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diff --git a/contrib/diff-highlight/README b/contrib/diff-highlight/README new file mode 100644 index 000000000000..d4c234317520 --- /dev/null +++ b/contrib/diff-highlight/README @@ -0,0 +1,223 @@ +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. |