of lines, and highlights the differing segments. It's currently very
simple and stupid about doing these tasks. In particular:
- 1. It will only highlight a pair of lines if they are the only two
- lines in a hunk. It could instead try to match up "before" and
- "after" lines for a given hunk into pairs of similar lines.
- However, this may end up visually distracting, as the paired
- lines would have other highlighted lines in between them. And in
- practice, the lines which most need attention called to their
- small, hard-to-see changes are touching only a single line.
+ 1. It will only highlight hunks in which the number of removed and
+ added lines is the same, and it will pair lines within the hunk by
+ position (so the first removed line is compared to the first added
+ line, and so forth). This is simple and tends to work well in
+ practice. More complex changes don't highlight well, so we tend to
+ exclude them due to the "same number of removed and added lines"
+ restriction. Or even if we do try to highlight them, they end up
+ not highlighting because of our "don't highlight if the whole line
+ would be highlighted" rule.
2. It will find the common prefix and suffix of two lines, and
consider everything in the middle to be "different". It could
show = diff-highlight | less
diff = diff-highlight | less
---------------------------------------------
+
+Bugs
+----
+
+Because diff-highlight relies on heuristics to guess which parts of
+changes are important, there are some cases where the highlighting is
+more distracting than useful. Fortunately, these cases are rare in
+practice, and when they do occur, the worst case is simply a little
+extra highlighting. This section documents some cases known to be
+sub-optimal, in case somebody feels like working on improving the
+heuristics.
+
+1. Two changes on the same line get highlighted in a blob. For example,
+ highlighting:
+
+----------------------------------------------
+-foo(buf, size);
++foo(obj->buf, obj->size);
+----------------------------------------------
+
+ yields (where the inside of "+{}" would be highlighted):
+
+----------------------------------------------
+-foo(buf, size);
++foo(+{obj->buf, obj->}size);
+----------------------------------------------
+
+ whereas a more semantically meaningful output would be:
+
+----------------------------------------------
+-foo(buf, size);
++foo(+{obj->}buf, +{obj->}size);
+----------------------------------------------
+
+ Note that doing this right would probably involve a set of
+ content-specific boundary patterns, similar to word-diff. Otherwise
+ you get junk like:
+
+-----------------------------------------------------
+-this line has some -{i}nt-{ere}sti-{ng} text on it
++this line has some +{fa}nt+{a}sti+{c} text on it
+-----------------------------------------------------
+
+ which is less readable than the current output.
+
+2. The multi-line matching assumes that lines in the pre- and post-image
+ match by position. This is often the case, but can be fooled when a
+ line is removed from the top and a new one added at the bottom (or
+ vice versa). Unless the lines in the middle are also changed, diffs
+ will show this as two hunks, and it will not get highlighted at all
+ (which is good). But if the lines in the middle are changed, the
+ highlighting can be misleading. Here's a pathological case:
+
+-----------------------------------------------------
+-one
+-two
+-three
+-four
++two 2
++three 3
++four 4
++five 5
+-----------------------------------------------------
+
+ which gets highlighted as:
+
+-----------------------------------------------------
+-one
+-t-{wo}
+-three
+-f-{our}
++two 2
++t+{hree 3}
++four 4
++f+{ive 5}
+-----------------------------------------------------
+
+ because it matches "two" to "three 3", and so forth. It would be
+ nicer as:
+
+-----------------------------------------------------
+-one
+-two
+-three
+-four
++two +{2}
++three +{3}
++four +{4}
++five 5
+-----------------------------------------------------
+
+ which would probably involve pre-matching the lines into pairs
+ according to some heuristic.
#!/usr/bin/perl
+use warnings FATAL => 'all';
+use strict;
+
# Highlight by reversing foreground and background. You could do
# other things like bold or underline if you prefer.
my $HIGHLIGHT = "\x1b[7m";
my $UNHIGHLIGHT = "\x1b[27m";
my $COLOR = qr/\x1b\[[0-9;]*m/;
+my $BORING = qr/$COLOR|\s/;
-my @window;
+my @removed;
+my @added;
+my $in_hunk;
while (<>) {
- # We highlight only single-line changes, so we need
- # a 4-line window to make a decision on whether
- # to highlight.
- push @window, $_;
- next if @window < 4;
- if ($window[0] =~ /^$COLOR*(\@| )/ &&
- $window[1] =~ /^$COLOR*-/ &&
- $window[2] =~ /^$COLOR*\+/ &&
- $window[3] !~ /^$COLOR*\+/) {
- print shift @window;
- show_pair(shift @window, shift @window);
+ if (!$in_hunk) {
+ print;
+ $in_hunk = /^$COLOR*\@/;
+ }
+ elsif (/^$COLOR*-/) {
+ push @removed, $_;
+ }
+ elsif (/^$COLOR*\+/) {
+ push @added, $_;
}
else {
- print shift @window;
+ show_hunk(\@removed, \@added);
+ @removed = ();
+ @added = ();
+
+ print;
+ $in_hunk = /^$COLOR*[\@ ]/;
}
# Most of the time there is enough output to keep things streaming,
}
}
-# Special case a single-line hunk at the end of file.
-if (@window == 3 &&
- $window[0] =~ /^$COLOR*(\@| )/ &&
- $window[1] =~ /^$COLOR*-/ &&
- $window[2] =~ /^$COLOR*\+/) {
- print shift @window;
- show_pair(shift @window, shift @window);
-}
-
-# And then flush any remaining lines.
-while (@window) {
- print shift @window;
-}
+# Flush any queued hunk (this can happen when there is no trailing context in
+# the final diff of the input).
+show_hunk(\@removed, \@added);
exit 0;
-sub show_pair {
+sub show_hunk {
+ my ($a, $b) = @_;
+
+ # If one side is empty, then there is nothing to compare or highlight.
+ if (!@$a || !@$b) {
+ print @$a, @$b;
+ return;
+ }
+
+ # If we have mismatched numbers of lines on each side, we could try to
+ # be clever and match up similar lines. But for now we are simple and
+ # stupid, and only handle multi-line hunks that remove and add the same
+ # number of lines.
+ if (@$a != @$b) {
+ print @$a, @$b;
+ return;
+ }
+
+ my @queue;
+ for (my $i = 0; $i < @$a; $i++) {
+ my ($rm, $add) = highlight_pair($a->[$i], $b->[$i]);
+ print $rm;
+ push @queue, $add;
+ }
+ print @queue;
+}
+
+sub highlight_pair {
my @a = split_line(shift);
my @b = split_line(shift);
}
}
- print highlight(\@a, $pa, $sa);
- print highlight(\@b, $pb, $sb);
+ if (is_pair_interesting(\@a, $pa, $sa, \@b, $pb, $sb)) {
+ return highlight_line(\@a, $pa, $sa),
+ highlight_line(\@b, $pb, $sb);
+ }
+ else {
+ return join('', @a),
+ join('', @b);
+ }
}
sub split_line {
split /($COLOR*)/;
}
-sub highlight {
+sub highlight_line {
my ($line, $prefix, $suffix) = @_;
return join('',
@{$line}[($suffix+1)..$#$line]
);
}
+
+# Pairs are interesting to highlight only if we are going to end up
+# highlighting a subset (i.e., not the whole line). Otherwise, the highlighting
+# is just useless noise. We can detect this by finding either a matching prefix
+# or suffix (disregarding boring bits like whitespace and colorization).
+sub is_pair_interesting {
+ my ($a, $pa, $sa, $b, $pb, $sb) = @_;
+ my $prefix_a = join('', @$a[0..($pa-1)]);
+ my $prefix_b = join('', @$b[0..($pb-1)]);
+ my $suffix_a = join('', @$a[($sa+1)..$#$a]);
+ my $suffix_b = join('', @$b[($sb+1)..$#$b]);
+
+ return $prefix_a !~ /^$COLOR*-$BORING*$/ ||
+ $prefix_b !~ /^$COLOR*\+$BORING*$/ ||
+ $suffix_a !~ /^$BORING*$/ ||
+ $suffix_b !~ /^$BORING*$/;
+}