more thoughts

doc/todo/hide_missing_files/comment_3_a9590ef65875a68eb1d1d7becc98b8b8._comment

@@ -0,0 +1,32 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 3"""
+ date="2018-10-17T15:50:46Z"
+ content="""
+This will need some state maintained, to allow efficiently querying for
+worktree files that have gained/lost content since the last sync.
+
+At least need to maintain a map of all keys that were gained/lost since
+last time.
+
+It would be easy to loop through `git ls-tree` of the master branch,
+look up all the keys with `git cat-file`, and find in the map.
+But slow...
+
+Better would be to maintain an additional map from filename to key.
+
+The keys database already maintains a map from key to worktree file
+(and back), but only in v6 mode, and only for unlocked files.
+Not useful for this.
+
+This would need anything that changes annex pointers
+(fix/unlock/lock/pre-commit) to update the map. Would also need to make
+sure that it gets updated with any changes to the checked out branch
+made by git commit or git-annex sync. Doable, but complicated.
+
+Or, the map could be of the sha1s of the annex pointers, then loop
+through `git ls-files --stage` and look up the sha1s in the map
+would not be too slow. On my laptop, with 85000 files in the tree, 
+that command takes 0.13s. Still needs to update the map whenever
+annex pointers are changed though.
+"""]]