Merge "Add README.md"

Author: Treehugger Robot

tools/repo_diff/README.md

@@ -0,0 +1,107 @@
+# Repo Diff Trees
+
+repo_diff_trees.py compares two repo source trees and outputs reports on the
+findings.
+
+The ouput is in CSV and is easily consumable in a spreadsheet.
+
+In addition to importing to a spreadsheet, you can also create your own
+Data Studio dashboard like [this one](https://datastudio.google.com/open/0Bz6OwjyDcWYDbDJoQWtmRl8telU).
+
+If you wish to create your own dashboard follow the instructions below:
+
+1. Sync the two repo workspaces you wish to compare. Example:
+
+```
+mkdir android-8.0.0_r1
+cd android-8.0.0_r1
+repo init \
+  --manifest-url=https://android.googlesource.com/platform/manifest \
+  --manifest-branch=android-8.0.0_r1
+# Adjust the number of parallel jobs to your needs
+repo sync --current-branch --no-clone-bundle --no-tags --jobs=8
+cd ..
+mkdir android-8.0.0_r11
+cd android-8.0.0_r11
+repo init \
+  --manifest-url=https://android.googlesource.com/platform/manifest \
+  --manifest-branch=android-8.0.0_r11
+# Adjust the number of parallel jobs to your needs
+repo sync --current-branch --no-clone-bundle --no-tags --jobs=8
+cd ..
+```
+
+2. Run repo_diff_trees.py. Example:
+
+```
+python repo_diff_trees.py --exclusions_file=android_exclusions.txt \
+  android-8.0.0_r1 android-8.0.0_r11
+```
+
+3. Create a [new Google spreadsheet](https://docs.google.com/spreadsheets/create).
+4. Import projects.csv to a new sheet.
+5. Create a [new data source in Data Studio](https://datastudio.google.com/datasources/create).
+6. Connect your new data source to the project.csv sheet in the Google spreadsheet.
+7. Add a "Count Diff Status" field by selecting the menu next to the "Diff
+   Status" field and selecting "Count".
+8. Copy the [Data Studio dashboard sample](https://datastudio.google.com/open/0Bz6OwjyDcWYDbDJoQWtmRl8telU).
+    Make sure you are logged into your Google account and you have agreed to Data Studio's terms of service. Once
+    this is done you should get a link to "Make a copy of this report".
+9. Select your own data source for your copy of the dashboard when prompted.
+10. You may see a "Configuration Incomplete" message under
+    the "Modified Projects" pie chart. To address this select the pie chart,
+    then replace the "Invalid Metric" field for "Count Diff Status".
+
+## Analysis method
+
+repo_diff_trees.py goes through several stages when comparing two repo
+source trees:
+
+1. Match projects in source tree A with projects in source tree B.
+2. Diff projects that have a match.
+3. Find commits in source tree B that are not in source tree A.
+
+The first two steps are self explanatory. The method
+of finding commits only in B is explaned below.
+
+## Finding commits not upstream
+
+After matching up projects in both source tree
+and diffing, the last stage is to iterate
+through each project matching pair and find
+the commits that exist in the downstream project (B) but not the
+upstream project (A).
+
+'git cherry' is a useful tool that finds changes
+which exist in one branch but not another. It does so by
+not only by finding which commits that were merged
+to both branches, but also by matching cherry picked
+commits.
+
+However, there are many instances where a change in one branch
+can have an equivalent in another branch without being a merge
+or a cherry pick. Some examples are:
+
+* Commits that were squashed with other commits
+* Commits that were reauthored
+
+Cherry pick will not recognize these commits as having an equivalent
+yet they clearly do.
+
+This is addressed in two steps:
+
+1. First listing the "git cherry" commits that will give us the
+   list of changes for which "git cherry" could not find an equivalent.
+2. Then we "git blame" the entire project's source tree and compile
+   a list of changes that actually have lines of code in the tree.
+3. Finally we find the intersection: 'git cherry' changes
+   that have lines of code in the final source tree.
+
+
+## Caveats
+
+The method described above has proven effective on Android
+source trees. It does have shortcomings.
+
+* It does not find commits that only delete lines of code.
+* It does take into accounts merge conflict resolutions.

tools/repo_diff/exclusions.txt renamed to tools/repo_diff/android_exclusions.txt

@@ -12,7 +12,7 @@
 import argparse
 import os
 import subprocess
-import repo_diff_downstream
+import repo_diff_trees
 
 HELP_MSG = "Diff a repo (downstream) and its upstream"
 
@@ -152,7 +152,7 @@ def diff(manifest_url, manifest_branch, tag, upstream_manifest_url,
       upstream_workspace)
 
   # do the comparison
-  repo_diff_downstream.diff(
+  repo_diff_trees.diff(
       upstream_workspace,
       workspace,
       os.path.abspath("project.csv"),