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/color.py

@@ -90,14 +90,16 b' except ImportError:'
     'branches.inactive': 'none',
     'diff.changed': 'white',
     'diff.deleted': 'red',
-    'diff.deleted.highlight': 'red bold underline',
+    'diff.deleted.changed': 'red',
+    'diff.deleted.unchanged': 'red dim',
     'diff.diffline': 'bold',
     'diff.extended': 'cyan bold',
     'diff.file_a': 'red bold',
     'diff.file_b': 'green bold',
     'diff.hunk': 'magenta',
     'diff.inserted': 'green',
-    'diff.inserted.highlight': 'green bold underline',
+    'diff.inserted.changed': 'green',
+    'diff.inserted.unchanged': 'green dim',
     'diff.tab': '',
     'diff.trailingwhitespace': 'bold red_background',
     'changeset.public': '',

mercurial/patch.py

@@ -50,7 +50,8 b' stringio = util.stringio'
 
 gitre = re.compile(br'diff --git a/(.*) b/(.*)')
 tabsplitter = re.compile(br'(\t+|[^\t]+)')
-_nonwordre = re.compile(br'([^a-zA-Z0-9_\x80-\xff])')
+wordsplitter = re.compile(br'(\t+| +|[a-zA-Z0-9_\x80-\xff]+|'
+                          '[^ \ta-zA-Z0-9_\x80-\xff])')
 
 PatchError = error.PatchError
 
@@ -2504,8 +2505,78 b' def diffsinglehunk(hunklines):'
         if chompline != line:
             yield (line[len(chompline):], '')
 
+def diffsinglehunkinline(hunklines):
+    """yield tokens for a list of lines in a single hunk, with inline colors"""
+    # prepare deleted, and inserted content
+    a = ''
+    b = ''
+    for line in hunklines:
+        if line[0] == '-':
+            a += line[1:]
+        elif line[0] == '+':
+            b += line[1:]
+        else:
+            raise error.ProgrammingError('unexpected hunk line: %s' % line)
+    # fast path: if either side is empty, use diffsinglehunk
+    if not a or not b:
+        for t in diffsinglehunk(hunklines):
+            yield t
+        return
+    # re-split the content into words
+    al = wordsplitter.findall(a)
+    bl = wordsplitter.findall(b)
+    # re-arrange the words to lines since the diff algorithm is line-based
+    aln = [s if s == '\n' else s + '\n' for s in al]
+    bln = [s if s == '\n' else s + '\n' for s in bl]
+    an = ''.join(aln)
+    bn = ''.join(bln)
+    # run the diff algorithm, prepare atokens and btokens
+    atokens = []
+    btokens = []
+    blocks = mdiff.allblocks(an, bn, lines1=aln, lines2=bln)
+    for (a1, a2, b1, b2), btype in blocks:
+        changed = btype == '!'
+        for token in mdiff.splitnewlines(''.join(al[a1:a2])):
+            atokens.append((changed, token))
+        for token in mdiff.splitnewlines(''.join(bl[b1:b2])):
+            btokens.append((changed, token))
+
+    # yield deleted tokens, then inserted ones
+    for prefix, label, tokens in [('-', 'diff.deleted', atokens),
+                                  ('+', 'diff.inserted', btokens)]:
+        nextisnewline = True
+        for changed, token in tokens:
+            if nextisnewline:
+                yield (prefix, label)
+                nextisnewline = False
+            # special handling line end
+            isendofline = token.endswith('\n')
+            if isendofline:
+                chomp = token[:-1] # chomp
+                token = chomp.rstrip() # detect spaces at the end
+                endspaces = chomp[len(token):]
+            # scan tabs
+            for maybetab in tabsplitter.findall(token):
+                if '\t' == maybetab[0]:
+                    currentlabel = 'diff.tab'
+                else:
+                    if changed:
+                        currentlabel = label + '.changed'
+                    else:
+                        currentlabel = label + '.unchanged'
+                yield (maybetab, currentlabel)
+            if isendofline:
+                if endspaces:
+                    yield (endspaces, 'diff.trailingwhitespace')
+                yield ('\n', '')
+                nextisnewline = True
+
 def difflabel(func, *args, **kw):
     '''yields 2-tuples of (output, label) based on the output of func()'''
+    if kw.get(r'opts') and kw[r'opts'].worddiff:
+        dodiffhunk = diffsinglehunkinline
+    else:
+        dodiffhunk = diffsinglehunk
     headprefixes = [('diff', 'diff.diffline'),
                     ('copy', 'diff.extended'),
                     ('rename', 'diff.extended'),
@@ -2525,7 +2596,7 b' def difflabel(func, *args, **kw):'
     hunkbuffer = []
     def consumehunkbuffer():
         if hunkbuffer:
-            for token in diffsinglehunk(hunkbuffer):
+            for token in dodiffhunk(hunkbuffer):
                 yield token
             hunkbuffer[:] = []
 

tests/test-diff-color.t

@@ -337,41 +337,39 b' test inline color diff'
   [diff.deleted|-(to see if it works)]
   [diff.inserted|+three of those lines have]
   [diff.inserted|+collapsed onto one]
-#if false
   $ hg diff --config experimental.worddiff=True --color=debug
   [diff.diffline|diff --git a/file1 b/file1]
   [diff.file_a|--- a/file1]
   [diff.file_b|+++ b/file1]
   [diff.hunk|@@ -1,16 +1,17 @@]
-  [diff.deleted|-this is the ][diff.deleted.highlight|first][diff.deleted| line]
-  [diff.deleted|-this is the second line]
-  [diff.deleted|-][diff.deleted.highlight|    ][diff.deleted|third line starts with space]
-  [diff.deleted|-][diff.deleted.highlight|+][diff.deleted| starts with a ][diff.deleted.highlight|plus][diff.deleted| sign]
-  [diff.deleted|-][diff.tab|	][diff.deleted|this one with ][diff.deleted.highlight|one][diff.deleted| tab]
-  [diff.deleted|-][diff.tab|		][diff.deleted|now with full ][diff.deleted.highlight|two][diff.deleted| tabs]
-  [diff.deleted|-][diff.tab|	][diff.deleted|now tabs][diff.tab|		][diff.deleted|everywhere, much fun]
-  [diff.inserted|+that is the first paragraph]
-  [diff.inserted|+][diff.inserted.highlight|    ][diff.inserted|this is the ][diff.inserted.highlight|second][diff.inserted| line]
-  [diff.inserted|+third line starts with space]
-  [diff.inserted|+][diff.inserted.highlight|-][diff.inserted| starts with a ][diff.inserted.highlight|minus][diff.inserted| sign]
-  [diff.inserted|+][diff.tab|	][diff.inserted|this one with ][diff.inserted.highlight|two][diff.inserted| tab]
-  [diff.inserted|+][diff.tab|			][diff.inserted|now with full ][diff.inserted.highlight|three][diff.inserted| tabs]
-  [diff.inserted|+][diff.tab|	][diff.inserted|now][diff.inserted.highlight| there are][diff.inserted| tabs][diff.tab|		][diff.inserted|everywhere, much fun]
+  [diff.deleted|-][diff.deleted.changed|this][diff.deleted.unchanged| is the first ][diff.deleted.changed|line]
+  [diff.deleted|-][diff.deleted.unchanged|this is the second line]
+  [diff.deleted|-][diff.deleted.changed|    ][diff.deleted.unchanged|third line starts with space]
+  [diff.deleted|-][diff.deleted.changed|+][diff.deleted.unchanged| starts with a ][diff.deleted.changed|plus][diff.deleted.unchanged| sign]
+  [diff.deleted|-][diff.tab|	][diff.deleted.unchanged|this one with ][diff.deleted.changed|one][diff.deleted.unchanged| tab]
+  [diff.deleted|-][diff.tab|		][diff.deleted.unchanged|now with full ][diff.deleted.changed|two][diff.deleted.unchanged| tabs]
+  [diff.deleted|-][diff.tab|	][diff.deleted.unchanged|now ][diff.deleted.unchanged|tabs][diff.tab|		][diff.deleted.unchanged|everywhere, much fun]
+  [diff.inserted|+][diff.inserted.changed|that][diff.inserted.unchanged| is the first ][diff.inserted.changed|paragraph]
+  [diff.inserted|+][diff.inserted.changed|    ][diff.inserted.unchanged|this is the second line]
+  [diff.inserted|+][diff.inserted.unchanged|third line starts with space]
+  [diff.inserted|+][diff.inserted.changed|-][diff.inserted.unchanged| starts with a ][diff.inserted.changed|minus][diff.inserted.unchanged| sign]
+  [diff.inserted|+][diff.tab|	][diff.inserted.unchanged|this one with ][diff.inserted.changed|two][diff.inserted.unchanged| tab]
+  [diff.inserted|+][diff.tab|			][diff.inserted.unchanged|now with full ][diff.inserted.changed|three][diff.inserted.unchanged| tabs]
+  [diff.inserted|+][diff.tab|	][diff.inserted.unchanged|now ][diff.inserted.changed|there are ][diff.inserted.unchanged|tabs][diff.tab|		][diff.inserted.unchanged|everywhere, much fun]
    
    this line won't change
    
    two lines are going to
-  [diff.deleted|-be changed into ][diff.deleted.highlight|three][diff.deleted|!]
-  [diff.inserted|+(entirely magically,]
-  [diff.inserted|+ assuming this works)]
-  [diff.inserted|+be changed into ][diff.inserted.highlight|four][diff.inserted|!]
+  [diff.deleted|-][diff.deleted.unchanged|be changed into ][diff.deleted.changed|three][diff.deleted.unchanged|!]
+  [diff.inserted|+][diff.inserted.changed|(entirely magically,]
+  [diff.inserted|+][diff.inserted.changed| assuming this works)]
+  [diff.inserted|+][diff.inserted.unchanged|be changed into ][diff.inserted.changed|four][diff.inserted.unchanged|!]
    
-  [diff.deleted|-three of those lines ][diff.deleted.highlight|will]
-  [diff.deleted|-][diff.deleted.highlight|collapse][diff.deleted| onto one]
-  [diff.deleted|-(to see if it works)]
-  [diff.inserted|+three of those lines ][diff.inserted.highlight|have]
-  [diff.inserted|+][diff.inserted.highlight|collapsed][diff.inserted| onto one]
-#endif
+  [diff.deleted|-][diff.deleted.unchanged|three of those lines ][diff.deleted.changed|will]
+  [diff.deleted|-][diff.deleted.changed|collapse][diff.deleted.unchanged| onto one]
+  [diff.deleted|-][diff.deleted.changed|(to see if it works)]
+  [diff.inserted|+][diff.inserted.unchanged|three of those lines ][diff.inserted.changed|have]
+  [diff.inserted|+][diff.inserted.changed|collapsed][diff.inserted.unchanged| onto one]
 
 multibyte character shouldn't be broken up in word diff:
 
@@ -386,12 +384,10 b" multibyte character shouldn't be broken "
   > EOF
   $ hg ci -m 'slightly change utf8 char' utf8
 
-#if false
   $ hg diff --config experimental.worddiff=True --color=debug -c.
   [diff.diffline|diff --git a/utf8 b/utf8]
   [diff.file_a|--- a/utf8]
   [diff.file_b|+++ b/utf8]
   [diff.hunk|@@ -1,1 +1,1 @@]
-  [diff.deleted|-blah ][diff.deleted.highlight|\xe3\x82\xa2][diff.deleted| blah] (esc)
-  [diff.inserted|+blah ][diff.inserted.highlight|\xe3\x82\xa4][diff.inserted| blah] (esc)
-#endif
+  [diff.deleted|-][diff.deleted.unchanged|blah ][diff.deleted.changed|\xe3\x82\xa2][diff.deleted.unchanged| blah] (esc)
+  [diff.inserted|+][diff.inserted.unchanged|blah ][diff.inserted.changed|\xe3\x82\xa4][diff.inserted.unchanged| blah] (esc)