[PATCH 00/11] Improve the readability of log --graph output

This series of patches are designed to improve the output of the log --graph command; their effect can be summed up in the following diagram:

    Before                    After
    ------                    -----

    *
    |\
    | *                       *
    | |\                      |\
    | | *                     | *
    | | |                     | |\
    | |  \                    | | *
    | *-. \                   | * |
    | |\ \ \                  |/|\|
    |/ / / /                  | | *
    | | | /                   | * |
    | | |/                    | |/
    | | *                     * /
    | * |                     |/
    | |/                      *
    * |
    |/
    *

These changes aim to make the edges in graph diagrams easier to read, by straightening lines and making certain kinds of topologies display more compactly. Three distinct changes are included.

First, if the first parent of a merge fuses with an edge to the left of the commit, then we display that by making the edges fuse immediately rather than by drawing a line straight down and then having it track to the left. That is, where we currently display these graphs:

    | *             | | | *
    | |\            | | | |\
    |/ /            | |_|/ /
    | |             |/| | |

We will now display these merges as follows:

    | *             | | | *
    |/|             | |_|/|
    | |             |/| | |

This transformation is applied to merges with any number of parents, for example we currently display 3-parent merges like this:

    | *-.           | | | *-.
    | |\ \          | | | |\ \
    |/ / /          | |_|/ / /
    | | |           |/| | | |

And we will now display them like this:

    | *             | | | *
    |/|\            | |_|/|\
    | | |           |/| | | |

If the edge the first parent fuses with is separated from the commit by multiple columns, a horizontal edge is drawn just as we currently do in the 'collapsing' state. This change also affects the display of commit and post-merge lines in subtle ways that are more thoroughly described in the relevant commits.

The second change is that if the final parent of a merge fuses with the edge to the right of the commit, then we can remove the zig-zag effect that currently results. We currently display these merges like this:

    * |
    |\ \
    | |/
    | *

After these changes, this merge will now be displayed like so:

    * |
    |\|
    | *

If the final parent fuses with an edge that's further to the right, its display is unchanged and it will still display like this:

    * | | |
    |\ \ \ \
    | | |_|/
    | |/| |
    | * | |

The final structural change smooths out lines that are collapsing through commit lines. For example, consider the following history:

    *-. \
    |\ \ \
    | | * |
    | * | |
    | |/ /
    * | |
    |/ /
    * |
    |/
    *

This is now rendered so that commit lines display an edge using / instead of
|, if that edge is tracking to the left both above and below the commit
line. That results in this improved display:

    *-. \
    |\ \ \
    | | * |
    | * | |
    | |/ /
    * / /
    |/ /
    * /
    |/
    *

Taken together, these changes produce the change shown in the first diagram above, with the current rendering on the left and the new rendering on the right.

A final addition to that set of changes fixes the coloring of dashes that are drawn next to octopus merges, in a manner compatible with all these changes. The early commits in this set are refactorings that make the functional changes easier to introduce.

James Coglan (11):
  graph: automatically track visible width of `strbuf`
  graph: reuse `find_new_column_by_commit()`
  graph: reduce duplication in `graph_insert_into_new_columns()`
  graph: remove `mapping_idx` and `graph_update_width()`
  graph: extract logic for moving to GRAPH_PRE_COMMIT state
  graph: tidy up display of left-skewed merges
  graph: commit and post-merge lines for left-skewed merges
  graph: rename `new_mapping` to `old_mapping`
  graph: smooth appearance of collapsing edges on commit lines
  graph: flatten edges that join to their right neighbor
  graph: fix coloring of octopus dashes

 graph.c                                    | 499 ++++++++++++---------
 strbuf.h                                   |   8 +-
 t/t3430-rebase-merges.sh                   |   2 +-
 t/t4202-log.sh                             |   2 +-
 t/t4214-log-graph-octopus.sh               |  85 +++-
 t/t4215-log-skewed-merges.sh               | 267 +++++++++++
 t/t6016-rev-list-graph-simplify-history.sh |  30 +-
 7 files changed, 649 insertions(+), 244 deletions(-)
 create mode 100755 t/t4215-log-skewed-merges.sh

base-commit: 5fa0f5238b0cd46cfe7f6fa76c3f526ea98148d9
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-383%2Fjcoglan%2Fjc%2Fsimplify-graph-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-383/jcoglan/jc/simplify-graph-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/383