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/usr/share/doc/git/contrib/diff-highlight/README
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diff-highlight ============== Line oriented diffs are great for reviewing code, because for most hunks, you want to see the old and the new segments of code next to each other. Sometimes, though, when an old line and a new line are very similar, it's hard to immediately see the difference. You can use "--color-words" to highlight only the changed portions of lines. However, this can often be hard to read for code, as it loses the line structure, and you end up with oddly formatted bits. Instead, this script post-processes the line-oriented diff, finds pairs 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 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 instead do a real diff of the characters between the two lines and find common subsequences. However, the point of the highlight is to call attention to a certain area. Even if some small subset of the highlighted area actually didn't change, that's OK. In practice it ends up being more readable to just have a single blob on the line showing the interesting bit. The goal of the script is therefore not to be exact about highlighting changes, but to call attention to areas of interest without being visually distracting. Non-diff lines and existing diff coloration is preserved; the intent is that the output should look exactly the same as the input, except for the occasional highlight. Use --- You can try out the diff-highlight program with: --------------------------------------------- git log -p --color | /path/to/diff-highlight --------------------------------------------- If you want to use it all the time, drop it in your $PATH and put the following in your git configuration: --------------------------------------------- [pager] log = diff-highlight | less show = diff-highlight | less diff = diff-highlight | less --------------------------------------------- Color Config ------------ You can configure the highlight colors and attributes using git's config. The colors for "old" and "new" lines can be specified independently. There are two "modes" of configuration: 1. You can specify a "highlight" color and a matching "reset" color. This will retain any existing colors in the diff, and apply the "highlight" and "reset" colors before and after the highlighted portion. 2. You can specify a "normal" color and a "highlight" color. In this case, existing colors are dropped from that line. The non-highlighted bits of the line get the "normal" color, and the highlights get the "highlight" color. If no "new" colors are specified, they default to the "old" colors. If no "old" colors are specified, the default is to reverse the foreground and background for highlighted portions. Examples: --------------------------------------------- # Underline highlighted portions [color "diff-highlight"] oldHighlight = ul oldReset = noul --------------------------------------------- --------------------------------------------- # Varying background intensities [color "diff-highlight"] oldNormal = "black #f8cbcb" oldHighlight = "black #ffaaaa" newNormal = "black #cbeecb" newHighlight = "black #aaffaa" --------------------------------------------- Using diff-highlight as a module -------------------------------- If you want to pre- or post- process the highlighted lines as part of another perl script, you can use the DiffHighlight module. You can either "require" it or just cat the module together with your script (to avoid run-time dependencies). Your script may set up one or more of the following variables: - $DiffHighlight::line_cb - this should point to a function which is called whenever DiffHighlight has lines (which may contain highlights) to output. The default function prints each line to stdout. Note that the function may be called with multiple lines. - $DiffHighlight::flush_cb - this should point to a function which flushes the output (because DiffHighlight believes it has completed processing a logical chunk of input). The default function flushes stdout. The script may then feed lines, one at a time, to DiffHighlight::handle_line(). When lines are done processing, they will be fed to $line_cb. Note that DiffHighlight may queue up many input lines (to analyze a whole hunk) before calling $line_cb. After providing all lines, call DiffHighlight::flush() to flush any unprocessed lines. If you just want to process stdin, DiffHighlight::highlight_stdin() is a convenience helper which will loop and flush for you. 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.
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