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| 1 | +diff-highlight |
| 2 | +============== |
| 3 | + |
| 4 | +Line oriented diffs are great for reviewing code, because for most |
| 5 | +hunks, you want to see the old and the new segments of code next to each |
| 6 | +other. Sometimes, though, when an old line and a new line are very |
| 7 | +similar, it's hard to immediately see the difference. |
| 8 | + |
| 9 | +You can use "--color-words" to highlight only the changed portions of |
| 10 | +lines. However, this can often be hard to read for code, as it loses |
| 11 | +the line structure, and you end up with oddly formatted bits. |
| 12 | + |
| 13 | +Instead, this script post-processes the line-oriented diff, finds pairs |
| 14 | +of lines, and highlights the differing segments. It's currently very |
| 15 | +simple and stupid about doing these tasks. In particular: |
| 16 | + |
| 17 | + 1. It will only highlight hunks in which the number of removed and |
| 18 | + added lines is the same, and it will pair lines within the hunk by |
| 19 | + position (so the first removed line is compared to the first added |
| 20 | + line, and so forth). This is simple and tends to work well in |
| 21 | + practice. More complex changes don't highlight well, so we tend to |
| 22 | + exclude them due to the "same number of removed and added lines" |
| 23 | + restriction. Or even if we do try to highlight them, they end up |
| 24 | + not highlighting because of our "don't highlight if the whole line |
| 25 | + would be highlighted" rule. |
| 26 | + |
| 27 | + 2. It will find the common prefix and suffix of two lines, and |
| 28 | + consider everything in the middle to be "different". It could |
| 29 | + instead do a real diff of the characters between the two lines and |
| 30 | + find common subsequences. However, the point of the highlight is to |
| 31 | + call attention to a certain area. Even if some small subset of the |
| 32 | + highlighted area actually didn't change, that's OK. In practice it |
| 33 | + ends up being more readable to just have a single blob on the line |
| 34 | + showing the interesting bit. |
| 35 | + |
| 36 | +The goal of the script is therefore not to be exact about highlighting |
| 37 | +changes, but to call attention to areas of interest without being |
| 38 | +visually distracting. Non-diff lines and existing diff coloration is |
| 39 | +preserved; the intent is that the output should look exactly the same as |
| 40 | +the input, except for the occasional highlight. |
| 41 | + |
| 42 | +Use |
| 43 | +--- |
| 44 | + |
| 45 | +You can try out the diff-highlight program with: |
| 46 | + |
| 47 | +--------------------------------------------- |
| 48 | +git log -p --color | /path/to/diff-highlight |
| 49 | +--------------------------------------------- |
| 50 | + |
| 51 | +If you want to use it all the time, drop it in your $PATH and put the |
| 52 | +following in your git configuration: |
| 53 | + |
| 54 | +--------------------------------------------- |
| 55 | +[pager] |
| 56 | + log = diff-highlight | less |
| 57 | + show = diff-highlight | less |
| 58 | + diff = diff-highlight | less |
| 59 | +--------------------------------------------- |
| 60 | + |
| 61 | + |
| 62 | +Color Config |
| 63 | +------------ |
| 64 | + |
| 65 | +You can configure the highlight colors and attributes using git's |
| 66 | +config. The colors for "old" and "new" lines can be specified |
| 67 | +independently. There are two "modes" of configuration: |
| 68 | + |
| 69 | + 1. You can specify a "highlight" color and a matching "reset" color. |
| 70 | + This will retain any existing colors in the diff, and apply the |
| 71 | + "highlight" and "reset" colors before and after the highlighted |
| 72 | + portion. |
| 73 | + |
| 74 | + 2. You can specify a "normal" color and a "highlight" color. In this |
| 75 | + case, existing colors are dropped from that line. The non-highlighted |
| 76 | + bits of the line get the "normal" color, and the highlights get the |
| 77 | + "highlight" color. |
| 78 | + |
| 79 | +If no "new" colors are specified, they default to the "old" colors. If |
| 80 | +no "old" colors are specified, the default is to reverse the foreground |
| 81 | +and background for highlighted portions. |
| 82 | + |
| 83 | +Examples: |
| 84 | + |
| 85 | +--------------------------------------------- |
| 86 | +# Underline highlighted portions |
| 87 | +[color "diff-highlight"] |
| 88 | +oldHighlight = ul |
| 89 | +oldReset = noul |
| 90 | +--------------------------------------------- |
| 91 | + |
| 92 | +--------------------------------------------- |
| 93 | +# Varying background intensities |
| 94 | +[color "diff-highlight"] |
| 95 | +oldNormal = "black #f8cbcb" |
| 96 | +oldHighlight = "black #ffaaaa" |
| 97 | +newNormal = "black #cbeecb" |
| 98 | +newHighlight = "black #aaffaa" |
| 99 | +--------------------------------------------- |
| 100 | + |
| 101 | + |
| 102 | +Bugs |
| 103 | +---- |
| 104 | + |
| 105 | +Because diff-highlight relies on heuristics to guess which parts of |
| 106 | +changes are important, there are some cases where the highlighting is |
| 107 | +more distracting than useful. Fortunately, these cases are rare in |
| 108 | +practice, and when they do occur, the worst case is simply a little |
| 109 | +extra highlighting. This section documents some cases known to be |
| 110 | +sub-optimal, in case somebody feels like working on improving the |
| 111 | +heuristics. |
| 112 | + |
| 113 | +1. Two changes on the same line get highlighted in a blob. For example, |
| 114 | + highlighting: |
| 115 | + |
| 116 | +---------------------------------------------- |
| 117 | +-foo(buf, size); |
| 118 | ++foo(obj->buf, obj->size); |
| 119 | +---------------------------------------------- |
| 120 | + |
| 121 | + yields (where the inside of "+{}" would be highlighted): |
| 122 | + |
| 123 | +---------------------------------------------- |
| 124 | +-foo(buf, size); |
| 125 | ++foo(+{obj->buf, obj->}size); |
| 126 | +---------------------------------------------- |
| 127 | + |
| 128 | + whereas a more semantically meaningful output would be: |
| 129 | + |
| 130 | +---------------------------------------------- |
| 131 | +-foo(buf, size); |
| 132 | ++foo(+{obj->}buf, +{obj->}size); |
| 133 | +---------------------------------------------- |
| 134 | + |
| 135 | + Note that doing this right would probably involve a set of |
| 136 | + content-specific boundary patterns, similar to word-diff. Otherwise |
| 137 | + you get junk like: |
| 138 | + |
| 139 | +----------------------------------------------------- |
| 140 | +-this line has some -{i}nt-{ere}sti-{ng} text on it |
| 141 | ++this line has some +{fa}nt+{a}sti+{c} text on it |
| 142 | +----------------------------------------------------- |
| 143 | + |
| 144 | + which is less readable than the current output. |
| 145 | + |
| 146 | +2. The multi-line matching assumes that lines in the pre- and post-image |
| 147 | + match by position. This is often the case, but can be fooled when a |
| 148 | + line is removed from the top and a new one added at the bottom (or |
| 149 | + vice versa). Unless the lines in the middle are also changed, diffs |
| 150 | + will show this as two hunks, and it will not get highlighted at all |
| 151 | + (which is good). But if the lines in the middle are changed, the |
| 152 | + highlighting can be misleading. Here's a pathological case: |
| 153 | + |
| 154 | +----------------------------------------------------- |
| 155 | +-one |
| 156 | +-two |
| 157 | +-three |
| 158 | +-four |
| 159 | ++two 2 |
| 160 | ++three 3 |
| 161 | ++four 4 |
| 162 | ++five 5 |
| 163 | +----------------------------------------------------- |
| 164 | + |
| 165 | + which gets highlighted as: |
| 166 | + |
| 167 | +----------------------------------------------------- |
| 168 | +-one |
| 169 | +-t-{wo} |
| 170 | +-three |
| 171 | +-f-{our} |
| 172 | ++two 2 |
| 173 | ++t+{hree 3} |
| 174 | ++four 4 |
| 175 | ++f+{ive 5} |
| 176 | +----------------------------------------------------- |
| 177 | + |
| 178 | + because it matches "two" to "three 3", and so forth. It would be |
| 179 | + nicer as: |
| 180 | + |
| 181 | +----------------------------------------------------- |
| 182 | +-one |
| 183 | +-two |
| 184 | +-three |
| 185 | +-four |
| 186 | ++two +{2} |
| 187 | ++three +{3} |
| 188 | ++four +{4} |
| 189 | ++five 5 |
| 190 | +----------------------------------------------------- |
| 191 | + |
| 192 | + which would probably involve pre-matching the lines into pairs |
| 193 | + according to some heuristic. |
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