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Re: [PATCH 6/8] gitweb: Highlight interesting parts of diff

From
Jakub Narebski <jnareb@gmail.com>
Date
Feb 13, 2012, 18:44 UTC
Message-ID
<201202131944.50886.jnareb@gmail.com>
In-Reply-To
<20120213074125.3e8793ad@mkiedrowicz.ivo.pl>
On Mon, 13 Feb 2012, Michal Kiedrowicz wrote:
Show 25 quoted lines
> Jakub Narebski <jnareb@gmail.com> wrote:
> 
> > Michał Kiedrowicz <michal.kiedrowicz@gmail.com> writes:
> > 
> > > Reading diff output is sometimes very hard, even if it's colored,
> > > especially if lines differ only in few characters.  This is often
> > > true when a commit fixes a typo or renames some variables or
> > > functions.
> > >
> > This is certainly nice feature to have.  I think most tools that
> > implement side-by-side diff implement this too.
> > 
> > > This commit teaches gitweb to highlight characters that are
> > > different between old and new line.  This should work in the
> > > similar manner as in Trac or GitHub.
> > >
> > Doe you have URLs with good examples of such diff refinement
> > highlighting (GNU Emacs ediff/emerge terminology) / highlighting
> > differing segments (diff-highlight terminology)?
> 
> I haven't found *examples* on GitHub and Trac sites, but what about
> these ones:
> 
> https://github.com/gitster/git/commit/8cad4744ee37ebec1d9491a1381ec1771a1ba795
> http://trac.edgewall.org/changeset/10973

Thanks. That is what I meant by "good examples". Perhaps they should be put in the commit message?

BTW GitHub is closed source, but we can check what algorithm does Trac use for diff refinement highlighting (highlighting changed portions of diff).

Show 20 quoted lines
> > > The code that compares lines is based on
> > > contrib/diff-highlight/diff-highlight, except that it works with
> > > multiline changes too.  It also won't highlight lines that are
> > > completely different because that would only make the output
> > > unreadable.
> > >
> > So what does it look like?  From the contrib/diff-highlight/README I
> > guess that it finds common prefix and common suffix, and highlights
> > the rest, which includes change.  
> 
> Yes.
> 
> > It doesn't implement LCS / diff
> > algorithm like e.g. tkdiff does for its diff refinement highlighting,
> > isn't it?
> 
> I doesn't. I share the Jeff's opinion that:
> a) Jeff's approach is "good enough"
> b) LCS on bytes could be very confusing if it marked few sets of
> characters.

I wonder if we can use --diff-words for diff refinement highlighting, i.e. LCS on words.

Anyway Jeff's approach is a bit limited, in that it would work only for change that does not involve adding newlines, for example splitting overly long line when changing something.

See for example line 1786 (in pre-image) in http://trac.edgewall.org/changeset/10973
 
> > By completly different you mean that they do not have common prefix or
> > common suffix (at least one of them), isn't it?

BTW. is it "at least one of prefix or suffix are non-empty" or "both prefix and suffix are non-empty"?

> Yes, but I also don't highlight lines which prefix/suffix consists only
> of whitespace (This is quite common). 
O.K., that is quite sensible.
> I would also consider ignoring prefixes/suffixes with punctuation, like:
> 
> 	- * I like you.
> 	+ * Alice had a little lamb.
But this patch doesn't implement this feature yet, isn't it?

Well, here is another idea: do not highlight if sum of prefix and suffix lengths are less than some threshold, e.g. 2 characters not including whitespace, or some percentage with respect to total line length.

Show 13 quoted lines
> > > +# Highlight characters from $prefix to $suffix and escape HTML.
> > > +# $str is a reference to the array of characters.
> > > +sub esc_html_mark_range {
> > > +	my ($str, $prefix, $suffix) = @_;
> > > +
> > > +	# Don't generate empty <span> element.
> > > +	if ($prefix == $suffix + 1) {
> > > +		return esc_html(join('', @$str), -nbsp=>1);
> > > +	}
> > > +
> > > +	my $before = join('', @{$str}[0..($prefix - 1)]);
> > > +	my $marked = join('', @{$str}[$prefix..$suffix]);
> > > +	my $after  = join('', @{$str}[($suffix + 1)..$#{$str}]);

It would be nicer with 'part' from List::MoreUtils... but that module is unfortunately not in core.

Show 7 quoted lines
> > Eeeeeek!  First you split into letters, in caller at that, then join?
> > Why not pass striung ($str suggests string not array of characters),
> > and use substr instead?
> > 
> > [Please disregard this and the next paragraph at first reading]
> 
> I will rename $str to something more self describing.
Please do.
BTW. don't you assume here that both common prefix and common suffix
are non-empty?
 
[...]
Show 8 quoted lines
> > > +
> > > +# Format removed and added line, mark changed part and HTML-format
> > > them.
> > 
> > You should probably add here that this code is taken from
> > diff-highlight in contrib area, isn't it?
> 
> True.

And perhaps not the changes from diff-highlight, unless you meant to send them upstream for inclusion.

> > > +sub format_rem_add_line {
> > > +	my ($rem, $add) = @_;
> > > +	my @r = split(//, $rem);
> > > +	my @a = split(//, $add);
BTW the name of variable can be just @add and @rem.
Show 15 quoted lines
> > > +	my ($esc_rem, $esc_add);
> > > +	my ($prefix, $suffix_rem, $suffix_add) = (1, $#r, $#a);
> > 
> > It is not as much $prefix, as $prefix_len, isn't it?  
> 
> Yes.
> 
> > Shouldn't
> > $prefix / $prefix_len start from 0, not from 1?
> 
> It starts from 1 because it skips first +/-. It should become obvious
> after reading the comment from last patch :).
> 
> +	# In combined diff we must ignore two +/- characters.
> +	$prefix = 2 if ($is_combined);
Anyway comment about that fact would be nice.
Show 26 quoted lines
> > > +	my ($prefix_is_space, $suffix_is_space) = (1, 1);
> > > +
> > > +	while ($prefix < @r && $prefix < @a) {
> > > +		last if ($r[$prefix] ne $a[$prefix]);
> > > +
> > > +		$prefix_is_space = 0 if ($r[$prefix] !~ /\s/);
> > > +		$prefix++;
> > > +	}
> > 
> > Ah, I see that it is easier to find common prefix by treating string
> > as array of characters.
> > 
> > Though I wonder if it wouldn't be easier to use trick of XOR-ing two
> > strings (see "Bitwise String Operators" in perlop(1)):
> > 
> >         my $xor = "$rem" ^ "$add";
> > 
> > and finding starting sequence of "\0", which denote common prefix.
> > 
> > 
> > Though this and the following is a nice implementation of
> > algorithm... as it would be implemented in C.  Nevermind, it might be
> > good enough...
> 
> The splitting and comparing by characters is taken from diff-highlight.
> I don't think it's worth changing here.
You are right.
I'll try to come with hacky algorithm using string bitwise xor and regexp,
and benchmark it comparing to your C-like solution, but it can be left for
later (simple is better than clever, usually).
Show 13 quoted lines
> > > +	while ($suffix_rem >= $prefix && $suffix_add >= $prefix) {
> > > +		last if ($r[$suffix_rem] ne $a[$suffix_add]);
> > > +
> > > +		$suffix_is_space = 0 if ($r[$suffix_rem] !~ /\s/);
> > > +		$suffix_rem--;
> > > +		$suffix_add--;
> > > +	}
> > 
> > BTW., perhaps using single negative $suffix_len instead of separate
> > $suffix_rem_pos and $suffix_add_pos would make code simpler and easier
> > to understand?
> 
> I'll try that.
I think this should just work, and it would be one variable less to
track.
Show 14 quoted lines
> > >  # HTML-format diff context, removed and added lines.
> > >  sub format_ctx_rem_add_lines {
> > > -	my ($ctx, $rem, $add) = @_;
> > > +	my ($ctx, $rem, $add, $is_combined) = @_;
> > >  	my (@new_ctx, @new_rem, @new_add);
> > > +	my $num_add_lines = @$add;
> > 
> > Why is this temporary variable needed?  If you are not sure that ==
> > operator enforces scalar context on both arguments you can always
> > write
> > 
> >   scalar @$add == scalar @$rem
> 
> You read my mind.
BTW. the same comment applies to patch adding support for highlighting
changed part in combined diff.
Show 5 quoted lines
> > The original contrib/diff-highlight works only for single changed line
> > (single removed and single added).  You make this code work also for
> > the case where number of aded lines is equal to the number of removed
> > lines, and assume that subsequent changed lines in preimage
> > correcponds to subsequent changed lines in postimage, [...]
[...]
> > This is not described in commit message, I think.
-- 
Jakub Narebski
Poland
Previous: Michal KiedrowiczNext: Michał Kiedrowicz
Message 53 of 67 in “gitweb: Highlight interesting parts of diff”
  1. 0/8 gitweb: Highlight interesting parts of diffMichał Kiedrowicz, Feb 10, 2012
  2. 1/8 gitweb: Extract print_sidebyside_diff_lines()Michał Kiedrowicz, Feb 10, 2012
  3. Jakub NarebskiFeb 11, 2012
  4. Michał KiedrowiczFeb 11, 2012
  5. 2/8 gitweb: Use print_diff_chunk() for both side-by-side and inline diffsMichał Kiedrowicz, Feb 10, 2012
  6. Jakub NarebskiFeb 11, 2012
  7. Michał KiedrowiczFeb 11, 2012
  8. Michał KiedrowiczFeb 25, 2012
  9. 3/8 gitweb: Move HTML-formatting diff line back to process_diff_line()Michał Kiedrowicz, Feb 10, 2012
  10. Jakub NarebskiFeb 11, 2012
  11. 4/8 gitweb: Push formatting diff lines to print_diff_chunk()Michał Kiedrowicz, Feb 10, 2012
  12. Jakub NarebskiFeb 11, 2012
  13. Michał KiedrowiczFeb 11, 2012
  14. Michał KiedrowiczFeb 11, 2012
  15. 5/8 gitweb: Format diff lines just before printingMichał Kiedrowicz, Feb 10, 2012
  16. Jakub NarebskiFeb 11, 2012
  17. Michał KiedrowiczFeb 11, 2012
  18. 6/8 gitweb: Highlight interesting parts of diffMichał Kiedrowicz, Feb 10, 2012
  19. Jakub NarebskiFeb 10, 2012
  20. Michał KiedrowiczFeb 10, 2012
  21. Jakub NarebskiFeb 10, 2012
  22. Michał KiedrowiczFeb 10, 2012
  23. Jakub NarebskiFeb 10, 2012
  24. Jeff KingFeb 10, 2012
  25. Michal KiedrowiczFeb 14, 2012
  26. Junio C HamanoFeb 14, 2012
  27. Jeff KingFeb 14, 2012
  28. Jeff KingFeb 10, 2012
  29. Michał KiedrowiczFeb 10, 2012
  30. Jeff KingFeb 10, 2012
  31. Michał KiedrowiczFeb 10, 2012
  32. diff-highlight: Work for multiline changes tooMichał Kiedrowicz, Feb 10, 2012
  33. Jeff KingFeb 13, 2012
  34. 1/5 diff-highlight: make perl strict and warnings fatalJeff King, Feb 13, 2012
  35. 2/5 diff-highlight: don't highlight whole linesJeff King, Feb 13, 2012
  36. Michal KiedrowiczFeb 14, 2012
  37. 3/5 diff-highlight: refactor to prepare for multi-line hunksJeff King, Feb 13, 2012
  38. 4/5 diff-highlight: match multi-line hunksJeff King, Feb 13, 2012
  39. 5/5 diff-highlight: document some non-optimal casesJeff King, Feb 13, 2012
  40. Michal KiedrowiczFeb 14, 2012
  41. Junio C HamanoFeb 14, 2012
  42. Jeff KingFeb 14, 2012
  43. Junio C HamanoFeb 14, 2012
  44. Jeff KingFeb 14, 2012
  45. Michal KiedrowiczFeb 14, 2012
  46. Jakub NarebskiFeb 10, 2012
  47. Jakub NarebskiFeb 11, 2012
  48. Jakub NarebskiFeb 12, 2012
  49. Michal KiedrowiczFeb 13, 2012
  50. Jakub NarebskiFeb 13, 2012
  51. Michał KiedrowiczFeb 13, 2012
  52. Michal KiedrowiczFeb 13, 2012
  53. Jakub NarebskiFeb 13, 2012
  54. Michał KiedrowiczFeb 13, 2012
  55. Jakub NarebskiFeb 14, 2012
  56. Michał KiedrowiczFeb 14, 2012
  57. Jeff KingFeb 14, 2012
  58. Michał KiedrowiczFeb 14, 2012
  59. Jeff KingFeb 14, 2012
  60. 7/8 gitweb: Use different colors to present marked changesMichał Kiedrowicz, Feb 10, 2012
  61. Jakub NarebskiFeb 12, 2012
  62. Michal KiedrowiczFeb 13, 2012
  63. 8/8 gitweb: Highlight combined diffsMichał Kiedrowicz, Feb 10, 2012
  64. Jakub NarebskiFeb 12, 2012
  65. Michal KiedrowiczFeb 13, 2012
  66. Jakub NarebskiFeb 11, 2012
  67. Michał KiedrowiczFeb 11, 2012

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