[PATCH 4/13] Add a topological sort procedure to commit.c [rev 4]

This patch introduces an in-place topological sort procedure to commit.c.

Given a list of commits, sort_in_topological_order() will perform an in-place
topological sort of that list.

The invariant that applies to the resulting list is:

       a reachable from b => ord(b) < ord(a)

This invariant is weaker than the --merge-order invariant, but is cheaper
to calculate (assuming the list has been identified) and will serve any
purpose where only a minimal topological order guarantee is required.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
---
Note: this patch currently has no observable consequences since nothing
in this patch calls it. A future patch will use this algorithm to provide
support for a --topo-order flag.

This patch is a complete replacement for earlier revisions of this patch.

[rev 2]
  * incorporates Junio's questions/comments as commentary,
  * adds object.util save/restore functionality so that no
    assumption is made about the pre-existing state of object.util
    upon entry to the procedure.
[rev 3]
  * removed object.util save/restore
  * added more documentation to header about pre-conditions
[rev 4]
  * re-applied rev 3 to new patch series - no other change
---

 commit.c |  107 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 commit.h |   13 ++++++++
 2 files changed, 120 insertions(+), 0 deletions(-)

155c21a7e0295deab710345d22f0551706a7f84a
diff --git a/commit.c b/commit.c
--- a/commit.c
+++ b/commit.c
@@ -3,6 +3,22 @@
 #include "commit.h"
 #include "cache.h"
 
+struct sort_node
+{
+	/*
+         * the number of children of the associated commit
+         * that also occur in the list being sorted.
+         */
+	unsigned int indegree;
+
+	/*
+         * reference to original list item that we will re-use
+         * on output.
+         */
+	struct commit_list * list_item;
+
+};
+
 const char *commit_type = "commit";
 
 enum cmit_fmt get_commit_format(const char *arg)
@@ -346,3 +362,94 @@ int count_parents(struct commit * commit
         return count;
 }
 
+/*
+ * Performs an in-place topological sort on the list supplied.
+ */
+void sort_in_topological_order(struct commit_list ** list)
+{
+	struct commit_list * next = *list;
+	struct commit_list * work = NULL;
+	struct commit_list ** pptr = list;
+	struct sort_node * nodes;
+	struct sort_node * next_nodes;
+	int count = 0;
+
+	/* determine the size of the list */
+	while (next) {
+		next = next->next;
+		count++;
+	}
+	/* allocate an array to help sort the list */
+	nodes = xcalloc(count, sizeof(*nodes));
+	/* link the list to the array */
+	next_nodes = nodes;
+	next=*list;
+	while (next) {
+		next_nodes->list_item = next;
+		next->item->object.util = next_nodes;
+		next_nodes++;
+		next = next->next;
+	}
+	/* update the indegree */
+	next=*list;
+	while (next) {
+		struct commit_list * parents = next->item->parents;
+		while (parents) {
+			struct commit * parent=parents->item;
+			struct sort_node * pn = (struct sort_node *)parent->object.util;
+			
+			if (pn)
+				pn->indegree++;
+			parents=parents->next;
+		}
+		next=next->next;
+	}
+	/* 
+         * find the tips
+         *
+         * tips are nodes not reachable from any other node in the list 
+         * 
+         * the tips serve as a starting set for the work queue.
+         */
+	next=*list;
+	while (next) {
+		struct sort_node * node = (struct sort_node *)next->item->object.util;
+
+		if (node->indegree == 0) {
+			commit_list_insert(next->item, &work);
+		}
+		next=next->next;
+	}
+	/* process the list in topological order */
+	while (work) {
+		struct commit * work_item = pop_commit(&work);
+		struct sort_node * work_node = (struct sort_node *)work_item->object.util;
+		struct commit_list * parents = work_item->parents;
+
+		while (parents) {
+			struct commit * parent=parents->item;
+			struct sort_node * pn = (struct sort_node *)parent->object.util;
+			
+			if (pn) {
+				/* 
+				 * parents are only enqueued for emission 
+                                 * when all their children have been emitted thereby
+                                 * guaranteeing topological order.
+                                 */
+				pn->indegree--;
+				if (!pn->indegree) 
+					commit_list_insert(parent, &work);
+			}
+			parents=parents->next;
+		}
+		/*
+                 * work_item is a commit all of whose children
+                 * have already been emitted. we can emit it now.
+                 */
+		*pptr = work_node->list_item;
+		pptr = &(*pptr)->next;
+		*pptr = NULL;
+		work_item->object.util = NULL;
+	}
+	free(nodes);
+}
diff --git a/commit.h b/commit.h
--- a/commit.h
+++ b/commit.h
@@ -59,4 +59,17 @@ struct commit *pop_most_recent_commit(st
 struct commit *pop_commit(struct commit_list **stack);
 
 int count_parents(struct commit * commit);
+
+/*
+ * Performs an in-place topological sort of list supplied.
+ *
+ * Pre-conditions:
+ *   all commits in input list and all parents of those
+ *   commits must have object.util == NULL
+ *        
+ * Post-conditions: 
+ *   invariant of resulting list is:
+ *      a reachable from b => ord(b) < ord(a)
+ */
+void sort_in_topological_order(struct commit_list ** list);
 #endif /* COMMIT_H */
------------
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