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git-commit-vandalism/Documentation/git-range-diff.txt
Johannes Schindelin a7be92acd9 range-diff: use dim/bold cues to improve dual color mode 
It *is* a confusing thing to look at a diff of diffs. All too easy is it
to mix up whether the -/+ markers refer to the "inner" or the "outer"
diff, i.e. whether a `+` indicates that a line was added by either the
old or the new diff (or both), or whether the new diff does something
different than the old diff.

To make things easier to process for normal developers, we introduced
the dual color mode which colors the lines according to the commit diff,
i.e. lines that are added by a commit (whether old, new, or both) are
colored in green. In non-dual color mode, the lines would be colored
according to the outer diff: if the old commit added a line, it would be
colored red (because that line addition is only present in the first
commit range that was specified on the command-line, i.e. the "old"
commit, but not in the second commit range, i.e. the "new" commit).

However, this dual color mode is still not making things clear enough,
as we are looking at two levels of diffs, and we still only pick a color
according to *one* of them (the outer diff marker is colored
differently, of course, but in particular with deep indentation, it is
easy to lose track of that outer diff marker's background color).

Therefore, let's add another dimension to the mix. Still use
green/red/normal according to the commit diffs, but now also dim the
lines that were only in the old commit, and use bold face for the lines
that are only in the new commit.

That way, it is much easier not to lose track of, say, when we are
looking at a line that was added in the previous iteration of a patch
series but the new iteration adds a slightly different version: the
obsolete change will be dimmed, the current version of the patch will be
bold.

At least this developer has a much easier time reading the range-diffs
that way.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-08-13 10:44:52 -07:00

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git-range-diff(1)
=================
NAME
----
git-range-diff - Compare two commit ranges (e.g. two versions of a branch)
SYNOPSIS
--------
[verse]
'git range-diff' [--color=[<when>]] [--no-color] [<diff-options>]
[--no-dual-color] [--creation-factor=<factor>]
( <range1> <range2> | <rev1>...<rev2> | <base> <rev1> <rev2> )
DESCRIPTION
-----------
This command shows the differences between two versions of a patch
series, or more generally, two commit ranges (ignoring merge commits).
To that end, it first finds pairs of commits from both commit ranges
that correspond with each other. Two commits are said to correspond when
the diff between their patches (i.e. the author information, the commit
message and the commit diff) is reasonably small compared to the
patches' size. See ``Algorithm`` below for details.
Finally, the list of matching commits is shown in the order of the
second commit range, with unmatched commits being inserted just after
all of their ancestors have been shown.
OPTIONS
-------
--no-dual-color::
When the commit diffs differ, `git range-diff` recreates the
original diffs' coloring, and adds outer -/+ diff markers with
the *background* being red/green to make it easier to see e.g.
when there was a change in what exact lines were added.
+
Additionally, the commit diff lines that are only present in the first commit
range are shown "dimmed" (this can be overridden using the `color.diff.<slot>`
config setting where `<slot>` is one of `contextDimmed`, `oldDimmed` and
`newDimmed`), and the commit diff lines that are only present in the second
commit range are shown in bold (which can be overridden using the config
settings `color.diff.<slot>` with `<slot>` being one of `contextBold`,
`oldBold` or `newBold`).
+
This is known to `range-diff` as "dual coloring". Use `--no-dual-color`
to revert to color all lines according to the outer diff markers
(and completely ignore the inner diff when it comes to color).
--creation-factor=<percent>::
Set the creation/deletion cost fudge factor to `<percent>`.
Defaults to 60. Try a larger value if `git range-diff` erroneously
considers a large change a total rewrite (deletion of one commit
and addition of another), and a smaller one in the reverse case.
See the ``Algorithm`` section below for an explanation why this is
needed.
<range1> <range2>::
Compare the commits specified by the two ranges, where
`<range1>` is considered an older version of `<range2>`.
<rev1>...<rev2>::
Equivalent to passing `<rev2>..<rev1>` and `<rev1>..<rev2>`.
<base> <rev1> <rev2>::
Equivalent to passing `<base>..<rev1>` and `<base>..<rev2>`.
Note that `<base>` does not need to be the exact branch point
of the branches. Example: after rebasing a branch `my-topic`,
`git range-diff my-topic@{u} my-topic@{1} my-topic` would
show the differences introduced by the rebase.
`git range-diff` also accepts the regular diff options (see
linkgit:git-diff[1]), most notably the `--color=[<when>]` and
`--no-color` options. These options are used when generating the "diff
between patches", i.e. to compare the author, commit message and diff of
corresponding old/new commits. There is currently no means to tweak the
diff options passed to `git log` when generating those patches.
CONFIGURATION
-------------
This command uses the `diff.color.*` and `pager.range-diff` settings
(the latter is on by default).
See linkgit:git-config[1].
EXAMPLES
--------
When a rebase required merge conflicts to be resolved, compare the changes
introduced by the rebase directly afterwards using:
------------
$ git range-diff @{u} @{1} @
------------
A typical output of `git range-diff` would look like this:
------------
-: ------- > 1: 0ddba11 Prepare for the inevitable!
1: c0debee = 2: cab005e Add a helpful message at the start
2: f00dbal ! 3: decafe1 Describe a bug
@@ -1,3 +1,3 @@
Author: A U Thor <author@example.com>
-TODO: Describe a bug
+Describe a bug
@@ -324,5 +324,6
This is expected.
-+What is unexpected is that it will also crash.
++Unexpectedly, it also crashes. This is a bug, and the jury is
++still out there how to fix it best. See ticket #314 for details.
Contact
3: bedead < -: ------- TO-UNDO
------------
In this example, there are 3 old and 3 new commits, where the developer
removed the 3rd, added a new one before the first two, and modified the
commit message of the 2nd commit as well its diff.
When the output goes to a terminal, it is color-coded by default, just
like regular `git diff`'s output. In addition, the first line (adding a
commit) is green, the last line (deleting a commit) is red, the second
line (with a perfect match) is yellow like the commit header of `git
show`'s output, and the third line colors the old commit red, the new
one green and the rest like `git show`'s commit header.
A naive color-coded diff of diffs is actually a bit hard to read,
though, as it colors the entire lines red or green. The line that added
"What is unexpected" in the old commit, for example, is completely red,
even if the intent of the old commit was to add something.
To help with that, `range` uses the `--dual-color` mode by default. In
this mode, the diff of diffs will retain the original diff colors, and
prefix the lines with -/+ markers that have their *background* red or
green, to make it more obvious that they describe how the diff itself
changed.
Algorithm
---------
The general idea is this: we generate a cost matrix between the commits
in both commit ranges, then solve the least-cost assignment.
The cost matrix is populated thusly: for each pair of commits, both
diffs are generated and the "diff of diffs" is generated, with 3 context
lines, then the number of lines in that diff is used as cost.
To avoid false positives (e.g. when a patch has been removed, and an
unrelated patch has been added between two iterations of the same patch
series), the cost matrix is extended to allow for that, by adding
fixed-cost entries for wholesale deletes/adds.
Example: Let commits `1--2` be the first iteration of a patch series and
`A--C` the second iteration. Let's assume that `A` is a cherry-pick of
`2,` and `C` is a cherry-pick of `1` but with a small modification (say,
a fixed typo). Visualize the commits as a bipartite graph:
------------
1 A
2 B
C
------------
We are looking for a "best" explanation of the new series in terms of
the old one. We can represent an "explanation" as an edge in the graph:
------------
1 A
/
2 --------' B
C
------------
This explanation comes for "free" because there was no change. Similarly
`C` could be explained using `1`, but that comes at some cost c>0
because of the modification:
------------
1 ----. A
| /
2 ----+---' B
|
`----- C
c>0
------------
In mathematical terms, what we are looking for is some sort of a minimum
cost bipartite matching; `1` is matched to `C` at some cost, etc. The
underlying graph is in fact a complete bipartite graph; the cost we
associate with every edge is the size of the diff between the two
commits' patches. To explain also new commits, we introduce dummy nodes
on both sides:
------------
1 ----. A
| /
2 ----+---' B
|
o `----- C
c>0
o o
o o
------------
The cost of an edge `o--C` is the size of `C`'s diff, modified by a
fudge factor that should be smaller than 100%. The cost of an edge
`o--o` is free. The fudge factor is necessary because even if `1` and
`C` have nothing in common, they may still share a few empty lines and
such, possibly making the assignment `1--C`, `o--o` slightly cheaper
than `1--o`, `o--C` even if `1` and `C` have nothing in common. With the
fudge factor we require a much larger common part to consider patches as
corresponding.
The overall time needed to compute this algorithm is the time needed to
compute n+m commit diffs and then n*m diffs of patches, plus the time
needed to compute the least-cost assigment between n and m diffs. Git
uses an implementation of the Jonker-Volgenant algorithm to solve the
assignment problem, which has cubic runtime complexity. The matching
found in this case will look like this:
------------
1 ----. A
| /
2 ----+---' B
.--+-----'
o -' `----- C
c>0
o ---------- o
o ---------- o
------------
SEE ALSO
--------
linkgit:git-log[1]
GIT
---
Part of the linkgit:git[1] suite
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