range-diff: populate the man page
The bulk of this patch consists of a heavily butchered version of tbdiff's README written by Thomas Rast and Thomas Gummerer, lifted from https://github.com/trast/tbdiff. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Johannes Schindelin
Junio C Hamano
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@ -5,6 +5,235 @@ NAME
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----
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git-range-diff - Compare two commit ranges (e.g. two versions of a branch)
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SYNOPSIS
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--------
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[verse]
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'git range-diff' [--color=[<when>]] [--no-color] [<diff-options>]
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[--dual-color] [--creation-factor=<factor>]
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( <range1> <range2> | <rev1>...<rev2> | <base> <rev1> <rev2> )
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DESCRIPTION
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-----------
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This command shows the differences between two versions of a patch
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series, or more generally, two commit ranges (ignoring merge commits).
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To that end, it first finds pairs of commits from both commit ranges
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that correspond with each other. Two commits are said to correspond when
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the diff between their patches (i.e. the author information, the commit
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message and the commit diff) is reasonably small compared to the
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patches' size. See ``Algorithm`` below for details.
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Finally, the list of matching commits is shown in the order of the
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second commit range, with unmatched commits being inserted just after
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all of their ancestors have been shown.
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OPTIONS
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-------
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--dual-color::
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When the commit diffs differ, recreate the original diffs'
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coloring, and add outer -/+ diff markers with the *background*
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being red/green to make it easier to see e.g. when there was a
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change in what exact lines were added.
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--creation-factor=<percent>::
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Set the creation/deletion cost fudge factor to `<percent>`.
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Defaults to 60. Try a larger value if `git range-diff` erroneously
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considers a large change a total rewrite (deletion of one commit
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and addition of another), and a smaller one in the reverse case.
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See the ``Algorithm`` section below for an explanation why this is
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needed.
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<range1> <range2>::
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Compare the commits specified by the two ranges, where
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`<range1>` is considered an older version of `<range2>`.
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<rev1>...<rev2>::
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Equivalent to passing `<rev2>..<rev1>` and `<rev1>..<rev2>`.
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<base> <rev1> <rev2>::
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Equivalent to passing `<base>..<rev1>` and `<base>..<rev2>`.
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Note that `<base>` does not need to be the exact branch point
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of the branches. Example: after rebasing a branch `my-topic`,
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`git range-diff my-topic@{u} my-topic@{1} my-topic` would
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show the differences introduced by the rebase.
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`git range-diff` also accepts the regular diff options (see
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linkgit:git-diff[1]), most notably the `--color=[<when>]` and
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`--no-color` options. These options are used when generating the "diff
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between patches", i.e. to compare the author, commit message and diff of
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corresponding old/new commits. There is currently no means to tweak the
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diff options passed to `git log` when generating those patches.
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CONFIGURATION
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-------------
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This command uses the `diff.color.*` and `pager.range-diff` settings
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(the latter is on by default).
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See linkgit:git-config[1].
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EXAMPLES
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--------
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When a rebase required merge conflicts to be resolved, compare the changes
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introduced by the rebase directly afterwards using:
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------------
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$ git range-diff @{u} @{1} @
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------------
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A typical output of `git range-diff` would look like this:
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------------
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-: ------- > 1: 0ddba11 Prepare for the inevitable!
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1: c0debee = 2: cab005e Add a helpful message at the start
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2: f00dbal ! 3: decafe1 Describe a bug
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@@ -1,3 +1,3 @@
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Author: A U Thor <author@example.com>
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-TODO: Describe a bug
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+Describe a bug
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@@ -324,5 +324,6
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This is expected.
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-+What is unexpected is that it will also crash.
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++Unexpectedly, it also crashes. This is a bug, and the jury is
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++still out there how to fix it best. See ticket #314 for details.
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Contact
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3: bedead < -: ------- TO-UNDO
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------------
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In this example, there are 3 old and 3 new commits, where the developer
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removed the 3rd, added a new one before the first two, and modified the
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commit message of the 2nd commit as well its diff.
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When the output goes to a terminal, it is color-coded by default, just
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like regular `git diff`'s output. In addition, the first line (adding a
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commit) is green, the last line (deleting a commit) is red, the second
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line (with a perfect match) is yellow like the commit header of `git
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show`'s output, and the third line colors the old commit red, the new
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one green and the rest like `git show`'s commit header.
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The color-coded diff is actually a bit hard to read, though, as it
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colors the entire lines red or green. The line that added "What is
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unexpected" in the old commit, for example, is completely red, even if
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the intent of the old commit was to add something.
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To help with that, use the `--dual-color` mode. In this mode, the diff
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of diffs will retain the original diff colors, and prefix the lines with
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-/+ markers that have their *background* red or green, to make it more
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obvious that they describe how the diff itself changed.
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Algorithm
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---------
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The general idea is this: we generate a cost matrix between the commits
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in both commit ranges, then solve the least-cost assignment.
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The cost matrix is populated thusly: for each pair of commits, both
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diffs are generated and the "diff of diffs" is generated, with 3 context
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lines, then the number of lines in that diff is used as cost.
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To avoid false positives (e.g. when a patch has been removed, and an
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unrelated patch has been added between two iterations of the same patch
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series), the cost matrix is extended to allow for that, by adding
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fixed-cost entries for wholesale deletes/adds.
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Example: Let commits `1--2` be the first iteration of a patch series and
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`A--C` the second iteration. Let's assume that `A` is a cherry-pick of
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`2,` and `C` is a cherry-pick of `1` but with a small modification (say,
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a fixed typo). Visualize the commits as a bipartite graph:
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------------
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1 A
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2 B
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C
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------------
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We are looking for a "best" explanation of the new series in terms of
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the old one. We can represent an "explanation" as an edge in the graph:
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------------
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1 A
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/
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2 --------' B
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C
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------------
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This explanation comes for "free" because there was no change. Similarly
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`C` could be explained using `1`, but that comes at some cost c>0
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because of the modification:
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------------
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1 ----. A
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| /
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2 ----+---' B
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`----- C
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c>0
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------------
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In mathematical terms, what we are looking for is some sort of a minimum
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cost bipartite matching; `1` is matched to `C` at some cost, etc. The
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underlying graph is in fact a complete bipartite graph; the cost we
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associate with every edge is the size of the diff between the two
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commits' patches. To explain also new commits, we introduce dummy nodes
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on both sides:
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------------
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1 ----. A
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| /
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2 ----+---' B
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o `----- C
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c>0
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o o
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o o
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------------
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The cost of an edge `o--C` is the size of `C`'s diff, modified by a
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fudge factor that should be smaller than 100%. The cost of an edge
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`o--o` is free. The fudge factor is necessary because even if `1` and
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`C` have nothing in common, they may still share a few empty lines and
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such, possibly making the assignment `1--C`, `o--o` slightly cheaper
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than `1--o`, `o--C` even if `1` and `C` have nothing in common. With the
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fudge factor we require a much larger common part to consider patches as
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corresponding.
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The overall time needed to compute this algorithm is the time needed to
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compute n+m commit diffs and then n*m diffs of patches, plus the time
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needed to compute the least-cost assigment between n and m diffs. Git
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uses an implementation of the Jonker-Volgenant algorithm to solve the
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assignment problem, which has cubic runtime complexity. The matching
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found in this case will look like this:
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------------
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1 ----. A
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| /
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2 ----+---' B
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.--+-----'
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o -' `----- C
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c>0
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o ---------- o
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o ---------- o
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------------
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SEE ALSO
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--------
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linkgit:git-log[1]
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GIT
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---
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Part of the linkgit:git[1] suite
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