Re: [PATCH v8 4/4] graph: indent visual root in graph

> When rendering a graph, if the history contains multiple "visual roots",
> actual roots or commits that look like roots (i.e. have their parents
> filtered out) can end up being vertically adjacent to unrelated commits,
> falsely appearing to be related.
> 
> A fix for this issue was already attempted [1] a while ago.
> 
> This happens because the commits fill the space from left to right and
> when a visual root ends, its column becomes free for the following
> commit even if they are not related. Once this happens the unrelated
> commit is rendered below the visual root. Because there is no special
> character or way to identify when a visual root is rendered making the
> graph confusing.
> 
> By indenting the visual roots when there are still commits to show the
> vertical adjacency can be avoided.
> 
> Add is_visual_root flag to git_graph making it visible in all graph states,
> give graph_update() a new function, graph_is_visual_root() to know if the
> current commit is a visual root and set is_visual_root.
> The different handled cases are:
> 
> - If a visual root has children: similar to GRAPH_PRE_COMMIT state when
>   octopus merges need space, an edge row needs to be printed to connect
>   the child with the indented visual root. A new state GRAPH_PRE_ROOT is
>   needed to connect the child with the visual root:
> 
>     * child of the visual root
>      \ GRAPH_PRE_ROOT
>       * visual root indented
> 
> - If a visual root is child-less we can skip GRAPH_PRE_ROOT state and
>   render the indented commit directly.
> 
>       * visual root indented
>     * unrelated commit
> 
> - If two or more visual roots are adjacent: by having a lookahead to the
>   next commit that will be rendered, if the next commit is also a visual
>   root and we are on a visual root, meaning two visual root adjacent in
>   the history, the top one can omit the indent, making the one below to
>   indent only once, if there are more adjacent visual commits, the
>   indentation will increase for each adjacent one, cascading.
> 
>     * visual root
>       * visual root
>         * visual root
>     * last commit
> 
>   Even if the last commit is a root, because there is nothing that will be
>   rendered below we can omit the indentation on purpose.
> 
> [1]: https://lore.kernel.org/git/xmqqwnwajbuj.fsf@gitster.c.googlers.com/
> 
> Helped-by: Kristofer Karlsson <krka@spotify.com>
> Mentored-by: Karthik Nayak <karthik.188@gmail.com>
> Mentored-by: Chandra Pratap <chandrapratap3519@gmail.com>
> Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
> ---
>  graph.c                          | 235 +++++++++++++++++++
>  t/meson.build                    |   1 +
>  t/t4218-log-graph-indentation.sh | 473 +++++++++++++++++++++++++++++++++++++++
>  3 files changed, 709 insertions(+)

> +/*
> + * A commit can be a visual root when:
> + *
> + * - It has no parents.
> + *
> + * - It has parents but they are all filtered out and
> + *   commit->parents arrives NULL.
> + *
> + * - It is not a boundary commit. Boundary commits also have no visible
> + *   parents, but they are not selected as visual roots because they cannot
> + *   cause the ambiguity of being vertically adjacent because:
> + *
> + *   1. A boundary only appears because an included commit is its child.
> + *      Children are always above, and the renderer draws an edge down to
> + *      the boundary from that child. Rather than starting a column like a
> + *      visual root would do, it inherits its child column.
> + *
> + *   2. Included commits cannot appear below a boundary. Boundaries are
> + *      ancestors of the exclusion point; if an included commit were an
> + *      ancestor of the boundary it would be excluded and not rendered.
> + *      Boundaries therefore always sink to the bottom.
> + */
> +static int graph_is_visual_root_candidate(struct commit *c)
> +{
> +	return c->parents == NULL && !(c->object.flags & BOUNDARY);
> +}