doc/README

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

Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:

  http://docutils.sourceforge.net/rst.html

It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:

  http://docutils.sourceforge.net/

Use the Makefile in this directory to generate the man and HTML pages.