pack-objects: refactor code into compute_layer_order()

In a following commit, as we will use delta islands, we will
have to compute the write order for different layers, not just
for one.

Let's prepare for that by refactoring the code that will be
used to compute the write order for a given layer into a new
compute_layer_order() function.

This will make it easier to see and understand what the
following changes are doing.

Helped-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -667,48 +667,15 @@ static void add_family_to_write_order(struct object_entry **wo,
 	add_descendants_to_write_order(wo, endp, root);
 }
 
-static struct object_entry **compute_write_order(void)
+static void compute_layer_order(struct object_entry **wo, unsigned int *wo_end)
 {
-	unsigned int i, wo_end, last_untagged;
-
-	struct object_entry **wo;
+	unsigned int i, last_untagged;
 	struct object_entry *objects = to_pack.objects;
 
 	for (i = 0; i < to_pack.nr_objects; i++) {
-		objects[i].tagged = 0;
-		objects[i].filled = 0;
-		SET_DELTA_CHILD(&objects[i], NULL);
-		SET_DELTA_SIBLING(&objects[i], NULL);
-	}
-
-	/*
-	 * Fully connect delta_child/delta_sibling network.
-	 * Make sure delta_sibling is sorted in the original
-	 * recency order.
-	 */
-	for (i = to_pack.nr_objects; i > 0;) {
-		struct object_entry *e = &objects[--i];
-		if (!DELTA(e))
-			continue;
-		/* Mark me as the first child */
-		e->delta_sibling_idx = DELTA(e)->delta_child_idx;
-		SET_DELTA_CHILD(DELTA(e), e);
-	}
-
-	/*
-	 * Mark objects that are at the tip of tags.
-	 */
-	for_each_tag_ref(mark_tagged, NULL);
-
-	/*
-	 * Give the objects in the original recency order until
-	 * we see a tagged tip.
-	 */
-	ALLOC_ARRAY(wo, to_pack.nr_objects);
-	for (i = wo_end = 0; i < to_pack.nr_objects; i++) {
 		if (objects[i].tagged)
 			break;
-		add_to_write_order(wo, &wo_end, &objects[i]);
+		add_to_write_order(wo, wo_end, &objects[i]);
 	}
 	last_untagged = i;
 
@@ -717,7 +684,7 @@ static struct object_entry **compute_write_order(void)
 	 */
 	for (; i < to_pack.nr_objects; i++) {
 		if (objects[i].tagged)
-			add_to_write_order(wo, &wo_end, &objects[i]);
+			add_to_write_order(wo, wo_end, &objects[i]);
 	}
 
 	/*
@@ -727,7 +694,7 @@ static struct object_entry **compute_write_order(void)
 		if (oe_type(&objects[i]) != OBJ_COMMIT &&
 		    oe_type(&objects[i]) != OBJ_TAG)
 			continue;
-		add_to_write_order(wo, &wo_end, &objects[i]);
+		add_to_write_order(wo, wo_end, &objects[i]);
 	}
 
 	/*
@@ -736,7 +703,7 @@ static struct object_entry **compute_write_order(void)
 	for (i = last_untagged; i < to_pack.nr_objects; i++) {
 		if (oe_type(&objects[i]) != OBJ_TREE)
 			continue;
-		add_to_write_order(wo, &wo_end, &objects[i]);
+		add_to_write_order(wo, wo_end, &objects[i]);
 	}
 
 	/*
@@ -744,8 +711,51 @@ static struct object_entry **compute_write_order(void)
 	 */
 	for (i = last_untagged; i < to_pack.nr_objects; i++) {
 		if (!objects[i].filled)
-			add_family_to_write_order(wo, &wo_end, &objects[i]);
+			add_family_to_write_order(wo, wo_end, &objects[i]);
 	}
+}
+
+static struct object_entry **compute_write_order(void)
+{
+	unsigned int i, wo_end;
+
+	struct object_entry **wo;
+	struct object_entry *objects = to_pack.objects;
+
+	for (i = 0; i < to_pack.nr_objects; i++) {
+		objects[i].tagged = 0;
+		objects[i].filled = 0;
+		SET_DELTA_CHILD(&objects[i], NULL);
+		SET_DELTA_SIBLING(&objects[i], NULL);
+	}
+
+	/*
+	 * Fully connect delta_child/delta_sibling network.
+	 * Make sure delta_sibling is sorted in the original
+	 * recency order.
+	 */
+	for (i = to_pack.nr_objects; i > 0;) {
+		struct object_entry *e = &objects[--i];
+		if (!DELTA(e))
+			continue;
+		/* Mark me as the first child */
+		e->delta_sibling_idx = DELTA(e)->delta_child_idx;
+		SET_DELTA_CHILD(DELTA(e), e);
+	}
+
+	/*
+	 * Mark objects that are at the tip of tags.
+	 */
+	for_each_tag_ref(mark_tagged, NULL);
+
+	/*
+	 * Give the objects in the original recency order until
+	 * we see a tagged tip.
+	 */
+	ALLOC_ARRAY(wo, to_pack.nr_objects);
+	wo_end = 0;
+
+	compute_layer_order(wo, &wo_end);
 
 	if (wo_end != to_pack.nr_objects)
 		die("ordered %u objects, expected %"PRIu32, wo_end, to_pack.nr_objects);