copytrace: move fast heuristic copytracing algorithm to core

copytrace extension in fb-hgext has a heuristic implementation of copy tracing
which is faster than the current copy tracing. The heuristic limits the search
of copies to just files that are either:

1) Renames in the same directory
2) Moved to other directory with same name

The default copytrace implementation is very slow as it finds all the new files
that were added from merge base up to the head commit and for each file it
checks whether it this was copied or moved version of a different file.

Stash@fb did analysis for the above heuristics on the fb repo and found that
among 2,443,768 moves/copies there are only 32,234 moves/copies which does not
fall under the above heuristics which is approx. 0.013 of total copies.

This patch moves the heuristics algorithm under config
`experimental.copytrace=heuristics`.

While moving fbext to core, this patch removes couple of less useful config
options named `sourcecommitlimit` and `maxmovescandidatestocheck`.

Tests are also added for the heuristics algorithm, which are basically copied
from fbext/tests/test-copytrace.t. The tests follow a pattern creating a server
repo and then cloning to a local repo to create public and draft changesets, the
distinction which will be useful in upcoming patches.

After this patch `experimental.copytrace` has the following behaviour:

1) `off`: turns off copytracing
2) `heuristics`: use the heuristic algorithm added in this patch.
3) everything else: use the full copytracing algorithm

.. feature::

   A new fast heuristic algorithm for copytracing which assumes that the files
   moves are either::
   1) Renames in the same directory
   2) Moves in other directories with same names
   You can use this algorithm by setting `experimental.copytrace=heuristics`.

Differential Revision: https://phab.mercurial-scm.org/D623

tests/test-copytrace-heuristics.t

@@ -0,0 +1,591 b''
+Test for the heuristic copytracing algorithm
+============================================
+
+  $ cat >> $TESTTMP/copytrace.sh << '__EOF__'
+  > initclient() {
+  > cat >> $1/.hg/hgrc <<EOF
+  > [experimental]
+  > copytrace = heuristics
+  > EOF
+  > }
+  > __EOF__
+  $ . "$TESTTMP/copytrace.sh"
+
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > rebase=
+  > shelve=
+  > EOF
+
+Check filename heuristics (same dirname and same basename)
+  $ hg init server
+  $ cd server
+  $ echo a > a
+  $ mkdir dir
+  $ echo a > dir/file.txt
+  $ hg addremove
+  adding a
+  adding dir/file.txt
+  $ hg ci -m initial
+  $ hg mv a b
+  $ hg mv -q dir dir2
+  $ hg ci -m 'mv a b, mv dir/ dir2/'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 0
+  $ echo b > a
+  $ echo b > dir/file.txt
+  $ hg ci -qm 'mod a, mod dir/file.txt'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: 557f403c0afd2a3cf15d7e2fb1f1001a8b85e081
+  |   desc: mod a, mod dir/file.txt, phase: draft
+  | o  changeset: 928d74bc9110681920854d845c06959f6dfc9547
+  |/    desc: mv a b, mv dir/ dir2/, phase: public
+  o  changeset: 3c482b16e54596fed340d05ffaf155f156cda7ee
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 1
+  rebasing 2:557f403c0afd "mod a, mod dir/file.txt" (tip)
+  merging b and a to b
+  merging dir2/file.txt and dir/file.txt to dir2/file.txt
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/557f403c0afd-9926eeff-rebase.hg (glob)
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Make sure filename heuristics do not when they are not related
+  $ hg init server
+  $ cd server
+  $ echo 'somecontent' > a
+  $ hg add a
+  $ hg ci -m initial
+  $ hg rm a
+  $ echo 'completelydifferentcontext' > b
+  $ hg add b
+  $ hg ci -m 'rm a, add b'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 0
+  $ printf 'somecontent\nmoarcontent' > a
+  $ hg ci -qm 'mode a'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: d526312210b9e8f795d576a77dc643796384d86e
+  |   desc: mode a, phase: draft
+  | o  changeset: 46985f76c7e5e5123433527f5c8526806145650b
+  |/    desc: rm a, add b, phase: public
+  o  changeset: e5b71fb099c29d9172ef4a23485aaffd497e4cc0
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 1
+  rebasing 2:d526312210b9 "mode a" (tip)
+  other [source] changed a which local [dest] deleted
+  use (c)hanged version, leave (d)eleted, or leave (u)nresolved? u
+  unresolved conflicts (see hg resolve, then hg rebase --continue)
+  [1]
+
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Test when lca didn't modified the file that was moved
+  $ hg init server
+  $ cd server
+  $ echo 'somecontent' > a
+  $ hg add a
+  $ hg ci -m initial
+  $ echo c > c
+  $ hg add c
+  $ hg ci -m randomcommit
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 1
+  $ echo b > a
+  $ hg ci -qm 'mod a'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: 9d5cf99c3d9f8e8b05ba55421f7f56530cfcf3bc
+  |   desc: mod a, phase: draft
+  | o  changeset: d760186dd240fc47b91eb9f0b58b0002aaeef95d
+  |/    desc: mv a b, phase: public
+  o  changeset: 48e1b6ba639d5d7fb313fa7989eebabf99c9eb83
+  |   desc: randomcommit, phase: public
+  o  changeset: e5b71fb099c29d9172ef4a23485aaffd497e4cc0
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 2
+  rebasing 3:9d5cf99c3d9f "mod a" (tip)
+  merging b and a to b
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/9d5cf99c3d9f-f02358cc-rebase.hg (glob)
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Rebase "backwards"
+  $ hg init server
+  $ cd server
+  $ echo 'somecontent' > a
+  $ hg add a
+  $ hg ci -m initial
+  $ echo c > c
+  $ hg add c
+  $ hg ci -m randomcommit
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 2
+  $ echo b > b
+  $ hg ci -qm 'mod b'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: fbe97126b3969056795c462a67d93faf13e4d298
+  |   desc: mod b, phase: draft
+  o  changeset: d760186dd240fc47b91eb9f0b58b0002aaeef95d
+  |   desc: mv a b, phase: public
+  o  changeset: 48e1b6ba639d5d7fb313fa7989eebabf99c9eb83
+  |   desc: randomcommit, phase: public
+  o  changeset: e5b71fb099c29d9172ef4a23485aaffd497e4cc0
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 0
+  rebasing 3:fbe97126b396 "mod b" (tip)
+  merging a and b to a
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/fbe97126b396-cf5452a1-rebase.hg (glob)
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Rebase draft commit on top of draft commit
+  $ hg init repo
+  $ initclient repo
+  $ cd repo
+  $ echo 'somecontent' > a
+  $ hg add a
+  $ hg ci -m initial
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  $ hg up -q ".^"
+  $ echo b > a
+  $ hg ci -qm 'mod a'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: 5268f05aa1684cfb5741e9eb05eddcc1c5ee7508
+  |   desc: mod a, phase: draft
+  | o  changeset: 542cb58df733ee48fa74729bd2cdb94c9310d362
+  |/    desc: mv a b, phase: draft
+  o  changeset: e5b71fb099c29d9172ef4a23485aaffd497e4cc0
+      desc: initial, phase: draft
+
+  $ hg rebase -s . -d 1
+  rebasing 2:5268f05aa168 "mod a" (tip)
+  merging b and a to b
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/5268f05aa168-284f6515-rebase.hg (glob)
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Check a few potential move candidates
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ mkdir dir
+  $ echo a > dir/a
+  $ hg add dir/a
+  $ hg ci -qm initial
+  $ hg mv dir/a dir/b
+  $ hg ci -qm 'mv dir/a dir/b'
+  $ mkdir dir2
+  $ echo b > dir2/a
+  $ hg add dir2/a
+  $ hg ci -qm 'create dir2/a'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 0
+  $ echo b > dir/a
+  $ hg ci -qm 'mod dir/a'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: 6b2f4cece40fd320f41229f23821256ffc08efea
+  |   desc: mod dir/a, phase: draft
+  | o  changeset: 4494bf7efd2e0dfdd388e767fb913a8a3731e3fa
+  | |   desc: create dir2/a, phase: public
+  | o  changeset: b1784dfab6ea6bfafeb11c0ac50a2981b0fe6ade
+  |/    desc: mv dir/a dir/b, phase: public
+  o  changeset: 36859b8907c513a3a87ae34ba5b1e7eea8c20944
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 2
+  rebasing 3:6b2f4cece40f "mod dir/a" (tip)
+  merging dir/b and dir/a to dir/b
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/6b2f4cece40f-503efe60-rebase.hg (glob)
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Move file in one branch and delete it in another
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  $ hg up -q ".^"
+  $ hg rm a
+  $ hg ci -m 'del a'
+  created new head
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: 7d61ee3b1e48577891a072024968428ba465c47b
+  |   desc: del a, phase: draft
+  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  |/    desc: mv a b, phase: draft
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+
+  $ hg rebase -s 1 -d 2
+  rebasing 1:472e38d57782 "mv a b"
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/472e38d57782-17d50e29-rebase.hg (glob)
+  $ hg up -q c492ed3c7e35dcd1dc938053b8adf56e2cfbd062
+  $ ls
+  b
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Move a directory in draft branch
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ mkdir dir
+  $ echo a > dir/a
+  $ hg add dir/a
+  $ hg ci -qm initial
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ echo b > dir/a
+  $ hg ci -qm 'mod dir/a'
+  $ hg up -q ".^"
+  $ hg mv -q dir/ dir2
+  $ hg ci -qm 'mv dir/ dir2/'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: a33d80b6e352591dfd82784e1ad6cdd86b25a239
+  |   desc: mv dir/ dir2/, phase: draft
+  | o  changeset: 6b2f4cece40fd320f41229f23821256ffc08efea
+  |/    desc: mod dir/a, phase: draft
+  o  changeset: 36859b8907c513a3a87ae34ba5b1e7eea8c20944
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 1
+  rebasing 2:a33d80b6e352 "mv dir/ dir2/" (tip)
+  merging dir/a and dir2/a to dir2/a
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/a33d80b6e352-fecb9ada-rebase.hg (glob)
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Move file twice and rebase mod on top of moves
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  $ hg mv b c
+  $ hg ci -m 'mv b c'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 0
+  $ echo c > a
+  $ hg ci -m 'mod a'
+  created new head
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: d413169422167a3fa5275fc5d71f7dea9f5775f3
+  |   desc: mod a, phase: draft
+  | o  changeset: d3efd280421d24f9f229997c19e654761c942a71
+  | |   desc: mv b c, phase: public
+  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  |/    desc: mv a b, phase: public
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+  $ hg rebase -s . -d 2
+  rebasing 3:d41316942216 "mod a" (tip)
+  merging c and a to c
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/d41316942216-2b5949bc-rebase.hg (glob)
+
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Move file twice and rebase moves on top of mods
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  $ hg mv b c
+  $ hg ci -m 'mv b c'
+  $ hg up -q 0
+  $ echo c > a
+  $ hg ci -m 'mod a'
+  created new head
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: d413169422167a3fa5275fc5d71f7dea9f5775f3
+  |   desc: mod a, phase: draft
+  | o  changeset: d3efd280421d24f9f229997c19e654761c942a71
+  | |   desc: mv b c, phase: draft
+  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  |/    desc: mv a b, phase: draft
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+  $ hg rebase -s 1 -d .
+  rebasing 1:472e38d57782 "mv a b"
+  merging a and b to b
+  rebasing 2:d3efd280421d "mv b c"
+  merging b and c to c
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/472e38d57782-ab8d3c58-rebase.hg (glob)
+
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Move one file and add another file in the same folder in one branch, modify file in another branch
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  $ echo c > c
+  $ hg add c
+  $ hg ci -m 'add c'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 0
+  $ echo b > a
+  $ hg ci -m 'mod a'
+  created new head
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
+  |   desc: mod a, phase: draft
+  | o  changeset: b1a6187e79fbce851bb584eadcb0cc4a80290fd9
+  | |   desc: add c, phase: public
+  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  |/    desc: mv a b, phase: public
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 2
+  rebasing 3:ef716627c70b "mod a" (tip)
+  merging b and a to b
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/ef716627c70b-24681561-rebase.hg (glob)
+  $ ls
+  b
+  c
+  $ cat b
+  b
+
+Merge test
+  $ hg init server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ echo b > a
+  $ hg ci -m 'modify a'
+  $ hg up -q 0
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+  created new head
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 2
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  |   desc: mv a b, phase: public
+  | o  changeset: b0357b07f79129a3d08a68621271ca1352ae8a09
+  |/    desc: modify a, phase: public
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+
+  $ hg merge 1
+  merging b and a to b
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci -m merge
+  $ ls
+  b
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Copy and move file
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ hg cp a c
+  $ hg mv a b
+  $ hg ci -m 'cp a c, mv a b'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q 0
+  $ echo b > a
+  $ hg ci -m 'mod a'
+  created new head
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
+  |   desc: mod a, phase: draft
+  | o  changeset: 4fc3fd13fbdb89ada6b75bfcef3911a689a0dde8
+  |/    desc: cp a c, mv a b, phase: public
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+
+  $ hg rebase -s . -d 1
+  rebasing 2:ef716627c70b "mod a" (tip)
+  merging b and a to b
+  merging c and a to c
+  saved backup bundle to $TESTTMP/repo/repo/.hg/strip-backup/ef716627c70b-24681561-rebase.hg (glob)
+  $ ls
+  b
+  c
+  $ cat b
+  b
+  $ cat c
+  b
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Do a merge commit with many consequent moves in one branch
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ echo b > a
+  $ hg ci -qm 'mod a'
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ hg up -q ".^"
+  $ hg mv a b
+  $ hg ci -qm 'mv a b'
+  $ hg mv b c
+  $ hg ci -qm 'mv b c'
+  $ hg up -q 1
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  o  changeset: d3efd280421d24f9f229997c19e654761c942a71
+  |   desc: mv b c, phase: draft
+  o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  |   desc: mv a b, phase: draft
+  | @  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
+  |/    desc: mod a, phase: public
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+
+  $ hg merge 3
+  merging a and c to c
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci -qm 'merge'
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @    changeset: cd29b0d08c0f39bfed4cde1b40e30f419db0c825
+  |\    desc: merge, phase: draft
+  | o  changeset: d3efd280421d24f9f229997c19e654761c942a71
+  | |   desc: mv b c, phase: draft
+  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  | |   desc: mv a b, phase: draft
+  o |  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
+  |/    desc: mod a, phase: public
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+  $ ls
+  c
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo
+
+Test shelve/unshelve
+  $ hg init server
+  $ initclient server
+  $ cd server
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m initial
+  $ cd ..
+  $ hg clone -q server repo
+  $ initclient repo
+  $ cd repo
+  $ echo b > a
+  $ hg shelve
+  shelved as default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg mv a b
+  $ hg ci -m 'mv a b'
+
+  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
+  @  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
+  |   desc: mv a b, phase: draft
+  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
+      desc: initial, phase: public
+  $ hg unshelve
+  unshelving change 'default'
+  rebasing shelved changes
+  rebasing 2:45f63161acea "changes to: initial" (tip)
+  merging b and a to b
+  $ ls
+  b
+  $ cat b
+  b
+  $ cd ..
+  $ rm -rf server
+  $ rm -rf repo

mercurial/copies.py

@@ -7,7 +7,9 b''
 
 from __future__ import absolute_import
 
+import collections
 import heapq
+import os
 
 from . import (
     match as matchmod,
@@ -364,6 +366,8 b' def mergecopies(repo, c1, c2, base):'
     # rebase.
     if copytracing == 'off':
         return {}, {}, {}, {}, {}
+    elif copytracing == 'heuristics':
+        return _heuristicscopytracing(repo, c1, c2, base)
     else:
         return _fullcopytracing(repo, c1, c2, base)
 
@@ -599,6 +603,94 b' def _fullcopytracing(repo, c1, c2, base)'
 
     return copy, movewithdir, diverge, renamedelete, dirmove
 
+def _heuristicscopytracing(repo, c1, c2, base):
+    """ Fast copytracing using filename heuristics
+
+    Assumes that moves or renames are of following two types:
+
+    1) Inside a directory only (same directory name but different filenames)
+    2) Move from one directory to another
+                    (same filenames but different directory names)
+
+    Works only when there are no merge commits in the "source branch".
+    Source branch is commits from base up to c2 not including base.
+
+    If merge is involved it fallbacks to _fullcopytracing().
+
+    Can be used by setting the following config:
+
+        [experimental]
+        copytrace = heuristics
+    """
+
+    if c1.rev() is None:
+        c1 = c1.p1()
+    if c2.rev() is None:
+        c2 = c2.p1()
+
+    copies = {}
+
+    changedfiles = set()
+    m1 = c1.manifest()
+    if not repo.revs('%d::%d', base.rev(), c2.rev()):
+        # If base is not in c2 branch, we switch to fullcopytracing
+        repo.ui.debug("switching to full copytracing as base is not "
+                      "an ancestor of c2\n")
+        return _fullcopytracing(repo, c1, c2, base)
+
+    ctx = c2
+    while ctx != base:
+        if len(ctx.parents()) == 2:
+            # To keep things simple let's not handle merges
+            repo.ui.debug("switching to full copytracing because of merges\n")
+            return _fullcopytracing(repo, c1, c2, base)
+        changedfiles.update(ctx.files())
+        ctx = ctx.p1()
+
+    cp = _forwardcopies(base, c2)
+    for dst, src in cp.iteritems():
+        if src in m1:
+            copies[dst] = src
+
+    # file is missing if it isn't present in the destination, but is present in
+    # the base and present in the source.
+    # Presence in the base is important to exclude added files, presence in the
+    # source is important to exclude removed files.
+    missingfiles = filter(lambda f: f not in m1 and f in base and f in c2,
+                          changedfiles)
+
+    if missingfiles:
+        basenametofilename = collections.defaultdict(list)
+        dirnametofilename = collections.defaultdict(list)
+
+        for f in m1.filesnotin(base.manifest()):
+            basename = os.path.basename(f)
+            dirname = os.path.dirname(f)
+            basenametofilename[basename].append(f)
+            dirnametofilename[dirname].append(f)
+
+        # in case of a rebase/graft, base may not be a common ancestor
+        anc = c1.ancestor(c2)
+
+        for f in missingfiles:
+            basename = os.path.basename(f)
+            dirname = os.path.dirname(f)
+            samebasename = basenametofilename[basename]
+            samedirname = dirnametofilename[dirname]
+            movecandidates = samebasename + samedirname
+            # f is guaranteed to be present in c2, that's why
+            # c2.filectx(f) won't fail
+            f2 = c2.filectx(f)
+            for candidate in movecandidates:
+                f1 = c1.filectx(candidate)
+                if _related(f1, f2, anc.rev()):
+                    # if there are a few related copies then we'll merge
+                    # changes into all of them. This matches the behaviour
+                    # of upstream copytracing
+                    copies[candidate] = f
+
+    return copies, {}, {}, {}, {}
+
 def _related(f1, f2, limit):
     """return True if f1 and f2 filectx have a common ancestor