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  • Jun Wu's avatar
    patch: implement a new worddiff algorithm · 35632d392279
    Jun Wu authored
    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
    35632d392279