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contrib: add diff highlight script
This is a simple and stupid script for highlighting differing parts of lines in a unified diff. See the README for a discussion of the limitations. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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‎contrib/diff-highlight/README‎

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diff-highlight
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==============
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Line oriented diffs are great for reviewing code, because for most
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hunks, you want to see the old and the new segments of code next to each
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other. Sometimes, though, when an old line and a new line are very
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similar, it's hard to immediately see the difference.
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You can use "--color-words" to highlight only the changed portions of
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lines. However, this can often be hard to read for code, as it loses
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the line structure, and you end up with oddly formatted bits.
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Instead, this script post-processes the line-oriented diff, finds pairs
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of lines, and highlights the differing segments. It's currently very
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simple and stupid about doing these tasks. In particular:
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1. It will only highlight a pair of lines if they are the only two
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lines in a hunk. It could instead try to match up "before" and
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"after" lines for a given hunk into pairs of similar lines.
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However, this may end up visually distracting, as the paired
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lines would have other highlighted lines in between them. And in
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practice, the lines which most need attention called to their
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small, hard-to-see changes are touching only a single line.
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2. It will find the common prefix and suffix of two lines, and
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consider everything in the middle to be "different". It could
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instead do a real diff of the characters between the two lines and
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find common subsequences. However, the point of the highlight is to
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call attention to a certain area. Even if some small subset of the
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highlighted area actually didn't change, that's OK. In practice it
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ends up being more readable to just have a single blob on the line
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showing the interesting bit.
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The goal of the script is therefore not to be exact about highlighting
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changes, but to call attention to areas of interest without being
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visually distracting. Non-diff lines and existing diff coloration is
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preserved; the intent is that the output should look exactly the same as
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the input, except for the occasional highlight.
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Use
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---
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You can try out the diff-highlight program with:
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---------------------------------------------
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git log -p --color | /path/to/diff-highlight
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---------------------------------------------
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If you want to use it all the time, drop it in your $PATH and put the
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following in your git configuration:
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---------------------------------------------
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[pager]
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log = diff-highlight | less
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show = diff-highlight | less
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diff = diff-highlight | less
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---------------------------------------------

‎contrib/diff-highlight/diff-highlight‎

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#!/usr/bin/perl
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# Highlight by reversing foreground and background. You could do
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# other things like bold or underline if you prefer.
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my $HIGHLIGHT = "\x1b[7m";
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my $UNHIGHLIGHT = "\x1b[27m";
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my $COLOR = qr/\x1b\[[0-9;]*m/;
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my @window;
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while (<>) {
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# We highlight only single-line changes, so we need
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# a 4-line window to make a decision on whether
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# to highlight.
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push @window, $_;
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next if @window < 4;
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if ($window[0] =~ /^$COLOR*(\@| )/ &&
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$window[1] =~ /^$COLOR*-/ &&
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$window[2] =~ /^$COLOR*\+/ &&
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$window[3] !~ /^$COLOR*\+/) {
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print shift @window;
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show_pair(shift @window, shift @window);
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}
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else {
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print shift @window;
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}
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# Most of the time there is enough output to keep things streaming,
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# but for something like "git log -Sfoo", you can get one early
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# commit and then many seconds of nothing. We want to show
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# that one commit as soon as possible.
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#
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# Since we can receive arbitrary input, there's no optimal
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# place to flush. Flushing on a blank line is a heuristic that
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# happens to match git-log output.
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if (!length) {
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local $| = 1;
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}
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}
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# Special case a single-line hunk at the end of file.
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if (@window == 3 &&
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$window[0] =~ /^$COLOR*(\@| )/ &&
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$window[1] =~ /^$COLOR*-/ &&
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$window[2] =~ /^$COLOR*\+/) {
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print shift @window;
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show_pair(shift @window, shift @window);
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}
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# And then flush any remaining lines.
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while (@window) {
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print shift @window;
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}
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exit 0;
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sub show_pair {
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my @a = split_line(shift);
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my @b = split_line(shift);
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# Find common prefix, taking care to skip any ansi
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# color codes.
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my $seen_plusminus;
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my ($pa, $pb) = (0, 0);
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while ($pa < @a && $pb < @b) {
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if ($a[$pa] =~ /$COLOR/) {
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$pa++;
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}
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elsif ($b[$pb] =~ /$COLOR/) {
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$pb++;
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}
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elsif ($a[$pa] eq $b[$pb]) {
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$pa++;
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$pb++;
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}
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elsif (!$seen_plusminus && $a[$pa] eq '-' && $b[$pb] eq '+') {
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$seen_plusminus = 1;
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$pa++;
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$pb++;
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}
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else {
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last;
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}
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}
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# Find common suffix, ignoring colors.
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my ($sa, $sb) = ($#a, $#b);
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while ($sa >= $pa && $sb >= $pb) {
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if ($a[$sa] =~ /$COLOR/) {
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$sa--;
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}
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elsif ($b[$sb] =~ /$COLOR/) {
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$sb--;
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}
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elsif ($a[$sa] eq $b[$sb]) {
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$sa--;
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$sb--;
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}
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else {
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last;
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}
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}
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print highlight(\@a, $pa, $sa);
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print highlight(\@b, $pb, $sb);
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}
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sub split_line {
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local $_ = shift;
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return map { /$COLOR/ ? $_ : (split //) }
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split /($COLOR*)/;
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}
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sub highlight {
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my ($line, $prefix, $suffix) = @_;
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return join('',
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@{$line}[0..($prefix-1)],
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$HIGHLIGHT,
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@{$line}[$prefix..$suffix],
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$UNHIGHLIGHT,
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@{$line}[($suffix+1)..$#$line]
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);
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}

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