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>]
        [--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
-------
--dual-color::
        When the commit diffs differ, recreate the original diffs'
        coloring, and add 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.

--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.

The color-coded diff 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, use the `--dual-color` mode. 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