On Sat, Jun 23, 2018 at 05:45:19PM -0400, Noam Postavsky wrote:
> On 20 May 2016 at 18:12, Noam Postavsky <npostavs@xxxxxxxxxxxxxxxxxxxxx> wrote:
My, this is a blast from the past. :)
> Subject: [PATCH v1] log: Fix coloring of certain octupus merge shapes
>
> For octopus merges where the first parent edge immediately merges into
> the next column to the left:
>
> | | *-.
> | | |\ \
> | |/ / /
>
> then the number of columns should be one less than the usual case:
>
> | *-.
> | |\ \
> | | | *
These diagrams confused me for a minute, because I see two differences:
1. The first one has an extra apparently unrelated parallel branch on
the far left.
2. The first has the first-parent of the "*" merge commit immediately
join the branch.
But if I understand correctly, we only care about the second property.
So would it be accurate to show them as:
| *-.
| |\ \
|/ / /
| *-.
| |\ \
| | | *
?
I think that makes it easier to compare them.
I don't remember much about our prior discussion, so let me try to talk
myself through the patch itself:
> diff --git a/graph.c b/graph.c
> index e1f6d3bdd..c919c86e8 100644
> --- a/graph.c
> +++ b/graph.c
> @@ -856,12 +856,16 @@ static int graph_draw_octopus_merge(struct git_graph *graph,
> int col_num, i;
> int num_dashes =
> ((graph->num_parents - dashless_commits) * 2) - 1;
> - for (i = 0; i < num_dashes; i++) {
> - col_num = (i / 2) + dashless_commits + graph->commit_index;
OK, so the old code emitted num_dashes, and every pair was done with the
same column. Our highest iteration of this loop would use the column at
(num_dashes-1) / 2. We know that num_dashes is always odd, so:
num_dashes = 1 puts our last column at 0
num_dashes = 3 puts our last column at 1
And so on. So far so good.
> + int first_col = dashless_commits + graph->commit_index;
This corresponds to the i=0 case, makes sense.
> + int last_col = first_col + (num_dashes / 2);
But here our last_col misses the "-1". I don't think it matters because
we know num_dashes is always odd, and therefore due to integer
truncation (num_dashes-1)/2 == (num_dashes/2).
> + if (last_col >= graph->num_new_columns) {
> + first_col--;
> + last_col--;
> + }
The shifting of last_col I expect as part of the fix. I was surprised by
shifting first_col, though. Wouldn't it always start at 0 (offset by the
previous commits)? It definitely seems to be necessary, but I'm not sure
I understand why.
> + for (i = 0, col_num = first_col; i < num_dashes; i++, col_num++) {
> strbuf_write_column(sb, &graph->new_columns[col_num], '-');
> }
> - col_num = (i / 2) + dashless_commits + graph->commit_index;
> - strbuf_write_column(sb, &graph->new_columns[col_num], '.');
> + strbuf_write_column(sb, &graph->new_columns[last_col], '.');
In this new loop we count up our dashes and our columns. But now we have
1-to-1 correspondence as we increment! I don't think that can be right.
And indeed, if I take your original problem report and add an extra "d"
branch and make the octopus "a b d", then the problem comes back. You
don't notice with a 3-parent merge because
We need to increment col_num only half as much as num_dashes. Should we
be doing:
for (col_num = first_col; col_num < last_col; col_num++) {
strbuf_write_column(sb, &graph->new_columns[col_num], '-');
strbuf_write_column(sb, &graph->new_columns[col_num], '-');
}
strbuf_write_column(sb, &graph->new_columns[last_col], '-');
strbuf_write_column(sb, &graph->new_columns[last_col], '.');
I.e., write "--" for each interior column, and then "-." for the last
one?
-Peff
