diff --git a/.gitignore b/.gitignore
index c0e00eb37b..efa8db0035 100644
--- a/.gitignore
+++ b/.gitignore
@@ -191,6 +191,7 @@
 /test-mktemp
 /test-parse-options
 /test-path-utils
+/test-prio-queue
 /test-read-cache
 /test-regex
 /test-revision-walking
diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index b462f17f62..e157ec3fe7 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -625,6 +625,10 @@ By default, the commits are shown in reverse chronological order.
 	Show no parents before all of its children are shown, but
 	otherwise show commits in the commit timestamp order.
 
+--author-date-order::
+	Show no parents before all of its children are shown, but
+	otherwise show commits in the author timestamp order.
+
 --topo-order::
 	Show no parents before all of its children are shown, and
 	avoid showing commits on multiple lines of history
diff --git a/Makefile b/Makefile
index e1583761df..5a68fe5431 100644
--- a/Makefile
+++ b/Makefile
@@ -569,6 +569,7 @@ TEST_PROGRAMS_NEED_X += test-mergesort
 TEST_PROGRAMS_NEED_X += test-mktemp
 TEST_PROGRAMS_NEED_X += test-parse-options
 TEST_PROGRAMS_NEED_X += test-path-utils
+TEST_PROGRAMS_NEED_X += test-prio-queue
 TEST_PROGRAMS_NEED_X += test-read-cache
 TEST_PROGRAMS_NEED_X += test-regex
 TEST_PROGRAMS_NEED_X += test-revision-walking
@@ -705,6 +706,7 @@ LIB_H += parse-options.h
 LIB_H += patch-ids.h
 LIB_H += pathspec.h
 LIB_H += pkt-line.h
+LIB_H += prio-queue.h
 LIB_H += progress.h
 LIB_H += prompt.h
 LIB_H += quote.h
@@ -846,6 +848,7 @@ LIB_OBJS += pathspec.o
 LIB_OBJS += pkt-line.o
 LIB_OBJS += preload-index.o
 LIB_OBJS += pretty.o
+LIB_OBJS += prio-queue.o
 LIB_OBJS += progress.o
 LIB_OBJS += prompt.o
 LIB_OBJS += quote.o
diff --git a/builtin/log.c b/builtin/log.c
index 9e2123295f..e3222ed9f9 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -237,7 +237,7 @@ 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)) {
diff --git a/builtin/show-branch.c b/builtin/show-branch.c
index 90fc6b1b9d..99ec4af224 100644
--- a/builtin/show-branch.c
+++ b/builtin/show-branch.c
@@ -630,7 +630,7 @@ 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;
@@ -665,15 +665,17 @@ 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"),
@@ -900,7 +902,7 @@ 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)
diff --git a/commit.c b/commit.c
index a1096a2b5a..521e49c309 100644
--- a/commit.c
+++ b/commit.c
@@ -9,6 +9,7 @@
 #include "gpg-interface.h"
 #include "mergesort.h"
 #include "commit-slab.h"
+#include "prio-queue.h"
 
 static struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
 
@@ -518,31 +519,124 @@ struct commit *pop_commit(struct commit_list **stack)
 /* count number of children that have not been emitted */
 define_commit_slab(indegree_slab, int);
 
+/* record author-date for each commit object */
+define_commit_slab(author_date_slab, unsigned long);
+
+static void record_author_date(struct author_date_slab *author_date,
+			       struct commit *commit)
+{
+	const char *buf, *line_end;
+	char *buffer = NULL;
+	struct ident_split ident;
+	char *date_end;
+	unsigned long date;
+
+	if (!commit->buffer) {
+		unsigned long size;
+		enum object_type type;
+		buffer = read_sha1_file(commit->object.sha1, &type, &size);
+		if (!buffer)
+			return;
+	}
+
+	for (buf = commit->buffer ? commit->buffer : buffer;
+	     buf;
+	     buf = line_end + 1) {
+		line_end = strchrnul(buf, '\n');
+		if (prefixcmp(buf, "author ")) {
+			if (!line_end[0] || line_end[1] == '\n')
+				return; /* end of header */
+			continue;
+		}
+		if (split_ident_line(&ident,
+				     buf + strlen("author "),
+				     line_end - (buf + strlen("author "))) ||
+		    !ident.date_begin || !ident.date_end)
+			goto fail_exit; /* malformed "author" line */
+		break;
+	}
+
+	date = strtoul(ident.date_begin, &date_end, 10);
+	if (date_end != ident.date_end)
+		goto fail_exit; /* malformed date */
+	*(author_date_slab_at(author_date, commit)) = date;
+
+fail_exit:
+	free(buffer);
+}
+
+static int compare_commits_by_author_date(const void *a_, const void *b_,
+					  void *cb_data)
+{
+	const struct commit *a = a_, *b = b_;
+	struct author_date_slab *author_date = cb_data;
+	unsigned long a_date = *(author_date_slab_at(author_date, a));
+	unsigned long b_date = *(author_date_slab_at(author_date, b));
+
+	/* newer commits with larger date first */
+	if (a_date < b_date)
+		return 1;
+	else if (a_date > b_date)
+		return -1;
+	return 0;
+}
+
+static int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused)
+{
+	const struct commit *a = a_, *b = b_;
+	/* newer commits with larger date first */
+	if (a->date < b->date)
+		return 1;
+	else if (a->date > b->date)
+		return -1;
+	return 0;
+}
+
 /*
  * 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;
 	struct commit_list **pptr;
 	struct indegree_slab indegree;
+	struct prio_queue queue;
+	struct commit *commit;
+	struct author_date_slab author_date;
 
 	if (!orig)
 		return;
 	*list = NULL;
 
 	init_indegree_slab(&indegree);
+	memset(&queue, '\0', sizeof(queue));
+
+	switch (sort_order) {
+	default: /* REV_SORT_IN_GRAPH_ORDER */
+		queue.compare = NULL;
+		break;
+	case REV_SORT_BY_COMMIT_DATE:
+		queue.compare = compare_commits_by_commit_date;
+		break;
+	case REV_SORT_BY_AUTHOR_DATE:
+		init_author_date_slab(&author_date);
+		queue.compare = compare_commits_by_author_date;
+		queue.cb_data = &author_date;
+		break;
+	}
 
 	/* Mark them and clear the indegree */
 	for (next = orig; next; next = next->next) {
 		struct commit *commit = next->item;
 		*(indegree_slab_at(&indegree, commit)) = 1;
+		/* also record the author dates, if needed */
+		if (sort_order == REV_SORT_BY_AUTHOR_DATE)
+			record_author_date(&author_date, commit);
 	}
 
 	/* update the indegree */
 	for (next = orig; next; next = next->next) {
-		struct commit_list * parents = next->item->parents;
+		struct commit_list *parents = next->item->parents;
 		while (parents) {
 			struct commit *parent = parents->item;
 			int *pi = indegree_slab_at(&indegree, parent);
@@ -560,30 +654,28 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 	 *
 	 * the tips serve as a starting set for the work queue.
 	 */
-	work = NULL;
-	insert = &work;
 	for (next = orig; next; next = next->next) {
 		struct commit *commit = next->item;
 
 		if (*(indegree_slab_at(&indegree, commit)) == 1)
-			insert = &commit_list_insert(commit, insert)->next;
+			prio_queue_put(&queue, commit);
 	}
 
-	/* process the list in topological order */
-	if (!lifo)
-		commit_list_sort_by_date(&work);
+	/*
+	 * This is unfortunate; the initial tips need to be shown
+	 * in the order given from the revision traversal machinery.
+	 */
+	if (sort_order == REV_SORT_IN_GRAPH_ORDER)
+		prio_queue_reverse(&queue);
+
+	/* We no longer need the commit list */
+	free_commit_list(orig);
 
 	pptr = list;
 	*list = NULL;
-	while (work) {
-		struct commit *commit;
-		struct commit_list *parents, *work_item;
+	while ((commit = prio_queue_get(&queue)) != NULL) {
+		struct commit_list *parents;
 
-		work_item = work;
-		work = work_item->next;
-		work_item->next = NULL;
-
-		commit = work_item->item;
 		for (parents = commit->parents; parents ; parents = parents->next) {
 			struct commit *parent = parents->item;
 			int *pi = indegree_slab_at(&indegree, parent);
@@ -596,23 +688,22 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 			 * when all their children have been emitted thereby
 			 * guaranteeing topological order.
 			 */
-			if (--(*pi) == 1) {
-				if (!lifo)
-					commit_list_insert_by_date(parent, &work);
-				else
-					commit_list_insert(parent, &work);
-			}
+			if (--(*pi) == 1)
+				prio_queue_put(&queue, parent);
 		}
 		/*
-		 * work_item is a commit all of whose children
-		 * have already been emitted. we can emit it now.
+		 * all children of commit have already been
+		 * emitted. we can emit it now.
 		 */
 		*(indegree_slab_at(&indegree, commit)) = 0;
-		*pptr = work_item;
-		pptr = &work_item->next;
+
+		pptr = &commit_list_insert(commit, pptr)->next;
 	}
 
 	clear_indegree_slab(&indegree);
+	clear_prio_queue(&queue);
+	if (sort_order == REV_SORT_BY_AUTHOR_DATE)
+		clear_author_date_slab(&author_date);
 }
 
 /* merge-base stuff */
diff --git a/commit.h b/commit.h
index 350472114b..4d452dc96d 100644
--- a/commit.h
+++ b/commit.h
@@ -142,15 +142,24 @@ void clear_commit_marks(struct commit *commit, unsigned int mark);
 void clear_commit_marks_many(int nr, 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,
+	REV_SORT_BY_AUTHOR_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];
diff --git a/prio-queue.c b/prio-queue.c
new file mode 100644
index 0000000000..c9f8c6d253
--- /dev/null
+++ b/prio-queue.c
@@ -0,0 +1,84 @@
+#include "cache.h"
+#include "commit.h"
+#include "prio-queue.h"
+
+void prio_queue_reverse(struct prio_queue *queue)
+{
+	int i, j;
+
+	if (queue->compare != NULL)
+		die("BUG: prio_queue_reverse() on non-LIFO queue");
+	for (i = 0; i <= (j = (queue->nr - 1) - i); i++) {
+		struct commit *swap = queue->array[i];
+		queue->array[i] = queue->array[j];
+		queue->array[j] = swap;
+	}
+}
+
+void clear_prio_queue(struct prio_queue *queue)
+{
+	free(queue->array);
+	queue->nr = 0;
+	queue->alloc = 0;
+	queue->array = NULL;
+}
+
+void prio_queue_put(struct prio_queue *queue, void *thing)
+{
+	prio_queue_compare_fn compare = queue->compare;
+	int ix, parent;
+
+	/* Append at the end */
+	ALLOC_GROW(queue->array, queue->nr + 1, queue->alloc);
+	queue->array[queue->nr++] = thing;
+	if (!compare)
+		return; /* LIFO */
+
+	/* Bubble up the new one */
+	for (ix = queue->nr - 1; ix; ix = parent) {
+		parent = (ix - 1) / 2;
+		if (compare(queue->array[parent], queue->array[ix],
+			    queue->cb_data) <= 0)
+			break;
+
+		thing = queue->array[parent];
+		queue->array[parent] = queue->array[ix];
+		queue->array[ix] = thing;
+	}
+}
+
+void *prio_queue_get(struct prio_queue *queue)
+{
+	void *result, *swap;
+	int ix, child;
+	prio_queue_compare_fn compare = queue->compare;
+
+	if (!queue->nr)
+		return NULL;
+	if (!compare)
+		return queue->array[--queue->nr]; /* LIFO */
+
+	result = queue->array[0];
+	if (!--queue->nr)
+		return result;
+
+	queue->array[0] = queue->array[queue->nr];
+
+	/* Push down the one at the root */
+	for (ix = 0; ix * 2 + 1 < queue->nr; ix = child) {
+		child = ix * 2 + 1; /* left */
+		if ((child + 1 < queue->nr) &&
+		    (compare(queue->array[child], queue->array[child + 1],
+			     queue->cb_data) >= 0))
+			child++; /* use right child */
+
+		if (compare(queue->array[ix], queue->array[child],
+			    queue->cb_data) <= 0)
+			break;
+
+		swap = queue->array[child];
+		queue->array[child] = queue->array[ix];
+		queue->array[ix] = swap;
+	}
+	return result;
+}
diff --git a/prio-queue.h b/prio-queue.h
new file mode 100644
index 0000000000..9c3cd1f875
--- /dev/null
+++ b/prio-queue.h
@@ -0,0 +1,48 @@
+#ifndef PRIO_QUEUE_H
+#define PRIO_QUEUE_H
+
+/*
+ * A priority queue implementation, primarily for keeping track of
+ * commits in the 'date-order' so that we process them from new to old
+ * as they are discovered, but can be used to hold any pointer to
+ * struct.  The caller is responsible for supplying a function to
+ * compare two "things".
+ *
+ * Alternatively, this data structure can also be used as a LIFO stack
+ * by specifying NULL as the comparison function.
+ */
+
+/*
+ * Compare two "things", one and two; the third parameter is cb_data
+ * in the prio_queue structure.  The result is returned as a sign of
+ * the return value, being the same as the sign of the result of
+ * subtracting "two" from "one" (i.e. negative if "one" sorts earlier
+ * than "two").
+ */
+typedef int (*prio_queue_compare_fn)(const void *one, const void *two, void *cb_data);
+
+struct prio_queue {
+	prio_queue_compare_fn compare;
+	void *cb_data;
+	int alloc, nr;
+	void **array;
+};
+
+/*
+ * Add the "thing" to the queue.
+ */
+extern void prio_queue_put(struct prio_queue *, void *thing);
+
+/*
+ * Extract the "thing" that compares the smallest out of the queue,
+ * or NULL.  If compare function is NULL, the queue acts as a LIFO
+ * stack.
+ */
+extern void *prio_queue_get(struct prio_queue *);
+
+extern void clear_prio_queue(struct prio_queue *);
+
+/* Reverse the LIFO elements */
+extern void prio_queue_reverse(struct prio_queue *);
+
+#endif /* PRIO_QUEUE_H */
diff --git a/revision.c b/revision.c
index f1bb731fd7..2f0142f22e 100644
--- a/revision.c
+++ b/revision.c
@@ -1296,7 +1296,7 @@ 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;
@@ -1638,7 +1638,7 @@ 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;
@@ -1656,7 +1656,10 @@ 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 (!strcmp(arg, "--author-date-order")) {
+		revs->sort_order = REV_SORT_BY_AUTHOR_DATE;
 		revs->topo_order = 1;
 	} else if (!prefixcmp(arg, "--early-output")) {
 		int count = 100;
@@ -2606,7 +2609,7 @@ 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->line_level_traverse)
 		line_log_filter(revs);
 	if (revs->simplify_merges)
@@ -2924,7 +2927,7 @@ 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)
diff --git a/revision.h b/revision.h
index eeea6fba3c..92d6614af6 100644
--- a/revision.h
+++ b/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)
@@ -62,6 +63,10 @@ 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;
 
@@ -72,7 +77,6 @@ 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,
diff --git a/t/lib-t6000.sh b/t/lib-t6000.sh
index ea25dd89e5..4ffd90127e 100644
--- a/t/lib-t6000.sh
+++ b/t/lib-t6000.sh
@@ -1,55 +1,50 @@
 : included from 6002 and others
 
-[ -d .git/refs/tags ] || mkdir -p .git/refs/tags
+mkdir -p .git/refs/tags
 
-:> sed.script
+>sed.script
 
-# Answer the sha1 has associated with the tag. The tag must exist in .git or .git/refs/tags
-tag()
-{
+# Answer the sha1 has associated with the tag. The tag must exist in .git/refs/tags
+tag () {
 	_tag=$1
-	[ -f .git/refs/tags/$_tag ] || error "tag: \"$_tag\" does not exist"
-	cat .git/refs/tags/$_tag
+	test -f ".git/refs/tags/$_tag" || error "tag: \"$_tag\" does not exist"
+	cat ".git/refs/tags/$_tag"
 }
 
 # Generate a commit using the text specified to make it unique and the tree
 # named by the tag specified.
-unique_commit()
-{
+unique_commit () {
 	_text=$1
-        _tree=$2
+	_tree=$2
 	shift 2
-	echo $_text | git commit-tree $(tag $_tree) "$@"
+	echo "$_text" | git commit-tree $(tag "$_tree") "$@"
 }
 
 # Save the output of a command into the tag specified. Prepend
 # a substitution script for the tag onto the front of sed.script
-save_tag()
-{
+save_tag () {
 	_tag=$1
-	[ -n "$_tag" ] || error "usage: save_tag tag commit-args ..."
+	test -n "$_tag" || error "usage: save_tag tag commit-args ..."
 	shift 1
-	"$@" >.git/refs/tags/$_tag
+	"$@" >".git/refs/tags/$_tag"
 
-        echo "s/$(tag $_tag)/$_tag/g" > sed.script.tmp
-	cat sed.script >> sed.script.tmp
+	echo "s/$(tag $_tag)/$_tag/g" >sed.script.tmp
+	cat sed.script >>sed.script.tmp
 	rm sed.script
 	mv sed.script.tmp sed.script
 }
 
 # Replace unhelpful sha1 hashses with their symbolic equivalents
-entag()
-{
+entag () {
 	sed -f sed.script
 }
 
 # Execute a command after first saving, then setting the GIT_AUTHOR_EMAIL
 # tag to a specified value. Restore the original value on return.
-as_author()
-{
+as_author () {
 	_author=$1
 	shift 1
-        _save=$GIT_AUTHOR_EMAIL
+	_save=$GIT_AUTHOR_EMAIL
 
 	GIT_AUTHOR_EMAIL="$_author"
 	export GIT_AUTHOR_EMAIL
@@ -63,45 +58,58 @@ as_author()
 	fi
 }
 
-commit_date()
-{
-        _commit=$1
-	git cat-file commit $_commit | sed -n "s/^committer .*> \([0-9]*\) .*/\1/p"
+commit_date () {
+	_commit=$1
+	git cat-file commit $_commit |
+	sed -n "s/^committer .*> \([0-9]*\) .*/\1/p"
 }
 
-on_committer_date()
-{
-    _date=$1
-    shift 1
-    GIT_COMMITTER_DATE="$_date"
-    export GIT_COMMITTER_DATE
-    "$@"
-    unset GIT_COMMITTER_DATE
+# Assign the value of fake date to a variable, but
+# allow fairly common "1971-08-16 00:00" to be omittd
+assign_fake_date () {
+	case "$2" in
+	??:??:??)	eval "$1='1971-08-16 $2'" ;;
+	??:??)		eval "$1='1971-08-16 00:$2'" ;;
+	??)		eval "$1='1971-08-16 00:00:$2'" ;;
+	*)		eval "$1='$2'" ;;
+	esac
+}
+
+on_committer_date () {
+	assign_fake_date GIT_COMMITTER_DATE "$1"
+	export GIT_COMMITTER_DATE
+	shift 1
+	"$@"
+}
+
+on_dates () {
+	assign_fake_date GIT_COMMITTER_DATE "$1"
+	assign_fake_date GIT_AUTHOR_DATE "$2"
+	export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
+	shift 2
+	"$@"
 }
 
 # Execute a command and suppress any error output.
-hide_error()
-{
+hide_error () {
 	"$@" 2>/dev/null
 }
 
-check_output()
-{
+check_output () {
 	_name=$1
 	shift 1
-	if eval "$*" | entag > $_name.actual
+	if eval "$*" | entag >"$_name.actual"
 	then
-		test_cmp $_name.expected $_name.actual
+		test_cmp "$_name.expected" "$_name.actual"
 	else
-		return 1;
+		return 1
 	fi
 }
 
 # Turn a reasonable test description into a reasonable test name.
 # All alphanums translated into -'s which are then compressed and stripped
 # from front and back.
-name_from_description()
-{
+name_from_description () {
 	perl -pe '
 		s/[^A-Za-z0-9.]/-/g;
 		s/-+/-/g;
@@ -119,9 +127,11 @@ name_from_description()
 test_output_expect_success()
 {
 	_description=$1
-        _test=$2
-        [ $# -eq 2 ] || error "usage: test_output_expect_success description test <<EOF ... EOF"
-        _name=$(echo $_description | name_from_description)
-	cat > $_name.expected
+	_test=$2
+	test $# -eq 2 ||
+	error "usage: test_output_expect_success description test <<EOF ... EOF"
+
+	_name=$(echo $_description | name_from_description)
+	cat >"$_name.expected"
 	test_expect_success "$_description" "check_output $_name \"$_test\""
 }
diff --git a/t/t0009-prio-queue.sh b/t/t0009-prio-queue.sh
new file mode 100755
index 0000000000..94045c3fad
--- /dev/null
+++ b/t/t0009-prio-queue.sh
@@ -0,0 +1,50 @@
+#!/bin/sh
+
+test_description='basic tests for priority queue implementation'
+. ./test-lib.sh
+
+cat >expect <<'EOF'
+1
+2
+3
+4
+5
+5
+6
+7
+8
+9
+10
+EOF
+test_expect_success 'basic ordering' '
+	test-prio-queue 2 6 3 10 9 5 7 4 5 8 1 dump >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<'EOF'
+2
+3
+4
+1
+5
+6
+EOF
+test_expect_success 'mixed put and get' '
+	test-prio-queue 6 2 4 get 5 3 get get 1 dump >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<'EOF'
+1
+2
+NULL
+1
+2
+NULL
+EOF
+test_expect_success 'notice empty queue' '
+	test-prio-queue 1 2 get get get 1 2 get get get >actual &&
+	test_cmp expect actual
+'
+
+test_done
diff --git a/t/t6002-rev-list-bisect.sh b/t/t6002-rev-list-bisect.sh
index fb07536a0f..43ad772484 100755
--- a/t/t6002-rev-list-bisect.sh
+++ b/t/t6002-rev-list-bisect.sh
@@ -39,25 +39,25 @@ test_bisection_diff()
 date >path0
 git update-index --add path0
 save_tag tree git write-tree
-on_committer_date "1971-08-16 00:00:00" hide_error save_tag root unique_commit root tree
-on_committer_date "1971-08-16 00:00:01" save_tag l0 unique_commit l0 tree -p root
-on_committer_date "1971-08-16 00:00:02" save_tag l1 unique_commit l1 tree -p l0
-on_committer_date "1971-08-16 00:00:03" save_tag l2 unique_commit l2 tree -p l1
-on_committer_date "1971-08-16 00:00:04" save_tag a0 unique_commit a0 tree -p l2
-on_committer_date "1971-08-16 00:00:05" save_tag a1 unique_commit a1 tree -p a0
-on_committer_date "1971-08-16 00:00:06" save_tag b1 unique_commit b1 tree -p a0
-on_committer_date "1971-08-16 00:00:07" save_tag c1 unique_commit c1 tree -p b1
-on_committer_date "1971-08-16 00:00:08" save_tag b2 unique_commit b2 tree -p b1
-on_committer_date "1971-08-16 00:00:09" save_tag b3 unique_commit b2 tree -p b2
-on_committer_date "1971-08-16 00:00:10" save_tag c2 unique_commit c2 tree -p c1 -p b2
-on_committer_date "1971-08-16 00:00:11" save_tag c3 unique_commit c3 tree -p c2
-on_committer_date "1971-08-16 00:00:12" save_tag a2 unique_commit a2 tree -p a1
-on_committer_date "1971-08-16 00:00:13" save_tag a3 unique_commit a3 tree -p a2
-on_committer_date "1971-08-16 00:00:14" save_tag b4 unique_commit b4 tree -p b3 -p a3
-on_committer_date "1971-08-16 00:00:15" save_tag a4 unique_commit a4 tree -p a3 -p b4 -p c3
-on_committer_date "1971-08-16 00:00:16" save_tag l3 unique_commit l3 tree -p a4
-on_committer_date "1971-08-16 00:00:17" save_tag l4 unique_commit l4 tree -p l3
-on_committer_date "1971-08-16 00:00:18" save_tag l5 unique_commit l5 tree -p l4
+on_committer_date "00:00" hide_error save_tag root unique_commit root tree
+on_committer_date "00:01" save_tag l0 unique_commit l0 tree -p root
+on_committer_date "00:02" save_tag l1 unique_commit l1 tree -p l0
+on_committer_date "00:03" save_tag l2 unique_commit l2 tree -p l1
+on_committer_date "00:04" save_tag a0 unique_commit a0 tree -p l2
+on_committer_date "00:05" save_tag a1 unique_commit a1 tree -p a0
+on_committer_date "00:06" save_tag b1 unique_commit b1 tree -p a0
+on_committer_date "00:07" save_tag c1 unique_commit c1 tree -p b1
+on_committer_date "00:08" save_tag b2 unique_commit b2 tree -p b1
+on_committer_date "00:09" save_tag b3 unique_commit b2 tree -p b2
+on_committer_date "00:10" save_tag c2 unique_commit c2 tree -p c1 -p b2
+on_committer_date "00:11" save_tag c3 unique_commit c3 tree -p c2
+on_committer_date "00:12" save_tag a2 unique_commit a2 tree -p a1
+on_committer_date "00:13" save_tag a3 unique_commit a3 tree -p a2
+on_committer_date "00:14" save_tag b4 unique_commit b4 tree -p b3 -p a3
+on_committer_date "00:15" save_tag a4 unique_commit a4 tree -p a3 -p b4 -p c3
+on_committer_date "00:16" save_tag l3 unique_commit l3 tree -p a4
+on_committer_date "00:17" save_tag l4 unique_commit l4 tree -p l3
+on_committer_date "00:18" save_tag l5 unique_commit l5 tree -p l4
 git update-ref HEAD $(tag l5)
 
 
@@ -90,29 +90,29 @@ git update-ref HEAD $(tag l5)
 #     F
 
 
-on_committer_date "1971-08-16 00:00:00" hide_error save_tag F unique_commit F tree
-on_committer_date "1971-08-16 00:00:01" save_tag e8 unique_commit e8 tree -p F
-on_committer_date "1971-08-16 00:00:02" save_tag e7 unique_commit e7 tree -p e8
-on_committer_date "1971-08-16 00:00:03" save_tag e6 unique_commit e6 tree -p e7
-on_committer_date "1971-08-16 00:00:04" save_tag e5 unique_commit e5 tree -p e6
-on_committer_date "1971-08-16 00:00:05" save_tag f4 unique_commit f4 tree -p F
-on_committer_date "1971-08-16 00:00:06" save_tag f3 unique_commit f3 tree -p f4
-on_committer_date "1971-08-16 00:00:07" save_tag f2 unique_commit f2 tree -p f3
-on_committer_date "1971-08-16 00:00:08" save_tag f1 unique_commit f1 tree -p f2
-on_committer_date "1971-08-16 00:00:09" save_tag e4 unique_commit e4 tree -p e5
-on_committer_date "1971-08-16 00:00:10" save_tag e3 unique_commit e3 tree -p e4
-on_committer_date "1971-08-16 00:00:11" save_tag e2 unique_commit e2 tree -p e3
-on_committer_date "1971-08-16 00:00:12" save_tag e1 unique_commit e1 tree -p e2
-on_committer_date "1971-08-16 00:00:13" save_tag E unique_commit E tree -p e1 -p f1
+on_committer_date "00:00" hide_error save_tag F unique_commit F tree
+on_committer_date "00:01" save_tag e8 unique_commit e8 tree -p F
+on_committer_date "00:02" save_tag e7 unique_commit e7 tree -p e8
+on_committer_date "00:03" save_tag e6 unique_commit e6 tree -p e7
+on_committer_date "00:04" save_tag e5 unique_commit e5 tree -p e6
+on_committer_date "00:05" save_tag f4 unique_commit f4 tree -p F
+on_committer_date "00:06" save_tag f3 unique_commit f3 tree -p f4
+on_committer_date "00:07" save_tag f2 unique_commit f2 tree -p f3
+on_committer_date "00:08" save_tag f1 unique_commit f1 tree -p f2
+on_committer_date "00:09" save_tag e4 unique_commit e4 tree -p e5
+on_committer_date "00:10" save_tag e3 unique_commit e3 tree -p e4
+on_committer_date "00:11" save_tag e2 unique_commit e2 tree -p e3
+on_committer_date "00:12" save_tag e1 unique_commit e1 tree -p e2
+on_committer_date "00:13" save_tag E unique_commit E tree -p e1 -p f1
 
-on_committer_date "1971-08-16 00:00:00" hide_error save_tag U unique_commit U tree
-on_committer_date "1971-08-16 00:00:01" save_tag u0 unique_commit u0 tree -p U
-on_committer_date "1971-08-16 00:00:01" save_tag u1 unique_commit u1 tree -p u0
-on_committer_date "1971-08-16 00:00:02" save_tag u2 unique_commit u2 tree -p u0
-on_committer_date "1971-08-16 00:00:03" save_tag u3 unique_commit u3 tree -p u0
-on_committer_date "1971-08-16 00:00:04" save_tag u4 unique_commit u4 tree -p u0
-on_committer_date "1971-08-16 00:00:05" save_tag u5 unique_commit u5 tree -p u0
-on_committer_date "1971-08-16 00:00:06" save_tag V unique_commit V tree -p u1 -p u2 -p u3 -p u4 -p u5
+on_committer_date "00:00" hide_error save_tag U unique_commit U tree
+on_committer_date "00:01" save_tag u0 unique_commit u0 tree -p U
+on_committer_date "00:01" save_tag u1 unique_commit u1 tree -p u0
+on_committer_date "00:02" save_tag u2 unique_commit u2 tree -p u0
+on_committer_date "00:03" save_tag u3 unique_commit u3 tree -p u0
+on_committer_date "00:04" save_tag u4 unique_commit u4 tree -p u0
+on_committer_date "00:05" save_tag u5 unique_commit u5 tree -p u0
+on_committer_date "00:06" save_tag V unique_commit V tree -p u1 -p u2 -p u3 -p u4 -p u5
 
 test_sequence()
 {
diff --git a/t/t6003-rev-list-topo-order.sh b/t/t6003-rev-list-topo-order.sh
index e4c52b0214..24d1836f41 100755
--- a/t/t6003-rev-list-topo-order.sh
+++ b/t/t6003-rev-list-topo-order.sh
@@ -16,39 +16,34 @@ list_duplicates()
 date >path0
 git update-index --add path0
 save_tag tree git write-tree
-on_committer_date "1971-08-16 00:00:00" hide_error save_tag root unique_commit root tree
-on_committer_date "1971-08-16 00:00:01" save_tag l0 unique_commit l0 tree -p root
-on_committer_date "1971-08-16 00:00:02" save_tag l1 unique_commit l1 tree -p l0
-on_committer_date "1971-08-16 00:00:03" save_tag l2 unique_commit l2 tree -p l1
-on_committer_date "1971-08-16 00:00:04" save_tag a0 unique_commit a0 tree -p l2
-on_committer_date "1971-08-16 00:00:05" save_tag a1 unique_commit a1 tree -p a0
-on_committer_date "1971-08-16 00:00:06" save_tag b1 unique_commit b1 tree -p a0
-on_committer_date "1971-08-16 00:00:07" save_tag c1 unique_commit c1 tree -p b1
-on_committer_date "1971-08-16 00:00:08" as_author foobar@example.com save_tag b2 unique_commit b2 tree -p b1
-on_committer_date "1971-08-16 00:00:09" save_tag b3 unique_commit b3 tree -p b2
-on_committer_date "1971-08-16 00:00:10" save_tag c2 unique_commit c2 tree -p c1 -p b2
-on_committer_date "1971-08-16 00:00:11" save_tag c3 unique_commit c3 tree -p c2
-on_committer_date "1971-08-16 00:00:12" save_tag a2 unique_commit a2 tree -p a1
-on_committer_date "1971-08-16 00:00:13" save_tag a3 unique_commit a3 tree -p a2
-on_committer_date "1971-08-16 00:00:14" save_tag b4 unique_commit b4 tree -p b3 -p a3
-on_committer_date "1971-08-16 00:00:15" save_tag a4 unique_commit a4 tree -p a3 -p b4 -p c3
-on_committer_date "1971-08-16 00:00:16" save_tag l3 unique_commit l3 tree -p a4
-on_committer_date "1971-08-16 00:00:17" save_tag l4 unique_commit l4 tree -p l3
-on_committer_date "1971-08-16 00:00:18" save_tag l5 unique_commit l5 tree -p l4
-on_committer_date "1971-08-16 00:00:19" save_tag m1 unique_commit m1 tree -p a4 -p c3
-on_committer_date "1971-08-16 00:00:20" save_tag m2 unique_commit m2 tree -p c3 -p a4
-on_committer_date "1971-08-16 00:00:21" hide_error save_tag alt_root unique_commit alt_root tree
-on_committer_date "1971-08-16 00:00:22" save_tag r0 unique_commit r0 tree -p alt_root
-on_committer_date "1971-08-16 00:00:23" save_tag r1 unique_commit r1 tree -p r0
-on_committer_date "1971-08-16 00:00:24" save_tag l5r1 unique_commit l5r1 tree -p l5 -p r1
-on_committer_date "1971-08-16 00:00:25" save_tag r1l5 unique_commit r1l5 tree -p r1 -p l5
+on_dates "00:00" "00:00" hide_error save_tag root unique_commit root tree
+on_dates "00:01" "00:01" save_tag l0 unique_commit l0 tree -p root
+on_dates "00:02" "00:02" save_tag l1 unique_commit l1 tree -p l0
+on_dates "00:03" "00:03" save_tag l2 unique_commit l2 tree -p l1
+on_dates "00:04" "00:04" save_tag a0 unique_commit a0 tree -p l2
+on_dates "00:05" "00:05" save_tag a1 unique_commit a1 tree -p a0
+on_dates "00:06" "00:06" save_tag b1 unique_commit b1 tree -p a0
+on_dates "00:07" "00:07" save_tag c1 unique_commit c1 tree -p b1
+on_dates "00:08" "00:08" as_author foobar@example.com save_tag b2 unique_commit b2 tree -p b1
+on_dates "00:09" "00:09" save_tag b3 unique_commit b3 tree -p b2
+on_dates "00:10" "00:10" save_tag c2 unique_commit c2 tree -p c1 -p b2
+on_dates "00:11" "00:11" save_tag c3 unique_commit c3 tree -p c2
+on_dates "00:12" "00:00" save_tag a2 unique_commit a2 tree -p a1
+on_dates "00:13" "00:01" save_tag a3 unique_commit a3 tree -p a2
+on_dates "00:14" "00:14" save_tag b4 unique_commit b4 tree -p b3 -p a3
+on_dates "00:15" "00:15" save_tag a4 unique_commit a4 tree -p a3 -p b4 -p c3
+on_dates "00:16" "00:16" save_tag l3 unique_commit l3 tree -p a4
+on_dates "00:17" "00:17" save_tag l4 unique_commit l4 tree -p l3
+on_dates "00:18" "00:18" save_tag l5 unique_commit l5 tree -p l4
+on_dates "00:19" "00:19" save_tag m1 unique_commit m1 tree -p a4 -p c3
+on_dates "00:20" "00:20" save_tag m2 unique_commit m2 tree -p c3 -p a4
+on_dates "00:21" "00:21" hide_error save_tag alt_root unique_commit alt_root tree
+on_dates "00:22" "00:22" save_tag r0 unique_commit r0 tree -p alt_root
+on_dates "00:23" "00:23" save_tag r1 unique_commit r1 tree -p r0
+on_dates "00:24" "00:24" save_tag l5r1 unique_commit l5r1 tree -p l5 -p r1
+on_dates "00:25" "00:25" save_tag r1l5 unique_commit r1l5 tree -p r1 -p l5
 
 
-#
-# note: as of 20/6, it isn't possible to create duplicate parents, so this
-# can't be tested.
-#
-#on_committer_date "1971-08-16 00:00:20" save_tag m3 unique_commit m3 tree -p c3 -p a4 -p c3
 hide_error save_tag e1 as_author e@example.com unique_commit e1 tree
 save_tag e2 as_author e@example.com unique_commit e2 tree -p e1
 save_tag f1 as_author f@example.com unique_commit f1 tree -p e1
@@ -105,6 +100,50 @@ l0
 root
 EOF
 
+test_output_expect_success 'simple date order' 'git rev-list --date-order  HEAD' <<EOF
+l5
+l4
+l3
+a4
+b4
+a3
+a2
+c3
+c2
+b3
+b2
+c1
+b1
+a1
+a0
+l2
+l1
+l0
+root
+EOF
+
+test_output_expect_success 'simple author-date order' 'git rev-list --author-date-order  HEAD' <<EOF
+l5
+l4
+l3
+a4
+b4
+c3
+c2
+b3
+b2
+c1
+b1
+a3
+a2
+a1
+a0
+l2
+l1
+l0
+root
+EOF
+
 test_output_expect_success 'two diamonds topo order (g6)' 'git rev-list --topo-order  g4' <<EOF
 g4
 h2
diff --git a/test-prio-queue.c b/test-prio-queue.c
new file mode 100644
index 0000000000..7be72f0086
--- /dev/null
+++ b/test-prio-queue.c
@@ -0,0 +1,39 @@
+#include "cache.h"
+#include "prio-queue.h"
+
+static int intcmp(const void *va, const void *vb, void *data)
+{
+	const int *a = va, *b = vb;
+	return *a - *b;
+}
+
+static void show(int *v)
+{
+	if (!v)
+		printf("NULL\n");
+	else
+		printf("%d\n", *v);
+	free(v);
+}
+
+int main(int argc, char **argv)
+{
+	struct prio_queue pq = { intcmp };
+
+	while (*++argv) {
+		if (!strcmp(*argv, "get"))
+			show(prio_queue_get(&pq));
+		else if (!strcmp(*argv, "dump")) {
+			int *v;
+			while ((v = prio_queue_get(&pq)))
+			       show(v);
+		}
+		else {
+			int *v = malloc(sizeof(*v));
+			*v = atoi(*argv);
+			prio_queue_put(&pq, v);
+		}
+	}
+
+	return 0;
+}