view CONTRIBUTORS @ 37732:35632d392279

patch: implement a new worddiff algorithm

The previous worddiff algorithm has many problems. The major problem is it
does a "similarity check" that selects a subset of matched lines to do
inline diffs. It is a bad idea because:

  - The "similarity check" is non-obvious to users. For example, a simple
    change from "long long x" to "int64_t x" will fail the similarity check
    and won't be diff-ed as expected.
  - Selecting "lines" to diff won't work as people expect if there are line
    wrapping changes.
  - It has a sad time complexity if lines do not match, could be O(N^2)-ish.

There are other problems in implementation details.

  - Lines can match across distant hunks (if the next hunk does not have
    "-" lines).
  - "difflib" is slow.

The solution would be removing the "similarity check", and just diff all
words in a same hunk. So no content will be missed and everything will be
diff-ed as expected. This is similar to what code review tool like
Phabricator does.

This diff implements the word diff algorithm as described above. It also
avoids difflib to be faster.

Note about colors: To be consistent, "changed inserted" parts and "purely
insertion blocks" should have a same color, since they do not exist in the
previous version. Instead of highlighting differences, this patch chooses to
dim common parts. This is also more consistent with Phabricator or GitHub
webpage. That said, the labels are defined in a way that people can still
highlight changed parts and leave purely inserted/deleted hunks use the
"non-highlighted" color.

As one example, running:

  hg log -pr df50b87d8f736aff8dc281f816bddcd6f306930c mercurial/commands.py \
    --config experimental.worddiff=1 --color=debug --config diff.unified=0

The previous algorithm outputs:

  [diff.file_a|--- a/mercurial/commands.py	Fri Mar 09 15:53:41 2018 +0100]
  [diff.file_b|+++ b/mercurial/commands.py	Sat Mar 10 12:33:19 2018 +0530]
  [diff.hunk|@@ -2039,1 +2039,4 @@]
  [diff.deleted|-][diff.deleted.highlight|@command('^forget',][diff.deleted| ][diff.deleted.highlight|walkopts,][diff.deleted| _('[OPTION]... FILE...'), inferrepo=True)]
  [diff.inserted|+@command(]
  [diff.inserted|+    '^forget',]
  [diff.inserted|+    walkopts + dryrunopts,]
  [diff.inserted|+ ][diff.inserted.highlight|  ][diff.inserted| _('[OPTION]... FILE...'), inferrepo=True)]
  [diff.hunk|@@ -2074,1 +2077,3 @@]
  [diff.deleted|-    rejected = cmdutil.forget(ui, repo, m, prefix="",][diff.deleted.highlight| explicitonly=False)[0]]
  [diff.inserted|+    dryrun = opts.get(r'dry_run')]
  [diff.inserted|+    rejected = cmdutil.forget(ui, repo, m, prefix="",]
  [diff.inserted|+                              explicitonly=False, dryrun=dryrun)[0]]

The new algorithm outputs:

  [diff.file_a|--- a/mercurial/commands.py	Fri Mar 09 15:53:41 2018 +0100]
  [diff.file_b|+++ b/mercurial/commands.py	Sat Mar 10 12:33:19 2018 +0530]
  [diff.hunk|@@ -2039,1 +2039,4 @@]
  [diff.deleted|-][diff.deleted.unchanged|@command(][diff.deleted.unchanged|'^forget',][diff.deleted.unchanged| ][diff.deleted.changed|walkopts][diff.deleted.unchanged|,][diff.deleted.changed| ][diff.deleted.unchanged|_('[OPTION]... FILE...'), inferrepo=True)]
  [diff.inserted|+][diff.inserted.unchanged|@command(]
  [diff.inserted|+][diff.inserted.changed|    ][diff.inserted.unchanged|'^forget',]
  [diff.inserted|+][diff.inserted.changed|    walkopts][diff.inserted.unchanged| ][diff.inserted.changed|+ dryrunopts][diff.inserted.unchanged|,]
  [diff.inserted|+][diff.inserted.changed|    ][diff.inserted.unchanged|_('[OPTION]... FILE...'), inferrepo=True)]
  [diff.hunk|@@ -2074,1 +2077,3 @@]
  [diff.deleted|-][diff.deleted.unchanged|    rejected = cmdutil.forget(ui, repo, m, prefix="",][diff.deleted.changed| ][diff.deleted.unchanged|explicitonly=False][diff.deleted.unchanged|)[0]]
  [diff.inserted|+][diff.inserted.changed|    dryrun = opts.get(r'dry_run')]
  [diff.inserted|+][diff.inserted.unchanged|    rejected = cmdutil.forget(ui, repo, m, prefix="",]
  [diff.inserted|+][diff.inserted.changed|                              ][diff.inserted.unchanged|explicitonly=False][diff.inserted.changed|, dryrun=dryrun][diff.inserted.unchanged|)[0]]

Practically, when diffing a 8k line change, the time spent on worddiff
reduces from 4 seconds to 0.14 seconds.

Differential Revision: https://phab.mercurial-scm.org/D3212

[This file is here for historical purposes, all recent contributors
should appear in the changelog directly]

Andrea Arcangeli <andrea at suse.de>
Thomas Arendsen Hein <thomas at intevation.de>
Goffredo Baroncelli <kreijack at libero.it>
Muli Ben-Yehuda <mulix at mulix.org>
Mikael Berthe <mikael at lilotux.net>
Benoit Boissinot <bboissin at gmail.com>
Brendan Cully <brendan at kublai.com>
Vincent Danjean <vdanjean.ml at free.fr>
Jake Edge <jake at edge2.net>
Michael Fetterman <michael.fetterman at intel.com>
Edouard Gomez <ed.gomez at free.fr>
Eric Hopper <hopper at omnifarious.org>
Alecs King <alecsk at gmail.com>
Volker Kleinfeld <Volker.Kleinfeld at gmx.de>
Vadim Lebedev <vadim at mbdsys.com>
Christopher Li <hg at chrisli.org>
Chris Mason <mason at suse.com>
Colin McMillen <mcmillen at cs.cmu.edu>
Wojciech Milkowski <wmilkowski at interia.pl>
Chad Netzer <chad.netzer at gmail.com>
Bryan O'Sullivan <bos at serpentine.com>
Vicent Seguí Pascual <vseguip at gmail.com>
Sean Perry <shaleh at speakeasy.net>
Nguyen Anh Quynh <aquynh at gmail.com>
Ollivier Robert <roberto at keltia.freenix.fr>
Alexander Schremmer <alex at alexanderweb.de>
Arun Sharma <arun at sharma-home.net>
Josef "Jeff" Sipek <jeffpc at optonline.net>
Kevin Smith <yarcs at qualitycode.com>
TK Soh <teekaysoh at yahoo.com>
Radoslaw Szkodzinski <astralstorm at gorzow.mm.pl>
Samuel Tardieu <sam at rfc1149.net>
K Thananchayan <thananck at yahoo.com>
Andrew Thompson <andrewkt at aktzero.com>
Michael S. Tsirkin <mst at mellanox.co.il>
Rafael Villar Burke <pachi at mmn-arquitectos.com>
Tristan Wibberley <tristan at wibberley.org>
Mark Williamson <mark.williamson at cl.cam.ac.uk>