Plug String.GetNonRandomizedHashCode to support cosmos memory model f…

…or strings allocated on the heap with no extra space
funroot-khan-wang · Jul 21, 2022 · 75f5245 · 75f5245
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diff --git a/Tests/Kernels/Cosmos.Compiler.Tests.Bcl/System/Collections/Generic/DictionaryTest.cs b/Tests/Kernels/Cosmos.Compiler.Tests.Bcl/System/Collections/Generic/DictionaryTest.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Numerics;
 using System.Threading.Tasks;
 using Cosmos.TestRunner;
 
@@ -566,6 +567,26 @@ public static void Execute()
 
             //TODO: Add GUID test once newGUID returns something other than a zero initialized guid.
 
+            //We need to ensure that string hashing is the same for constant strings and strings on the heap
+            {
+                var dictionary = new Dictionary<string, bool>
+                {
+                    {"a", true },
+                    {"ab", true },
+                    {"abab", true},
+                    {"ababa", true},
+                    {"aba", true }
+                };
+                string t = "aba";
+                string a = "ab";
+                string b = "a";
+                string alt = a + b;
+
+                Assert.IsTrue(dictionary.ContainsKey("a"), "Dictionary<string, bool> ContainsKey works for length 1");
+                Assert.IsTrue(dictionary.ContainsKey(t), "Dictionary<string, bool> ContainsKey works for length 3");
+				Assert.IsTrue(dictionary.ContainsKey(alt), "Dictionary<string, bool> ContainsKey works for length 3 allocated on the heap");
+				Assert.IsTrue(dictionary.ContainsKey("abab"), "Dictionary<string, bool> ContainsKey works for length 4");
+            }
         }
     }
 }
diff --git a/source/Cosmos.Core_Plugs/System/StringImpl.cs b/source/Cosmos.Core_Plugs/System/StringImpl.cs
@@ -974,5 +974,35 @@ public static int Compare(string strA, int indexA, string strB, int indexB, int
                     throw new ArgumentException("Not Supported StringComparison");
             }
         }
+
+        public static unsafe int GetNonRandomizedHashCode(string aString)
+        {
+            // the code is the same as the one used in .net except for the explicit == 2 and == 1 cases
+            // we need this since a new object can start directly behind the string in memory, so the standard
+            // implementation would read the allocated size of the next object and use it for the hash
+            fixed (char* ptr = &aString.AsSpan()[0])
+            {
+                uint num = 352654597u;
+                uint num2 = num;
+                uint* ptr2 = (uint*)ptr;
+                int num3 = aString.Length;
+                while (num3 > 2)
+                {
+                    num3 -= 4;
+                    num = (global::System.Numerics.BitOperations.RotateLeft(num, 5) + num) ^ *ptr2;
+                    num2 = (global::System.Numerics.BitOperations.RotateLeft(num2, 5) + num2) ^ ptr2[1];
+                    ptr2 += 2;
+                }
+                if (num3 == 2)
+                {
+                    num2 = (global::System.Numerics.BitOperations.RotateLeft(num2, 5) + num2) ^ *ptr2;
+                }
+                if (num3 == 1)
+                {
+                    num2 = (global::System.Numerics.BitOperations.RotateLeft(num2, 5) + num2) ^ *(char*)ptr2;
+                }
+                return (int)(num + num2 * 1566083941);
+            }
+        }
     }
 }