findrenames: Optimise "addremove -s100" by matching files by their SHA1 hashes.

We speed up 'findrenames' for the usecase when a user specifies they
want a similarity of 100% by matching files by their exact SHA1 hash
value. This reduces the number of comparisons required to find exact
matches from O(n^2) to O(n).

While it would be nice if we could just use mercurial's pre-calculated
SHA1 hash for existing files, this hash includes the file's ancestor
information making it unsuitable for our purposes. Instead, we calculate
the hash of old content from scratch.

The following benchmarks were taken on the current head of crew:

addremove 100% similarity:
  rm -rf *; hg up -C; mv tests tests.new
  hg --time addremove -s100 --dry-run

  before:  real 176.350 secs (user 128.890+0.000 sys 47.430+0.000)
  after:   real   2.130 secs (user   1.890+0.000 sys  0.240+0.000)

addremove 75% similarity:
  rm -rf *; hg up -C; mv tests tests.new; \
      for i in tests.new/*; do echo x >> $i; done
  hg --time addremove -s75  --dry-run

  before: real 264.560 secs (user 215.130+0.000 sys 49.410+0.000)
  after:  real 218.710 secs (user 172.790+0.000 sys 45.870+0.000)

mercurial/similar.py

@@ -10,29 +10,49 b' import util'
 import mdiff
 import bdiff
 
-def findrenames(repo, added, removed, threshold):
-    '''find renamed files -- yields (before, after, score) tuples'''
+def _findexactmatches(repo, added, removed):
+    '''find renamed files that have no changes
+
+    Takes a list of new filectxs and a list of removed filectxs, and yields
+    (before, after) tuples of exact matches.
+    '''
+    numfiles = len(added) + len(removed)
+
+    # Get hashes of removed files.
+    hashes = {}
+    for i, fctx in enumerate(removed):
+        repo.ui.progress(_('searching for exact renames'), i, total=numfiles)
+        h = util.sha1(fctx.data()).digest()
+        hashes[h] = fctx
+
+    # For each added file, see if it corresponds to a removed file.
+    for i, fctx in enumerate(added):
+        repo.ui.progress(_('searching for exact renames'), i + len(removed),
+                total=numfiles)
+        h = util.sha1(fctx.data()).digest()
+        if h in hashes:
+            yield (hashes[h], fctx)
+
+    # Done
+    repo.ui.progress(_('searching for exact renames'), None)
+
+def _findsimilarmatches(repo, added, removed, threshold):
+    '''find potentially renamed files based on similar file content
+
+    Takes a list of new filectxs and a list of removed filectxs, and yields
+    (before, after, score) tuples of partial matches.
+    '''
     copies = {}
-    ctx = repo['.']
     for i, r in enumerate(removed):
-        repo.ui.progress(_('searching'), i, total=len(removed))
-        if r not in ctx:
-            continue
-        fctx = ctx.filectx(r)
+        repo.ui.progress(_('searching for similar files'), i, total=len(removed))
 
         # lazily load text
         @util.cachefunc
         def data():
-            orig = fctx.data()
+            orig = r.data()
             return orig, mdiff.splitnewlines(orig)
 
         def score(text):
-            if not len(text):
-                return 0.0
-            if not fctx.cmp(text):
-                return 1.0
-            if threshold == 1.0:
-                return 0.0
             orig, lines = data()
             # bdiff.blocks() returns blocks of matching lines
             # count the number of bytes in each
@@ -47,7 +67,7 b' def findrenames(repo, added, removed, th'
 
         for a in added:
             bestscore = copies.get(a, (None, threshold))[1]
-            myscore = score(repo.wread(a))
+            myscore = score(a.data())
             if myscore >= bestscore:
                 copies[a] = (r, myscore)
     repo.ui.progress(_('searching'), None)
@@ -56,4 +76,28 b' def findrenames(repo, added, removed, th'
         source, score = v
         yield source, dest, score
 
+def findrenames(repo, added, removed, threshold):
+    '''find renamed files -- yields (before, after, score) tuples'''
+    parentctx = repo['.']
+    workingctx = repo[None]
 
+    # Zero length files will be frequently unrelated to each other, and
+    # tracking the deletion/addition of such a file will probably cause more
+    # harm than good. We strip them out here to avoid matching them later on.
+    addedfiles = set([workingctx[fp] for fp in added
+            if workingctx[fp].size() > 0])
+    removedfiles = set([parentctx[fp] for fp in removed
+            if fp in parentctx and parentctx[fp].size() > 0])
+
+    # Find exact matches.
+    for (a, b) in _findexactmatches(repo,
+            sorted(addedfiles),sorted( removedfiles)):
+        addedfiles.remove(b)
+        yield (a.path(), b.path(), 1.0)
+
+    # If the user requested similar files to be matched, search for them also.
+    if threshold < 1.0:
+        for (a, b, score) in _findsimilarmatches(repo,
+                sorted(addedfiles), sorted(removedfiles), threshold):
+            yield (a.path(), b.path(), score)
+