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graph: example of graph output that can be simplified

The commits following this one introduce a series of improvements to the
layout of graphs, tidying up a few edge cases, namely:

- merge whose first parent fuses with an existing column to the left
- merge whose last parent fuses with its immediate neighbor on the right
- edges that collapse to the left above and below a commit line

This test case exemplifies these cases and provides a motivating example
of the kind of history I'm aiming to clear up.

The first parent of merge E is the same as the parent of H, so those
edges fuse together.

        * H
        |
        | *-.   E
        | |\ \
        |/ / /
        |
        * B

We can "skew" the display of this merge so that it doesn't introduce
additional columns that immediately collapse:

        * H
        |
        | *   E
        |/|\
        |
        * B

The last parent of E is D, the same as the parent of F which is the edge
to the right of the merge.

            * F
            |
             \
          *-. \   E
          |\ \ \
         / / / /
            | /
            |/
            * D

The two edges leading to D could be fused sooner: rather than expanding
the F edge around the merge and then letting the edges collapse, the F
edge could fuse with the E edge in the post-merge line:

            * F
            |
             \
          *-. | E
          |\ \|
         / / /
            |
            * D

If this is combined with the "skew" effect above, we get a much cleaner
graph display for these edges:

            * F
            |
          * | E
         /|\|
            |
            * D

Finally, the edge leading from C to A appears jagged as it passes
through the commit line for B:

        | * | C
        | |/
        * | B
        |/
        * A

This can be smoothed out so that such edges are easier to read:

        | * | C
        | |/
        * / B
        |/
        * A

Signed-off-by: James Coglan <jcoglan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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jcoglan authored and gitster committed Oct 15, 2019
1 parent ee7abb5 commit 458152cce1a5544079456675a9cb7df00704c272
Showing with 43 additions and 0 deletions.
  1. +43 −0 t/t4215-log-skewed-merges.sh
@@ -0,0 +1,43 @@
#!/bin/sh

test_description='git log --graph of skewed merges'

. ./test-lib.sh

test_expect_success 'log --graph with merge fusing with its left and right neighbors' '
cat >expect <<-\EOF &&
* H
|\
| * G
| |\
| | * F
| | |
| | \
| *-. \ E
| |\ \ \
|/ / / /
| | | /
| | |/
| | * D
| * | C
| |/
* | B
|/
* A
EOF
git checkout --orphan _p &&
test_commit A &&
test_commit B &&
git checkout -b _q @^ && test_commit C &&
git checkout -b _r @^ && test_commit D &&
git checkout _p && git merge --no-ff _q _r -m E &&
git checkout _r && test_commit F &&
git checkout _p && git merge --no-ff _r -m G &&
git checkout @^^ && git merge --no-ff _p -m H &&
git log --graph --pretty=tformat:%s | sed "s/ *$//" >actual &&
test_cmp expect actual
'

test_done

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