlap_length2 = diff_commonOverlap(insertion, deletion);
          if (overlap_length1 >= overlap_length2) {
            if (overlap_length1 >= deletion.Length / 2.0 ||
                overlap_length1 >= insertion.Length / 2.0) {
              // Overlap found.
              // Insert an equality and trim the surrounding edits.
              diffs.Insert(pointer, new Diff(Operation.EQUAL,
                  insertion.Substring(0, overlap_length1)));
              diffs[pointer - 1].text =
                  deletion.Substring(0, deletion.Length - overlap_length1);
              diffs[pointer + 1].text = insertion.Substring(overlap_length1);
              pointer++;
            }
          } else {
            if (overlap_length2 >= deletion.Length / 2.0 ||
                overlap_length2 >= insertion.Length / 2.0) {
              // Reverse overlap found.
              // Insert an equality and swap and trim the surrounding edits.
              diffs.Insert(pointer, new Diff(Operation.EQUAL,
                  deletion.Substring(0, overlap_length2)));
              diffs[pointer - 1].operation = Operation.INSERT;
              diffs[pointer - 1].text =
                  insertion.Substring(0, insertion.Length - overlap_length2);
              diffs[pointer + 1].operation = Operation.DELETE;
              diffs[pointer + 1].text = deletion.Substring(overlap_length2);
              pointer++;
            }
          }
          pointer++;
        }
        pointer++;
      }
    }

    /**
     * Look for single edits surrounded on both sides by equalities
     * which can be shifted sideways to align the edit to a word boundary.
     * e.g: The c<ins>at c</ins>ame. -> The <ins>cat </ins>came.
     * @param diffs List of Diff objects.
     */
    public void diff_cleanupSemanticLossless(List<Diff> diffs) {
      int pointer = 1;
      // Intentionally ignore the first and last element (don't need checking).
      while (pointer < diffs.Count - 1) {
        if (diffs[pointer - 1].operation == Operation.EQUAL &&
          diffs[pointer + 1].operation == Operation.EQUAL) {
          // This is a single edit surrounded by equalities.
          string equality1 = diffs[pointer - 1].text;
          string edit = diffs[pointer].text;
          string equality2 = diffs[pointer + 1].text;

          // First, shift the edit as far left as possible.
          int commonOffset = this.diff_commonSuffix(equality1, edit);
          if (commonOffset > 0) {
            string commonString = edit.Substring(edit.Length - commonOffset);
            equality1 = equality1.Substring(0, equality1.Length - commonOffset);
            edit = commonString + edit.Substring(0, edit.Length - commonOffset);
            equality2 = commonString + equality2;
          }

          // Second, step character by character right,
          // looking for the best fit.
          string bestEquality1 = equality1;
          string bestEdit = edit;
          string bestEquality2 = equality2;
          int bestScore = diff_cleanupSemanticScore(equality1, edit) +
              diff_cleanupSemanticScore(edit, equality2);
          while (edit.Length != 0 && equality2.Length != 0
              && edit[0] == equality2[0]) {
            equality1 += edit[0];
            edit = edit.Substring(1) + equality2[0];
            equality2 = equality2.Substring(1);
            int score = diff_cleanupSemanticScore(equality1, edit) +
                diff_cleanupSemanticScore(edit, equality2);
            // The >= encourages trailing rather than leading whitespace on
            // edits.
            if (score >= bestScore) {
              bestScore = score;
              bestEquality1 = equality1;
              bestEdit = edit;
              bestEquality2 = equality2;
            }
          }

          if (diffs[pointer - 1].text != bestEquality1) {
            // We have an improvement, save it back to the diff.
            if (bestEquality1.Length != 0) {
              diffs[pointer - 1].text = bestEquality1;
            } else {
              diffs.RemoveAt(pointer - 1);
              pointer--;
            }
            diffs[pointer].text = bestEdit;
            if (bestEquality2.Length != 0) {
              diffs[pointer + 1].text = bestEquality2;
            } else {
              diffs.RemoveAt(pointer + 1);
              pointer--;
            }
          }
        }
        pointer++;
      }
    }

    /**
     * Given two strings, comAdde a score representing whether the internal
     * boundary falls on logical boundaries.
     * Scores range from 6 (best) to 0 (worst).
     * @param one First string.
     * @param two Second string.
     * @return The score.
     */
    private int diff_cleanupSemanticScore(string one, string two) {
      if (one.Length == 0 || two.Length == 0) {
        // Edges are the best.
        return 6;
      }

      // Each port of this function behaves slightly differently due to
      // subtle differences in each language's definition of things like
      // 'whitespace'.  Since this function's purpose is largely cosmetic,
      // the choice has been made to use each language's native features
      // rather than force total conformity.
      char char1 = one[one.Length - 1];
      char char2 = two[0];
      bool nonAlphaNumeric1 = !Char.IsLetterOrDigit(char1);
      bool nonAlphaNumeric2 = !Char.IsLetterOrDigit(char2);
      bool whitespace1 = nonAlphaNumeric1 && Char.IsWhiteSpace(char1);
      bool whitespace2 = nonAlphaNumeric2 && Char.IsWhiteSpace(char2);
      bool lineBreak1 = whitespace1 && Char.IsControl(char1);
      bool lineBreak2 = whitespace2 && Char.IsControl(char2);
      bool blankLine1 = lineBreak1 && BLANKLINEEND.IsMatch(one);
      bool blankLine2 = lineBreak2 && BLANKLINESTART.IsMatch(two);

      if (blankLine1 || blankLine2) {
        // Five points for blank lines.
        return 5;
      } else if (lineBreak1 || lineBreak2) {
        // Four points for line breaks.
        return 4;
      } else if (nonAlphaNumeric1 && !whitespace1 && whitespace2) {
        // Three points for end of sentences.
        return 3;
      } else if (whitespace1 || whitespace2) {
        // Two points for whitespace.
        return 2;
      } else if (nonAlphaNumeric1 || nonAlphaNumeric2) {
        // One point for non-alphanumeric.
        return 1;
      }
      return 0;
    }

    // Define some regex patterns for matching boundaries.
    private Regex BLANKLINEEND = new Regex("\\n\\r?\\n\\Z");
    private Regex BLANKLINESTART = new Regex("\\A\\r?\\n\\r?\\n");

    /**
     * Reduce the number of edits by eliminating operationally trivial
     * equalities.
     * @param diffs List of Diff objects.
     */
    public void diff_cleanupEfficiency(List<Diff> diffs) {
      bool changes = false;
      // Stack of indices where equalities are found.
      Stack<int> equalities = new Stack<int>();
      // Always equal to equalities[equalitiesLength-1][1]
      string lastequality = string.Empty;
      int pointer = 0;  // Index of current position.
      // Is there an insertion operation before the last equality.
      bool pre_ins = false;
      // Is there a deletion operation before the last equality.
      bool pre_del = false;
      // Is there an insertion operation after the last equality.
      bool post_ins = false;
      // Is there a deletion operation after the last equality.
      bool post_del = false;
      while (pointer < diffs.Count) {
        if (diffs[pointer].operation == Operation.EQUAL) {  // Equality found.
          if (diffs[pointer].text.Length < this.Diff_EditCost
              && (post_ins || post_del)) {
            // Candidate found.
            equalities.Push(pointer);
            pre_ins = post_ins;
            pre_del = post_del;
            lastequality = diffs[pointer].text;
          } else {
            // Not a candidate, and can never become one.
            equalities.Clear();
            lastequality = string.Empty;
          }
          post_ins = post_del = false;
        } else {  // An insertion or deletion.
          if (diffs[pointer].operation == Operation.DELETE) {
            post_del = true;
          } else {
            post_ins = true;
          }
          /*
           * Five types to be split:
           * <ins>A</ins><del>B</del>XY<ins>C</ins><del>D</del>
           * <ins>A</ins>X<ins>C</ins><del>D</del>
           * <ins>A</ins><del>B</del>X<ins>C</ins>
           * <ins>A</del>X<ins>C</ins><del>D</del>
           * <ins>A</ins><del>B</del>X<del>C</del>
           */
          if ((lastequality.Length != 0)
              && ((pre_ins && pre_del && post_ins && post_del)
              || ((lastequality.Length < this.Diff_EditCost / 2)
              && ((pre_ins ? 1 : 0) + (pre_del ? 1 : 0) + (post_ins ? 1 : 0)
              + (post_del ? 1 : 0)) == 3))) {
            // Duplicate record.
            diffs.Insert(equalities.Peek(),
                         new Diff(Operation.DELETE, lastequality));
            // Change second copy to insert.
            diffs[equalities.Peek() + 1].operation = Operation.INSERT;
            equalities.Pop();  // Throw away the equality we just deleted.
            lastequality = string.Empty;
            if (pre_ins && pre_del) {
              // No changes made which could affect previous entry, keep going.
              post_ins = post_del = true;
              equalities.Clear();
            } else {
              if (equalities.Count > 0) {
                equalities.Pop();
              }

              pointer = equalities.Count > 0 ? equalities.Peek() : -1;
              post_ins = post_del = false;
            }
            changes = true;
          }
        }
        pointer++;
      }

      if (changes) {
        diff_cleanupMerge(diffs);
      }
    }

    /**
     * Reorder and merge like edit sections.  Merge equalities.
     * Any edit section can move as long as it doesn't cross an equality.
     * @param diffs List of Diff objects.
     */
    public void diff_cleanupMerge(List<Diff> diffs) {
      // Add a dummy entry at the end.
      diffs.Add(new Diff(Operation.EQUAL, string.Empty));
      int pointer = 0;
      int count_delete = 0;
      int count_insert = 0;
      string text_delete = string.Empty;
      string text_insert = string.Empty;
      int commonlength;
      while (pointer < diffs.Count) {
        switch (diffs[pointer].operation) {
          case Operation.INSERT:
            count_insert++;
            text_insert += diffs[pointer].text;
            pointer++;
            break;
          case Operation.DELETE:
            count_delete++;
            text_delete += diffs[pointer].text;
            pointer++;
            break;
          case Operation.EQUAL:
            // Upon reaching an equality, check for prior redundancies.
            if (count_delete + count_insert > 1) {
              if (count_delete != 0 && count_insert != 0) {
                // Factor out any common prefixies.
                commonlength = this.diff_commonPrefix(text_insert, text_delete);
                if (commonlength != 0) {
                  if ((pointer - count_delete - count_insert) > 0 &&
                    diffs[pointer - count_delete - count_insert - 1].operation
                        == Operation.EQUAL) {
                    diffs[pointer - count_delete - count_insert - 1].text
                        += text_insert.Substring(0, commonlength);
                  } else {
                    diffs.Insert(0, new Diff(Operation.EQUAL,
                        text_insert.Substring(0, commonlength)));
                    pointer++;
                  }
                  text_insert = text_insert.Substring(commonlength);
                  text_delete = text_delete.Substring(commonlength);
                }
                // Factor out any common suffixies.
                commonlength = this.diff_commonSuffix(text_insert, text_delete);
                if (commonlength != 0) {
                  diffs[pointer].text = text_insert.Substring(text_insert.Length
                      - commonlength) + diffs[pointer].text;
                  text_insert = text_insert.Substring(0, text_insert.Length
                      - commonlength);
                  text_delete = text_delete.Substring(0, text_delete.Length
                      - commonlength);
                }
              }
              // Delete the offending records and add the merged ones.
              if (count_delete == 0) {
                diffs.Splice(pointer - count_insert,
                    count_delete + count_insert,
                    new Diff(Operation.INSERT, text_insert));
              } else if (count_insert == 0) {
                diffs.Splice(pointer - count_delete,
                    count_delete + count_insert,
                    new Diff(Operation.DELETE, text_delete));
              } else {
                diffs.Splice(pointer - count_delete - count_insert,
                    count_delete + count_insert,
                    new Diff(Operation.DELETE, text_delete),
                    new Diff(Operation.INSERT, text_insert));
              }
              pointer = pointer - count_delete - count_insert +
                  (count_delete != 0 ? 1 : 0) + (count_insert != 0 ? 1 : 0) + 1;
            } else if (pointer != 0
                && diffs[pointer - 1].operation == Operation.EQUAL) {
              // Merge this equality with the previous one.
              diffs[pointer - 1].text += diffs[pointer].text;
              diffs.RemoveAt(pointer);
            } else {
              pointer++;
            }
            count_insert = 0;
            count_delete = 0;
            text_delete = string.Empty;
            text_insert = string.Empty;
            break;
        }
      }
      if (diffs[diffs.Count - 1].text.Length == 0) {
        diffs.RemoveAt(diffs.Count - 1);  // Remove the dummy entry at the end.
      }

      // Second pass: look for single edits surrounded on both sides by
      // equalities which can be shifted sideways to eliminate an equality.
      // e.g: A<ins>BA</ins>C -> <ins>AB</ins>AC
      bool changes = false;
      pointer = 1;
      // Intentionally ignore the first and last element (don't need checking).
      while (pointer < (diffs.Count - 1)) {
        if (diffs[pointer - 1].operation == Operation.EQUAL &&
          diffs[pointer + 1].operation == Operation.EQUAL) {
          // This is a single edit surrounded by equalities.
          if (diffs[pointer].text.EndsWith(diffs[pointer - 1].text,
              StringComparison.Ordinal)) {
            // Shift the edit over the previous equality.
            diffs[pointer].text = diffs[pointer - 1].text +
                diffs[pointer].text.Substring(0, diffs[pointer].text.Length -
                                              diffs[pointer - 1].text.Length);
            diffs[pointer + 1].text = diffs[pointer - 1].text
                + diffs[pointer + 1].text;
            diffs.Splice(pointer - 1, 1);
            changes = true;
          } else if (diffs[pointer].text.StartsWith(diffs[pointer + 1].text,
              StringComparison.Ordinal)) {
            // Shift the edit over the next equality.
            diffs[pointer - 1].text += diffs[pointer + 1].text;
            diffs[pointer].text =
                diffs[pointer].text.Substring(diffs[pointer + 1].text.Length)
                + diffs[pointer + 1].text;
            diffs.Splice(pointer + 1, 1);
            changes = true;
          }
        }
        pointer++;
      }
      // If shifts were made, the diff needs reordering and another shift sweep.
      if (changes) {
        this.diff_cleanupMerge(diffs);
      }
    }

    /**
     * loc is a location in text1, comAdde and return the equivalent location in
     * text2.
     * e.g. "The cat" vs "The big cat", 1->1, 5->8
     * @param diffs List of Diff objects.
     * @param loc Location within text1.
     * @return Location within text2.
     */
    public int diff_xIndex(List<Diff> diffs, int loc) {
      int chars1 = 0;
      int chars2 = 0;
      int last_chars1 = 0;
      int last_chars2 = 0;
      Diff lastDiff = null;
      foreach (Diff aDiff in diffs) {
        if (aDiff.operation != Operation.INSERT) {
          // Equality or deletion.
          chars1 += aDiff.text.Length;
        }
        if (aDiff.operation != Operation.DELETE) {
          // Equality or insertion.
          chars2 += aDiff.text.Length;
        }
        if (chars1 > loc) {
          // Overshot the location.
          lastDiff = aDiff;
          break;
        }
        last_chars1 = chars1;
        last_chars2 = chars2;
      }
      if (lastDiff != null && lastDiff.operation == Operation.DELETE) {
        // The location was deleted.
        return last_chars2;
      }
      // Add the remaining character length.
      return last_chars2 + (loc - last_chars1);
    }

    /**
     * Convert a Diff list into a pretty HTML report.
     * @param diffs List of Diff objects.
     * @return HTML representation.
     */
    public string diff_prettyHtml(List<Diff> diffs) {
      StringBuilder html = new StringBuilder();
      foreach (Diff aDiff in diffs) {
        string text = aDiff.text.Replace("&", "&amp;").Replace("<", "&lt;")
          .Replace(">", "&gt;").Replace("\n", "&para;<br>");
        switch (aDiff.operation) {
          case Operation.INSERT:
            html.Append("<ins style=\"background:#e6ffe6;\">").Append(text)
                .Append("</ins>");
            break;
          case Operation.DELETE:
            html.Append("<del style=\"background:#ffe6e6;\">").Append(text)
                .Append("</del>");
            break;
          case Operation.EQUAL:
            html.Append("<span>").Append(text).Append("</span>");
            break;
        }
      }
      return html.ToString();
    }

    /**
     * Compute and return the source text (all equalities and deletions).
     * @param diffs List of Diff objects.
     * @return Source text.
     */
    public string diff_text1(List<Diff> diffs) {
      StringBuilder text = new StringBuilder();
      foreach (Diff aDiff in diffs) {
        if (aDiff.operation != Operation.INSERT) {
          text.Append(aDiff.text);
        }
      }
      return text.ToString();
    }

    /**
     * Compute and return the destination text (all equalities and insertions).
     * @param diffs List of Diff objects.
     * @return Destination text.
     */
    public string diff_text2(List<Diff> diffs) {
      StringBuilder text = new StringBuilder();
      foreach (Diff aDiff in diffs) {
        if (aDiff.operation != Operation.DELETE) {
          text.Append(aDiff.text);
        }
      }
      return text.ToString();
    }

    /**
     * Compute the Levenshtein distance; the number of inserted, deleted or
     * substituted characters.
     * @param diffs List of Diff objects.
     * @return Number of changes.
     */
    public int diff_levenshtein(List<Diff> diffs) {
      int levenshtein = 0;
      int insertions = 0;
      int deletions = 0;
      foreach (Diff aDiff in diffs) {
        switch (aDiff.operation) {
          case Operation.INSERT:
            insertions += aDiff.text.Length;
            break;
          case Operation.DELETE:
            deletions += aDiff.text.Length;
            break;
          case Operation.EQUAL:
            // A deletion and an insertion is one substitution.
            levenshtein += Math.Max(insertions, deletions);
            insertions = 0;
            deletions = 0;
            break;
        }
      }
      levenshtein += Math.Max(insertions, deletions);
      return levenshtein;
    }

    /**
     * Crush the diff into an encoded string which describes the operations
     * required to transform text1 into text2.
     * E.g. =3\t-2\t+ing  -> Keep 3 chars, delete 2 chars, insert 'ing'.
     * Operations are tab-separated.  Inserted text is escaped using %xx
     * notation.
     * @param diffs Array of Diff objects.
     * @return Delta text.
     */
    public string diff_toDelta(List<Diff> diffs) {
      StringBuilder text = new StringBuilder();
      foreach (Diff aDiff in diffs) {
        switch (aDiff.operation) {
          case Operation.INSERT:
            text.Append("+").Append(HttpUtility.UrlEncode(aDiff.text,
                new UTF8Encoding()).Replace('+', ' ')).Append("\t");
            break;
          case Operation.DELETE:
            text.Append("-").Append(aDiff.text.Length).Append("\t");
            break;
          case Operation.EQUAL:
            text.Append("=").Append(aDiff.text.Length).Append("\t");
            break;
        }
      }
      string delta = text.ToString();
      if (delta.Length != 0) {
        // Strip off trailing tab character.
        delta = delta.Substring(0, delta.Length - 1);
        delta = unescapeForEncodeUriCompatability(delta);
      }
      return delta;
    }

    /**
     * Given the original text1, and an encoded string which describes the
     * operations required to transform text1 into text2, comAdde the full diff.
     * @param text1 Source string for the diff.
     * @param delta Delta text.
     * @return Array of Diff objects or null if invalid.
     * @throws ArgumentException If invalid input.
     */
    public List<Diff> diff_fromDelta(string text1, string delta) {
      List<Diff> diffs = new List<Diff>();
      int pointer = 0;  // Cursor in text1
      string[] tokens = delta.Split(new string[] { "\t" },
          StringSplitOptions.None);
      foreach (string token in tokens) {
        if (token.Length == 0) {
          // Blank tokens are ok (from a trailing \t).
          continue;
        }
        // Each token begins with a one character parameter which specifies the
        // operation of this token (delete, insert, equality).
        string param = token.Substring(1);
        switch (token[0]) {
          case '+':
            // decode would change all "+" to " "
            param = param.Replace("+", "%2b");

            param = HttpUtility.UrlDecode(param, new UTF8Encoding(false, true));
            //} catch (UnsupportedEncodingException e) {
            //  // Not likely on modern system.
            //  throw new Error("This system does not support UTF-8.", e);
            //} catch (IllegalArgumentException e) {
            //  // Malformed URI sequence.
            //  throw new IllegalArgumentException(
            //      "Illegal escape in diff_fromDelta: " + param, e);
            //}
            diffs.Add(new Diff(Operation.INSERT, param));
            break;
          case '-':
            // Fall through.
          case '=':
            int n;
            try {
              n = Convert.ToInt32(param);
            } catch (FormatException e) {
              throw new ArgumentException(
                  "Invalid number in diff_fromDelta: " + param, e);
            }
            if (n < 0) {
              throw new ArgumentException(
                  "Negative number in diff_fromDelta: " + param);
            }
            string text;
            try {
              text = text1.Substring(pointer, n);
              pointer += n;
            } catch (ArgumentOutOfRangeException e) {
              throw new ArgumentException("Delta length (" + pointer
                  + ") larger than source text length (" + text1.Length
                  + ").", e);
            }
            if (token[0] == '=') {
              diffs.Add(new Diff(Operation.EQUAL, text));
            } else {
              diffs.Add(new Diff(Operation.DELETE, text));
            }
            break;
          default:
            // Anything else is an error.
            throw new ArgumentException(
                "Invalid diff operation in diff_fromDelta: " + token[0]);
        }
      }
      if (pointer != text1.Length) {
        throw new ArgumentException("Delta length (" + pointer
            + ") smaller than source text length (" + text1.Length + ").");
      }
      return diffs;
    }


    //  MATCH FUNCTIONS


    /**
     * Locate the best instance of 'pattern' in 'text' near 'loc'.
     * Returns -1 if no match found.
     * @param text The text to search.
     * @param pattern The pattern to search for.
     * @param loc The location to search around.
     * @return Best match index or -1.
     */
    public int match_main(string text, string pattern, int loc) {
      // Check for null inputs not needed since null can't be passed in C#.

      loc = Math.Max(0, Math.Min(loc, text.Length));
      if (text == pattern) {
        // Shortcut (potentially not guaranteed by the algorithm)
        return 0;
      } else if (text.Length == 0) {
        // Nothing to match.
        return -1;
      } else if (loc + pattern.Length <= text.Length
        && text.Substring(loc, pattern.Length) == pattern) {
        // Perfect match at the perfect spot!  (Includes case of null pattern)
        return loc;
      } else {
        // Do a fuzzy compare.
        return match_bitap(text, pattern, loc);
      }
    }

    /**
     * Locate the best instance of 'pattern' in 'text' near 'loc' using the
     * Bitap algorithm.  Returns -1 if no match found.
     * @param text The text to search.
     * @param pattern The pattern to search for.
     * @param loc The location to search around.
     * @return Best match index or -1.
     */
    protected int match_bitap(string text, string pattern, int loc) {
      // assert (Match_MaxBits == 0 || pattern.Length <= Match_MaxBits)
      //    : "Pattern too long for this application.";

      // Initialise the alphabet.
      Dictionary<char, int> s = match_alphabet(pattern);

      // Highest score beyond which we give up.
      double score_threshold = Match_Threshold;
      // Is there a nearby exact match? (speedup)
      int best_loc = text.IndexOf(pattern, loc, StringComparison.Ordinal);
      if (best_loc != -1) {
        score_threshold = Math.Min(match_bitapScore(0, best_loc, loc,
            pattern), score_threshold);
        // What about in the other direction? (speedup)
        best_loc = text.LastIndexOf(pattern,
            Math.Min(loc + pattern.Length, text.Length),
            StringComparison.Ordinal);
        if (best_loc != -1) {
          score_threshold = Math.Min(match_bitapScore(0, best_loc, loc,
              pattern), score_threshold);
        }
      }

      // Initialise the bit arrays.
      int matchmask = 1 << (pattern.Length - 1);
      best_loc = -1;

      int bin_min, bin_mid;
      int bin_max = pattern.Length + text.Length;
      // Empty initialization added to appease C# compiler.
      int[] last_rd = new int[0];
      for (int d = 0; d < pattern.Length; d++) {
        // Scan for the best match; each iteration allows for one more error.
        // Run a binary search to determine how far from 'loc' we can stray at
        // this error level.
        bin_min = 0;
        bin_mid = bin_max;
        while (bin_min < bin_mid) {
          if (match_bitapScore(d, loc + bin_mid, loc, pattern)
              <= score_threshold) {
            bin_min = bin_mid;
          } else {
            bin_max = bin_mid;
          }
          bin_mid = (bin_max - bin_min) / 2 + bin_min;
        }
        // Use the result from this iteration as the maximum for the next.
        bin_max = bin_mid;
        int start = Math.Max(1, loc - bin_mid + 1);
        int finish = Math.Min(loc + bin_mid, text.Length) + pattern.Length;

        int[] rd = new int[finish + 2];
        rd[finish + 1] = (1 << d) - 1;
        for (int j = finish; j >= start; j--) {
          int charMatch;
          if (text.Length <= j - 1 || !s.ContainsKey(text[j - 1])) {
            // Out of range.
            charMatch = 0;
          } else {
            charMatch = s[text[j - 1]];
          }
          if (d == 0) {
            // First pass: exact match.
            rd[j] = ((rd[j + 1] << 1) | 1) & charMatch;
          } else {
            // Subsequent passes: fuzzy match.
            rd[j] = ((rd[j + 1] << 1) | 1) & charMatch
                | (((last_rd[j + 1] | last_rd[j]) << 1) | 1) | last_rd[j + 1];
          }
          if ((rd[j] & matchmask) != 0) {
            double score = match_bitapScore(d, j - 1, loc, pattern);
            // This match will almost certainly be better than any existing
            // match.  But check anyway.
            if (score <= score_threshold) {
              // Told you so.
              score_threshold = score;
              best_loc = j - 1;
              if (best_loc > loc) {
                // When passing loc, don't exceed our current distance from loc.
                start = Math.Max(1, 2 * loc - best_loc);
              } else {
                // Already passed loc, downhill from here on in.
                break;
              }
            }
          }
        }
        if (match_bitapScore(d + 1, loc, loc, pattern) > score_threshold) {
          // No hope for a (better) match at greater error levels.
          break;
        }
        last_rd = rd;
      }
      return best_loc;
    }

    /**
     * Compute and return the score for a match with e errors and x location.
     * @param e Number of errors in match.
     * @param x Location of match.
     * @param loc Expected location of match.
     * @param pattern Pattern being sought.
     * @return Overall score for match (0.0 = good, 1.0 = bad).
     */
    private double match_bitapScore(int e, int x, int loc, string pattern) {
      float accuracy = (float)e / pattern.Length;
      int proximity = Math.Abs(loc - x);
      if (Match_Distance == 0) {
        // Dodge divide by zero error.
        return proximity == 0 ? accuracy : 1.0;
      }
      return accuracy + (proximity / (float) Match_Distance);
    }

    /**
     * Initialise the alphabet for the Bitap algorithm.
     * @param pattern The text to encode.
     * @return Hash of character locations.
     */
    protected Dictionary<char, int> match_alphabet(string pattern) {
      Dictionary<char, int> s = new Dictionary<char, int>();
      char[] char_pattern = pattern.ToCharArray();
      foreach (char c in char_pattern) {
        if (!s.ContainsKey(c)) {
          s.Add(c, 0);
        }
      }
      int i = 0;
      foreach (char c in char_pattern) {
        int value = s[c] | (1 << (pattern.Length - i - 1));
        s[c] = value;
        i++;
      }
      return s;
    }


    //  PATCH FUNCTIONS


    /**
     * Increase the context until it is unique,
     * but don't let the pattern expand beyond Match_MaxBits.
     * @param patch The patch to grow.
     * @param text Source text.
     */
    protected void patch_addContext(Patch patch, string text) {
      if (text.Length == 0) {
        return;
      }
      string pattern = text.Substring(patch.start2, patch.length1);
      int padding = 0;

      // Look for the first and last matches of pattern in text.  If two
      // different matches are found, increase the pattern length.
      while (text.IndexOf(pattern, StringComparison.Ordinal)
          != text.LastIndexOf(pattern, StringComparison.Ordinal)
          && pattern.Length < Match_MaxBits - Patch_Margin - Patch_Margin) {
        padding += Patch_Margin;
        pattern = text.JavaSubstring(Math.Max(0, patch.start2 - padding),
            Math.Min(text.Length, patch.start2 + patch.length1 + padding));
      }
      // Add one chunk for good luck.
      padding += Patch_Margin;

      // Add the prefix.
      string prefix = text.JavaSubstring(Math.Max(0, patch.start2 - padding),
        patch.start2);
      if (prefix.Length != 0) {
        patch.diffs.Insert(0, new Diff(Operation.EQUAL, prefix));
      }
      // Add the suffix.
      string suffix = text.JavaSubstring(patch.start2 + patch.length1,
          Math.Min(text.Length, patch.start2 + patch.length1 + padding));
      if (suffix.Length != 0) {
        patch.diffs.Add(new Diff(Operation.EQUAL, suffix));
      }

      // Roll back the start points.
      patch.start1 -= prefix.Length;
      patch.start2 -= prefix.Length;
      // Extend the lengths.
      patch.length1 += prefix.Length + suffix.Length;
      patch.length2 += prefix.Length + suffix.Length;
    }

    /**
     * Compute a list of patches to turn text1 into text2.
     * A set of diffs will be computed.
     * @param text1 Old text.
     * @param text2 New text.
     * @return List of Patch objects.
     */
    public List<Patch> patch_make(string text1, string text2) {
      // Check for null inputs not needed since null can't be passed in C#.
      // No diffs provided, comAdde our own.
      List<Diff> diffs = diff_main(text1, text2, true);
      if (diffs.Count > 2) {
        diff_cleanupSemantic(diffs);
        diff_cleanupEfficiency(diffs);
      }
      return patch_make(text1, diffs);
    }

    /**
     * Compute a list of patches to turn text1 into text2.
     * text1 will be derived from the provided diffs.
     * @param diffs Array of Diff objects for text1 to text2.
     * @return List of Patch objects.
     */
    public List<Patch> patch_make(List<Diff> diffs) {
      // Check for null inputs not needed since null can't be passed in C#.
      // No origin string provided, comAdde our own.
      string text1 = diff_text1(diffs);
      return patch_make(text1, diffs);
    }

    /**
     * Compute a list of patches to turn text1 into text2.
     * text2 is ignored, diffs are the delta between text1 and text2.
     * @param text1 Old text
     * @param text2 Ignored.
     * @param diffs Array of Diff objects for text1 to text2.
     * @return List of Patch objects.
     * @deprecated Prefer patch_make(string text1, List<Diff> diffs).
     */
    public List<Patch> patch_make(string text1, string text2,
        List<Diff> diffs) {
      return patch_make(text1, diffs);
    }

    /**
     * Compute a list of patches to turn text1 into text2.
     * text2 is not provided, diffs are the delta between text1 and text2.
     * @param text1 Old text.
     * @param diffs Array of Diff objects for text1 to text2.
     * @return List of Patch objects.
     */
    public List<Patch> patch_make(string text1, List<Diff> diffs) {
      // Check for null inputs not needed since null can't be passed in C#.
      List<Patch> patches = new List<Patch>();
      if (diffs.Count == 0) {
        return patches;  // Get rid of the null case.
      }
      Patch patch = new Patch();
      int char_count1 = 0;  // Number of characters into the text1 string.
      int char_count2 = 0;  // Number of characters into the text2 string.
      // Start with text1 (prepatch_text) and apply the diffs until we arrive at
      // text2 (postpatch_text). We recreate the patches one by one to determine
      // context info.
      string prepatch_text = text1;
      string postpatch_text = text1;
      foreach (Diff aDiff in diffs) {
        if (patch.diffs.Count == 0 && aDiff.operation != Operation.EQUAL) {
          // A new patch starts here.
          patch.start1 = char_count1;
          patch.start2 = char_count2;
        }

        switch (aDiff.operation) {
          case Operation.INSERT:
            patch.diffs.Add(aDiff);
            patch.length2 += aDiff.text.Length;
            postpatch_text = postpatch_text.Insert(char_count2, aDiff.text);
            break;
          case Operation.DELETE:
            patch.length1 += aDiff.text.Length;
            patch.diffs.Add(aDiff);
            postpatch_text = postpatch_text.Remove(char_count2,
                aDiff.text.Length);
            break;
          case Operation.EQUAL:
            if (aDiff.text.Length <= 2 * Patch_Margin
                && patch.diffs.Count() != 0 && aDiff != diffs.Last()) {
              // Small equality inside a patch.
              patch.diffs.Add(aDiff);
              patch.length1 += aDiff.text.Length;
              patch.length2 += aDiff.text.Length;
            }

            if (aDiff.text.Length >= 2 * Patch_Margin) {
              // Time for a new patch.
              if (patch.diffs.Count != 0) {
                patch_addContext(patch, prepatch_text);
                patches.Add(patch);
                patch = new Patch();
                // Unlike Unidiff, our patch lists have a rolling context.
                // http://code.google.com/p/google-diff-match-patch/wiki/Unidiff
                // Update prepatch text & pos to reflect the application of the
                // just completed patch.
                prepatch_text = postpatch_text;
                char_count1 = char_count2;
              }
            }
            break;
        }

        // Update the current character count.
        if (aDiff.operation != Operation.INSERT) {
          char_count1 += aDiff.text.Length;
        }
        if (aDiff.operation != Operation.DELETE) {
          char_count2 += aDiff.text.Length;
        }
      }
      // Pick up the leftover patch if not empty.
      if (patch.diffs.Count != 0) {
        patch_addContext(patch, prepatch_text);
        patches.Add(patch);
      }

      return patches;
    }

    /**
     * Given an array of patches, return another array that is identical.
     * @param patches Array of Patch objects.
     * @return Array of Patch objects.
     */
    public List<Patch> patch_deepCopy(List<Patch> patches) {
      List<Patch> patchesCopy = new List<Patch>();
      foreach (Patch aPatch in patches) {
        Patch patchCopy = new Patch();
        foreach (Diff aDiff in aPatch.diffs) {
          Diff diffCopy = new Diff(aDiff.operation, aDiff.text);
          patchCopy.diffs.Add(diffCopy);
        }
        patchCopy.start1 = aPatch.start1;
        patchCopy.start2 = aPatch.start2;
        patchCopy.length1 = aPatch.length1;
        patchCopy.length2 = aPatch.length2;
        patchesCopy.Add(patchCopy);
      }
      return patchesCopy;
    }

    /**
     * Merge a set of patches onto the text.  Return a patched text, as well
     * as an array of true/false values indicating which patches were applied.
     * @param patches Array of Patch objects
     * @param text Old text.
     * @return Two element Object array, containing the new text and an array of
     *      bool values.
     */
    public Object[] patch_apply(List<Patch> patches, string text) {
      if (patches.Count == 0) {
        return new Object[] { text, new bool[0] };
      }

      // Deep copy the patches so that no changes are made to originals.
      patches = patch_deepCopy(patches);

      string nullPadding = this.patch_addPadding(patches);
      text = nullPadding + text + nullPadding;
      patch_splitMax(patches);

      int x = 0;
      // delta keeps track of the offset between the expected and actual
      // location of the previous patch.  If there are patches expected at
      // positions 10 and 20, but the first patch was found at 12, delta is 2
      // and the second patch has an effective expected position of 22.
      int delta = 0;
      bool[] results = new bool[patches.Count];
      foreach (Patch aPatch in patches) {
        int expected_loc = aPatch.start2 + delta;
        string text1 = diff_text1(aPatch.diffs);
        int start_loc;
        int end_loc = -1;
        if (text1.Length > this.Match_MaxBits) {
          // patch_splitMax will only provide an oversized pattern
          // in the case of a monster delete.
          start_loc = match_main(text,
              text1.Substring(0, this.Match_MaxBits), expected_loc);
          if (start_loc != -1) {
            end_loc = match_main(text,
                text1.Substring(text1.Length - this.Match_MaxBits),
                expected_loc + text1.Length - this.Match_MaxBits);
            if (end_loc == -1 || start_loc >= end_loc) {
              // Can't find valid trailing context.  Drop this patch.
              start_loc = -1;
            }
          }
        } else {
          start_loc = this.match_main(text, text1, expected_loc);
        }
        if (start_loc == -1) {
          // No match found.  :(
          results[x] = false;
          // Subtract the delta for this failed patch from subsequent patches.
          delta -= aPatch.length2 - aPatch.length1;
        } else {
          // Found a match.  :)
          results[x] = true;
          delta = start_loc - expected_loc;
          string text2;
          if (end_loc == -1) {
            text2 = text.JavaSubstring(start_loc,
                Math.Min(start_loc + text1.Length, text.Length));
          } else {
            text2 = text.JavaSubstring(start_loc,
                Math.Min(end_loc + this.Match_MaxBits, text.Length));
          }
          if (text1 == text2) {
            // Perfect match, just shove the Replacement text in.
            text = text.Substring(0, start_loc) + diff_text2(aPatch.diffs)
                + text.Substring(start_loc + text1.Length);
          } else {
            // Imperfect match.  Run a diff to get a framework of equivalent
            // indices.
            List<Diff> diffs = diff_main(text1, text2, false);
            if (text1.Length > this.Match_MaxBits
                && this.diff_levenshtein(diffs) / (float) text1.Length
                > this.Patch_DeleteThreshold) {
              // The end points match, but the content is unacceptably bad.
              results[x] = false;
            } else {
              diff_cleanupSemanticLossless(diffs);
              int index1 = 0;
              foreach (Diff aDiff in aPatch.diffs) {
                if (aDiff.operation != Operation.EQUAL) {
                  int index2 = diff_xIndex(diffs, index1);
                  if (aDiff.operation == Operation.INSERT) {
                    // Insertion
                    text = text.Insert(start_loc + index2, aDiff.text);
                  } else if (aDiff.operation == Operation.DELETE) {
                    // Deletion
                    text = text.Remove(start_loc + index2, diff_xIndex(diffs,
                        index1 + aDiff.text.Length) - index2);
                  }
                }
                if (aDiff.operation != Operation.DELETE) {
                  index1 += aDiff.text.Length;
                }
              }
            }
          }
        }
        x++;
      }
      // Strip the padding off.
      text = text.Substring(nullPadding.Length, text.Length
          - 2 * nullPadding.Length);
      return new Object[] { text, results };
    }

    /**
     * Add some padding on text start and end so that edges can match something.
     * Intended to be called only from within patch_apply.
     * @param patches Array of Patch objects.
     * @return The padding string added to each side.
     */
    public string patch_addPadding(List<Patch> patches) {
      short paddingLength = this.Patch_Margin;
      string nullPadding = string.Empty;
      for (short x = 1; x <= paddingLength; x++) {
        nullPadding += (char)x;
      }

      // Bump all the patches forward.
      foreach (Patch aPatch in patches) {
        aPatch.start1 += paddingLength;
        aPatch.start2 += paddingLength;
      }

      // Add some padding on start of first diff.
      Patch patch = patches.First();
      List<Diff> diffs = patch.diffs;
      if (diffs.Count == 0 || diffs.First().operation != Operation.EQUAL) {
        // Add nullPadding equality.
        diffs.Insert(0, new Diff(Operation.EQUAL, nullPadding));
        patch.start1 -= paddingLength;  // Should be 0.
        patch.start2 -= paddingLength;  // Should be 0.
        patch.length1 += paddingLength;
        patch.length2 += paddingLength;
      } else if (paddingLength > diffs.First().text.Length) {
        // Grow first equality.
        Diff firstDiff = diffs.First();
        int extraLength = paddingLength - firstDiff.text.Length;
        firstDiff.text = nullPadding.Substring(firstDiff.text.Length)
            + firstDiff.text;
        patch.start1 -= extraLength;
        patch.start2 -= extraLength;
        patch.length1 += extraLength;
        patch.length2 += extraLength;
      }

      // Add some padding on end of last diff.
      patch = patches.Last();
      diffs = patch.diffs;
      if (diffs.Count == 0 || diffs.Last().operation != Operation.EQUAL) {
        // Add nullPadding equality.
        diffs.Add(new Diff(Operation.EQUAL, nullPadding));
        patch.length1 += paddingLength;
        patch.length2 += paddingLength;
      } else if (paddingLength > diffs.Last().text.Length) {
        // Grow last equality.
        Diff lastDiff = diffs.Last();
        int extraLength = paddingLength - lastDiff.text.Length;
        lastDiff.text += nullPadding.Substring(0, extraLength);
        patch.length1 += extraLength;
        patch.length2 += extraLength;
      }

      return nullPadding;
    }

    /**
     * Look through the patches and break up any which are longer than the
     * maximum limit of the match algorithm.
     * Intended to be called only from within patch_apply.
     * @param patches List of Patch objects.
     */
    public void patch_splitMax(List<Patch> patches) {
      short patch_size = this.Match_MaxBits;
      for (int x = 0; x < patches.Count; x++) {
        if (patches[x].length1 <= patch_size) {
          continue;
        }
        Patch bigpatch = patches[x];
        // Remove the big old patch.
        patches.Splice(x--, 1);
        int start1 = bigpatch.start1;
        int start2 = bigpatch.start2;
        string precontext = string.Empty;
        while (bigpatch.diffs.Count != 0) {
          // Create one of several smaller patches.
          Patch patch = new Patch();
          bool empty = true;
          patch.start1 = start1 - precontext.Length;
          patch.start2 = start2 - precontext.Length;
          if (precontext.Length != 0) {
            patch.length1 = patch.length2 = precontext.Length;
            patch.diffs.Add(new Diff(Operation.EQUAL, precontext));
          }
          while (bigpatch.diffs.Count != 0
              && patch.length1 < patch_size - this.Patch_Margin) {
            Operation diff_type = bigpatch.diffs[0].operation;
            string diff_text = bigpatch.diffs[0].text;
            if (diff_type == Operation.INSERT) {
              // Insertions are harmless.
              patch.length2 += diff_text.Length;
              start2 += diff_text.Length;
              patch.diffs.Add(bigpatch.diffs.First());
              bigpatch.diffs.RemoveAt(0);
              empty = false;
            } else if (diff_type == Operation.DELETE && patch.diffs.Count == 1
                && patch.diffs.First().operation == Operation.EQUAL
                && diff_text.Length > 2 * patch_size) {
              // This is a large deletion.  Let it pass in one chunk.
              patch.length1 += diff_text.Length;
              start1 += diff_text.Length;
              empty = false;
              patch.diffs.Add(new Diff(diff_type, diff_text));
              bigpatch.diffs.RemoveAt(0);
            } else {
              // Deletion or equality.  Only take as much as we can stomach.
              diff_text = diff_text.Substring(0, Math.Min(diff_text.Length,
                  patch_size - patch.length1 - Patch_Margin));
              patch.length1 += diff_text.Length;
              start1 += diff_text.Length;
              if (diff_type == Operation.EQUAL) {
                patch.length2 += diff_text.Length;
                start2 += diff_text.Length;
              } else {
                empty = false;
              }
              patch.diffs.Add(new Diff(diff_type, diff_text));
              if (diff_text == bigpatch.diffs[0].text) {
                bigpatch.diffs.RemoveAt(0);
              } else {
                bigpatch.diffs[0].text =
                    bigpatch.diffs[0].text.Substring(diff_text.Length);
              }
            }
          }
          // Compute the head context for the next patch.
          precontext = this.diff_text2(patch.diffs);
          precontext = precontext.Substring(Math.Max(0,
              precontext.Length - this.Patch_Margin));

          string postcontext = null;
          // Append the end context for this patch.
          if (diff_text1(bigpatch.diffs).Length > Patch_Margin) {
            postcontext = diff_text1(bigpatch.diffs)
                .Substring(0, Patch_Margin);
          } else {
            postcontext = diff_text1(bigpatch.diffs);
          }

          if (postcontext.Length != 0) {
            patch.length1 += postcontext.Length;
            patch.length2 += postcontext.Length;
            if (patch.diffs.Count != 0
                && patch.diffs[patch.diffs.Count - 1].operation
                == Operation.EQUAL) {
              patch.diffs[patch.diffs.Count - 1].text += postcontext;
            } else {
              patch.diffs.Add(new Diff(Operation.EQUAL, postcontext));
            }
          }
          if (!empty) {
            patches.Splice(++x, 0, patch);
          }
        }
      }
    }

    /**
     * Take a list of patches and return a textual representation.
     * @param patches List of Patch objects.
     * @return Text representation of patches.
     */
    public string patch_toText(List<Patch> patches) {
      StringBuilder text = new StringBuilder();
      foreach (Patch aPatch in patches) {
        text.Append(aPatch);
      }
      return text.ToString();
    }

    /**
     * Parse a textual representation of patches and return a List of Patch
     * objects.
     * @param textline Text representation of patches.
     * @return List of Patch objects.
     * @throws ArgumentException If invalid input.
     */
    public List<Patch> patch_fromText(string textline) {
      List<Patch> patches = new List<Patch>();
      if (textline.Length == 0) {
        return patches;
      }
      string[] text = textline.Split('\n');
      int textPointer = 0;
      Patch patch;
      Regex patchHeader
          = new Regex("^@@ -(\\d+),?(\\d*) \\+(\\d+),?(\\d*) @@$");
      Match m;
      char sign;
      string line;
      while (textPointer < text.Length) {
        m = patchHeader.Match(text[textPointer]);
        if (!m.Success) {
          throw new ArgumentException("Invalid patch string: "
              + text[textPointer]);
        }
        patch = new Patch();
        patches.Add(patch);
        patch.start1 = Convert.ToInt32(m.Groups[1].Value);
        if (m.Groups[2].Length == 0) {
          patch.start1--;
          patch.length1 = 1;
        } else if (m.Groups[2].Value == "0") {
          patch.length1 = 0;
        } else {
          patch.start1--;
          patch.length1 = Convert.ToInt32(m.Groups[2].Value);
        }

        patch.start2 = Convert.ToInt32(m.Groups[3].Value);
        if (m.Groups[4].Length == 0) {
          patch.start2--;
          patch.length2 = 1;
        } else if (m.Groups[4].Value == "0") {
          patch.length2 = 0;
        } else {
          patch.start2--;
          patch.length2 = Convert.ToInt32(m.Groups[4].Value);
        }
        textPointer++;

        while (textPointer < text.Length) {
          try {
            sign = text[textPointer][0];
          } catch (IndexOutOfRangeException) {
            // Blank line?  Whatever.
            textPointer++;
            continue;
          }
          line = text[textPointer].Substring(1);
          line = line.Replace("+", "%2b");
          line = HttpUtility.UrlDecode(line, new UTF8Encoding(false, true));
          if (sign == '-') {
            // Deletion.
            patch.diffs.Add(new Diff(Operation.DELETE, line));
          } else if (sign == '+') {
            // Insertion.
            patch.diffs.Add(new Diff(Operation.INSERT, line));
          } else if (sign == ' ') {
            // Minor equality.
            patch.diffs.Add(new Diff(Operation.EQUAL, line));
          } else if (sign == '@') {
            // Start of next patch.
            break;
          } else {
            // WTF?
            throw new ArgumentException(
                "Invalid patch mode '" + sign + "' in: " + line);
          }
          textPointer++;
        }
      }
      return patches;
    }

    /**
     * Unescape selected chars for compatability with JavaScript's encodeURI.
     * In speed critical applications this could be dropped since the
     * receiving application will certainly decode these fine.
     * Note that this function is case-sensitive.  Thus "%3F" would not be
     * unescaped.  But this is ok because it is only called with the output of
     * HttpUtility.UrlEncode which returns lowercase hex.
     *
     * Example: "%3f" -> "?", "%24" -> "$", etc.
     *
     * @param str The string to escape.
     * @return The escaped string.
     */
    public static string unescapeForEncodeUriCompatability(string str) {
      return str.Replace("%21", "!").Replace("%7e", "~")
          .Replace("%27", "'").Replace("%28", "(").Replace("%29", ")")
          .Replace("%3b", ";").Replace("%2f", "/").Replace("%3f", "?")
          .Replace("%3a", ":").Replace("%40", "@").Replace("%26", "&")
          .Replace("%3d", "=").Replace("%2b", "+").Replace("%24", "$")
          .Replace("%2c", ",").Replace("%23", "#");
    }
  }
}