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contrib/diff-highlight/README
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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 hunks in which the number of removed and
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added lines is the same, and it will pair lines within the hunk by
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position (so the first removed line is compared to the first added
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line, and so forth). This is simple and tends to work well in
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practice. More complex changes don't highlight well, so we tend to
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exclude them due to the "same number of removed and added lines"
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restriction. Or even if we do try to highlight them, they end up
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not highlighting because of our "don't highlight if the whole line
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would be highlighted" rule.
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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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---------------------------------------------
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Color Config
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------------
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You can configure the highlight colors and attributes using git's
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config. The colors for "old" and "new" lines can be specified
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independently. There are two "modes" of configuration:
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1. You can specify a "highlight" color and a matching "reset" color.
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This will retain any existing colors in the diff, and apply the
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"highlight" and "reset" colors before and after the highlighted
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portion.
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2. You can specify a "normal" color and a "highlight" color. In this
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case, existing colors are dropped from that line. The non-highlighted
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bits of the line get the "normal" color, and the highlights get the
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"highlight" color.
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If no "new" colors are specified, they default to the "old" colors. If
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no "old" colors are specified, the default is to reverse the foreground
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and background for highlighted portions.
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Examples:
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---------------------------------------------
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# Underline highlighted portions
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[color "diff-highlight"]
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oldHighlight = ul
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oldReset = noul
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---------------------------------------------
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---------------------------------------------
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# Varying background intensities
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[color "diff-highlight"]
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oldNormal = "black #f8cbcb"
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oldHighlight = "black #ffaaaa"
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newNormal = "black #cbeecb"
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newHighlight = "black #aaffaa"
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---------------------------------------------
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Using diff-highlight as a module
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--------------------------------
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If you want to pre- or post- process the highlighted lines as part of
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another perl script, you can use the DiffHighlight module. You can
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either "require" it or just cat the module together with your script (to
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avoid run-time dependencies).
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Your script may set up one or more of the following variables:
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- $DiffHighlight::line_cb - this should point to a function which is
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called whenever DiffHighlight has lines (which may contain
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highlights) to output. The default function prints each line to
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stdout. Note that the function may be called with multiple lines.
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- $DiffHighlight::flush_cb - this should point to a function which
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flushes the output (because DiffHighlight believes it has completed
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processing a logical chunk of input). The default function flushes
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stdout.
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The script may then feed lines, one at a time, to DiffHighlight::handle_line().
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When lines are done processing, they will be fed to $line_cb. Note that
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DiffHighlight may queue up many input lines (to analyze a whole hunk)
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before calling $line_cb. After providing all lines, call
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DiffHighlight::flush() to flush any unprocessed lines.
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If you just want to process stdin, DiffHighlight::highlight_stdin()
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is a convenience helper which will loop and flush for you.
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Bugs
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----
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Because diff-highlight relies on heuristics to guess which parts of
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changes are important, there are some cases where the highlighting is
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more distracting than useful. Fortunately, these cases are rare in
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practice, and when they do occur, the worst case is simply a little
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extra highlighting. This section documents some cases known to be
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sub-optimal, in case somebody feels like working on improving the
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heuristics.
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1. Two changes on the same line get highlighted in a blob. For example,
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highlighting:
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----------------------------------------------
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-foo(buf, size);
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+foo(obj->buf, obj->size);
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----------------------------------------------
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yields (where the inside of "+{}" would be highlighted):
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----------------------------------------------
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-foo(buf, size);
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+foo(+{obj->buf, obj->}size);
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----------------------------------------------
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whereas a more semantically meaningful output would be:
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----------------------------------------------
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-foo(buf, size);
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+foo(+{obj->}buf, +{obj->}size);
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----------------------------------------------
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Note that doing this right would probably involve a set of
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content-specific boundary patterns, similar to word-diff. Otherwise
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you get junk like:
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-----------------------------------------------------
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-this line has some -{i}nt-{ere}sti-{ng} text on it
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+this line has some +{fa}nt+{a}sti+{c} text on it
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-----------------------------------------------------
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which is less readable than the current output.
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2. The multi-line matching assumes that lines in the pre- and post-image
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match by position. This is often the case, but can be fooled when a
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line is removed from the top and a new one added at the bottom (or
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vice versa). Unless the lines in the middle are also changed, diffs
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will show this as two hunks, and it will not get highlighted at all
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(which is good). But if the lines in the middle are changed, the
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highlighting can be misleading. Here's a pathological case:
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-----------------------------------------------------
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-one
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-two
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-three
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-four
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+two 2
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+three 3
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+four 4
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+five 5
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-----------------------------------------------------
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which gets highlighted as:
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-----------------------------------------------------
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-one
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-t-{wo}
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-three
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-f-{our}
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+two 2
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+t+{hree 3}
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+four 4
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+f+{ive 5}
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-----------------------------------------------------
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because it matches "two" to "three 3", and so forth. It would be
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nicer as:
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-----------------------------------------------------
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-one
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-two
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-three
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-four
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+two +{2}
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+three +{3}
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+four +{4}
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+five 5
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-----------------------------------------------------
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which would probably involve pre-matching the lines into pairs
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according to some heuristic.
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