toposort: rename "lifo" field

The primary invariant of sort_in_topological_order() is that a
parent commit is not emitted until all children of it are.  When
traversing a forked history like this with "git log C E":

    A----B----C
     \
      D----E

we ensure that A is emitted after all of B, C, D, and E are done, B
has to wait until C is done, and D has to wait until E is done.

In some applications, however, we would further want to control how
these child commits B, C, D and E on two parallel ancestry chains
are shown.

Most of the time, we would want to see C and B emitted together, and
then E and D, and finally A (i.e. the --topo-order output).  The
"lifo" parameter of the sort_in_topological_order() function is used
to control this behaviour.  We start the traversal by knowing two
commits, C and E.  While keeping in mind that we also need to
inspect E later, we pick C first to inspect, and we notice and
record that B needs to be inspected.  By structuring the "work to be
done" set as a LIFO stack, we ensure that B is inspected next,
before other in-flight commits we had known that we will need to
inspect, e.g. E.

When showing in --date-order, we would want to see commits ordered
by timestamps, i.e. show C, E, B and D in this order before showing
A, possibly mixing commits from two parallel histories together.
When "lifo" parameter is set to false, the function keeps the "work
to be done" set sorted in the date order to realize this semantics.
After inspecting C, we add B to the "work to be done" set, but the
next commit we inspect from the set is E which is newer than B.

The name "lifo", however, is too strongly tied to the way how the
function implements its behaviour, and does not describe what the
behaviour _means_.

Replace this field with an enum rev_sort_order, with two possible
values: REV_SORT_IN_GRAPH_ORDER and REV_SORT_BY_COMMIT_DATE, and
update the existing code.  The mechanical replacement rule is:

  "lifo == 0" is equivalent to "sort_order == REV_SORT_BY_COMMIT_DATE"
  "lifo == 1" is equivalent to "sort_order == REV_SORT_IN_GRAPH_ORDER"

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -205,7 +205,7 @@ static void log_show_early(struct rev_info *revs, struct commit_list *list)
 	int i = revs->early_output;
 	int show_header = 1;
 
-	sort_in_topological_order(&list, revs->lifo);
+	sort_in_topological_order(&list, revs->sort_order);
 	while (list && i) {
 		struct commit *commit = list->item;
 		switch (simplify_commit(revs, commit)) {

builtin/show-branch.c

@@ -631,7 +631,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 	int num_rev, i, extra = 0;
 	int all_heads = 0, all_remotes = 0;
 	int all_mask, all_revs;
-	int lifo = 1;
+	enum rev_sort_order sort_order = REV_SORT_IN_GRAPH_ORDER;
 	char head[128];
 	const char *head_p;
 	int head_len;
@@ -666,15 +666,17 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			    N_("show possible merge bases")),
 		OPT_BOOLEAN(0, "independent", &independent,
 			    N_("show refs unreachable from any other ref")),
-		OPT_BOOLEAN(0, "topo-order", &lifo,
-			    N_("show commits in topological order")),
+		OPT_SET_INT(0, "topo-order", &sort_order,
+			    N_("show commits in topological order"),
+			    REV_SORT_IN_GRAPH_ORDER),
 		OPT_BOOLEAN(0, "topics", &topics,
 			    N_("show only commits not on the first branch")),
 		OPT_SET_INT(0, "sparse", &dense,
 			    N_("show merges reachable from only one tip"), 0),
-		OPT_SET_INT(0, "date-order", &lifo,
+		OPT_SET_INT(0, "date-order", &sort_order,
 			    N_("show commits where no parent comes before its "
-			       "children"), 0),
+			       "children"),
+			    REV_SORT_BY_COMMIT_DATE),
 		{ OPTION_CALLBACK, 'g', "reflog", &reflog_base, N_("<n>[,<base>]"),
 			    N_("show <n> most recent ref-log entries starting at "
 			       "base"),
@@ -901,7 +903,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 		exit(0);
 
 	/* Sort topologically */
-	sort_in_topological_order(&seen, lifo);
+	sort_in_topological_order(&seen, sort_order);
 
 	/* Give names to commits */
 	if (!sha1_name && !no_name)

commit.c

@@ -512,7 +512,7 @@ define_commit_slab(indegree_slab, int);
 /*
  * Performs an in-place topological sort on the list supplied.
  */
-void sort_in_topological_order(struct commit_list ** list, int lifo)
+void sort_in_topological_order(struct commit_list ** list, enum rev_sort_order sort_order)
 {
 	struct commit_list *next, *orig = *list;
 	struct commit_list *work, **insert;
@@ -561,7 +561,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 	}
 
 	/* process the list in topological order */
-	if (!lifo)
+	if (sort_order != REV_SORT_IN_GRAPH_ORDER)
 		commit_list_sort_by_date(&work);
 
 	pptr = list;
@@ -588,10 +588,14 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 			 * guaranteeing topological order.
 			 */
 			if (--(*pi) == 1) {
-				if (!lifo)
+				switch (sort_order) {
+				case REV_SORT_BY_COMMIT_DATE:
 					commit_list_insert_by_date(parent, &work);
-				else
+					break;
+				default: /* REV_SORT_IN_GRAPH_ORDER */
 					commit_list_insert(parent, &work);
+					break;
+				}
 			}
 		}
 		/*

commit.h

@@ -139,15 +139,23 @@ struct commit *pop_commit(struct commit_list **stack);
 void clear_commit_marks(struct commit *commit, unsigned int mark);
 void clear_commit_marks_for_object_array(struct object_array *a, unsigned mark);
 
+
+enum rev_sort_order {
+	REV_SORT_IN_GRAPH_ORDER = 0,
+	REV_SORT_BY_COMMIT_DATE
+};
+
 /*
  * Performs an in-place topological sort of list supplied.
  *
  *   invariant of resulting list is:
  *      a reachable from b => ord(b) < ord(a)
- *   in addition, when lifo == 0, commits on parallel tracks are
- *   sorted in the dates order.
+ *   sort_order further specifies:
+ *   REV_SORT_IN_GRAPH_ORDER: try to show a commit on a single-parent
+ *                            chain together.
+ *   REV_SORT_BY_COMMIT_DATE: show eligible commits in committer-date order.
  */
-void sort_in_topological_order(struct commit_list ** list, int lifo);
+void sort_in_topological_order(struct commit_list **, enum rev_sort_order);
 
 struct commit_graft {
 	unsigned char sha1[20];

revision.c

@@ -1038,7 +1038,7 @@ void init_revisions(struct rev_info *revs, const char *prefix)
 	DIFF_OPT_SET(&revs->pruning, QUICK);
 	revs->pruning.add_remove = file_add_remove;
 	revs->pruning.change = file_change;
-	revs->lifo = 1;
+	revs->sort_order = REV_SORT_IN_GRAPH_ORDER;
 	revs->dense = 1;
 	revs->prefix = prefix;
 	revs->max_age = -1;
@@ -1373,7 +1373,7 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--merge")) {
 		revs->show_merge = 1;
 	} else if (!strcmp(arg, "--topo-order")) {
-		revs->lifo = 1;
+		revs->sort_order = REV_SORT_IN_GRAPH_ORDER;
 		revs->topo_order = 1;
 	} else if (!strcmp(arg, "--simplify-merges")) {
 		revs->simplify_merges = 1;
@@ -1391,7 +1391,7 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->prune = 1;
 		load_ref_decorations(DECORATE_SHORT_REFS);
 	} else if (!strcmp(arg, "--date-order")) {
-		revs->lifo = 0;
+		revs->sort_order = REV_SORT_BY_COMMIT_DATE;
 		revs->topo_order = 1;
 	} else if (!prefixcmp(arg, "--early-output")) {
 		int count = 100;
@@ -2165,7 +2165,7 @@ int prepare_revision_walk(struct rev_info *revs)
 		if (limit_list(revs) < 0)
 			return -1;
 	if (revs->topo_order)
-		sort_in_topological_order(&revs->commits, revs->lifo);
+		sort_in_topological_order(&revs->commits, revs->sort_order);
 	if (revs->simplify_merges)
 		simplify_merges(revs);
 	if (revs->children.name)
@@ -2480,7 +2480,7 @@ static void create_boundary_commit_list(struct rev_info *revs)
 	 * If revs->topo_order is set, sort the boundary commits
 	 * in topological order
 	 */
-	sort_in_topological_order(&revs->commits, revs->lifo);
+	sort_in_topological_order(&revs->commits, revs->sort_order);
 }
 
 static struct commit *get_revision_internal(struct rev_info *revs)

revision.h

@@ -4,6 +4,7 @@
 #include "parse-options.h"
 #include "grep.h"
 #include "notes.h"
+#include "commit.h"
 
 #define SEEN		(1u<<0)
 #define UNINTERESTING   (1u<<1)
@@ -60,6 +61,10 @@ struct rev_info {
 	const char *prefix;
 	const char *def;
 	struct pathspec prune_data;
+
+	/* topo-sort */
+	enum rev_sort_order sort_order;
+
 	unsigned int	early_output:1,
 			ignore_missing:1;
 
@@ -70,7 +75,6 @@ struct rev_info {
 			show_all:1,
 			remove_empty_trees:1,
 			simplify_history:1,
-			lifo:1,
 			topo_order:1,
 			simplify_merges:1,
 			simplify_by_decoration:1,