[PATCH] log: add --show-linear-break to help see non-linear history

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 This is a more serious attempt to make non-linear history more
 visible without --graph. It looks like this

    commit e4ddb05720710213108cd13ddd5a115e12a6211d
    Author: Andy Spencer <andy753421@gmail.com>
    
        tree_entry_interesting: match against all pathspecs
    
                        ..........
    
    commit a0332337be53f266682279c72a5e553986638c87
    Author: Jeff King <peff@peff.net>
    
        t7700: do not use "touch" unnecessarily
    
    commit 088304bf73b9b4149e04d2246fe08a06eef6e795
    Author: Jeff King <peff@peff.net>
    
        t7501: fix "empty commit" test with NO_PERL
    
                        ..........
    
    commit 74b4f7f27736f3e196a4eb3db41c68e37a6e2160
    Author: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
    
        tree-walk.c: ignore trailing slash on submodule in tree_entry_interesting()

 You can use fancier break bars from [1] instead of plain dots. I'm
 not sure it works with all combinations of rev-list. But at least
 log (with/out pathspec) and log --reverse work.
 
 [1] http://www.ascii-art.de/ascii/ab/border.txt

 Documentation/rev-list-options.txt |  7 +++++
 log-tree.c                         |  4 +++
 revision.c                         | 52 +++++++++++++++++++++++++++++++++++---
 revision.h                         |  6 +++++
 4 files changed, 66 insertions(+), 3 deletions(-)

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 03533af..a0780be 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -750,6 +750,13 @@ This enables parent rewriting, see 'History Simplification' below.
 This implies the `--topo-order` option by default, but the
 `--date-order` option may also be specified.
 
+--show-linear-break[=<break bar>]::
+	When --graph is not used, all history branches are flatten and
+	could be hard to see that the two consecutive commits do not
+	belong to a linear branch. This option puts a break bar in
+	between them in that case. If `<break bar>` is specified, it
+	is the string that will be shown instead of the default one.
+
 ifdef::git-rev-list[]
 --count::
 	Print a number stating how many commits would have been
diff --git a/log-tree.c b/log-tree.c
index 08970bf..17862f6 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -805,12 +805,16 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit)
 	if (opt->line_level_traverse)
 		return line_log_print(opt, commit);
 
+	if (opt->track_linear && !opt->linear && !opt->reverse_output_stage)
+		printf("\n%s\n", opt->break_bar);
 	shown = log_tree_diff(opt, commit, &log);
 	if (!shown && opt->loginfo && opt->always_show_header) {
 		log.parent = NULL;
 		show_log(opt);
 		shown = 1;
 	}
+	if (opt->track_linear && !opt->linear && opt->reverse_output_stage)
+		printf("\n%s\n", opt->break_bar);
 	opt->loginfo = NULL;
 	maybe_flush_or_die(stdout, "stdout");
 	return shown;
diff --git a/revision.c b/revision.c
index a0df72f..ff3107f 100644
--- a/revision.c
+++ b/revision.c
@@ -1836,6 +1836,12 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->notes_opt.use_default_notes = 1;
 	} else if (!strcmp(arg, "--show-signature")) {
 		revs->show_signature = 1;
+	} else if (!strcmp(arg, "--show-linear-break")) {
+		revs->track_linear = 1;
+		revs->break_bar = "                    ..........";
+	} else if (starts_with(arg, "--show-linear-break=")) {
+		revs->track_linear = 1;
+		revs->break_bar = xstrdup(arg + 20);
 	} else if (starts_with(arg, "--show-notes=") ||
 		   starts_with(arg, "--notes=")) {
 		struct strbuf buf = STRBUF_INIT;
@@ -2901,6 +2907,32 @@ enum commit_action simplify_commit(struct rev_info *revs, struct commit *commit)
 	return action;
 }
 
+define_commit_slab(saved_linear, int);
+
+static void track_linear(struct rev_info *revs, struct commit *commit)
+{
+	struct commit_list *p = revs->previous_parents;
+
+	if (p) {
+		int got_parent = 0;
+		for (; p && !got_parent; p = p->next)
+			got_parent = !hashcmp(p->item->object.sha1,
+					      commit->object.sha1);
+		revs->linear = got_parent;
+		free_commit_list(revs->previous_parents);
+	} else
+		revs->linear = 1;
+	if (revs->reverse) {
+		if (!revs->saved_linear_slab) {
+			revs->saved_linear_slab = xmalloc(sizeof(struct saved_linear));
+			init_saved_linear(revs->saved_linear_slab);
+		}
+
+		*saved_linear_at(revs->saved_linear_slab, commit) = revs->linear;
+	}
+	revs->previous_parents = copy_commit_list(commit->parents);
+}
+
 static struct commit *get_revision_1(struct rev_info *revs)
 {
 	if (!revs->commits)
@@ -2933,6 +2965,9 @@ static struct commit *get_revision_1(struct rev_info *revs)
 				    sha1_to_hex(commit->object.sha1));
 		}
 
+		if (revs->track_linear)
+			track_linear(revs, commit);
+
 		switch (simplify_commit(revs, commit)) {
 		case commit_ignore:
 			continue;
@@ -3106,14 +3141,25 @@ struct commit *get_revision(struct rev_info *revs)
 		revs->reverse_output_stage = 1;
 	}
 
-	if (revs->reverse_output_stage)
-		return pop_commit(&revs->commits);
+	if (revs->reverse_output_stage) {
+		c = pop_commit(&revs->commits);
+		if (revs->track_linear)
+			revs->linear = *saved_linear_at(revs->saved_linear_slab, c);
+		return c;
+	}
 
 	c = get_revision_internal(revs);
 	if (c && revs->graph)
 		graph_update(revs->graph, c);
-	if (!c)
+	if (!c) {
 		free_saved_parents(revs);
+		if (revs->saved_linear_slab)
+			clear_saved_linear(revs->saved_linear_slab);
+		if (revs->previous_parents) {
+			free_commit_list(revs->previous_parents);
+			revs->previous_parents = NULL;
+		}
+	}
 	return c;
 }
 
diff --git a/revision.h b/revision.h
index 88967d6..bffdc15 100644
--- a/revision.h
+++ b/revision.h
@@ -134,6 +134,7 @@ struct rev_info {
 			use_terminator:1,
 			missing_newline:1,
 			date_mode_explicit:1,
+			track_linear:1,
 			preserve_subject:1;
 	unsigned int	disable_stdin:1;
 	unsigned int	leak_pending:1;
@@ -195,6 +196,11 @@ struct rev_info {
 
 	/* copies of the parent lists, for --full-diff display */
 	struct saved_parents *saved_parents_slab;
+
+	struct commit_list *previous_parents;
+	struct saved_linear *saved_linear_slab;
+	char *break_bar;
+	unsigned int linear;
 };
 
 extern int ref_excluded(struct string_list *, const char *path);
-- 
1.8.5.2.240.g8478abd