GHash, EdgeHash: add debugging function to measure the hash quality

Can use to check on improvements to hash functions.

1 files changed, 35 insertions, 0 deletions

diff --git a/source/blender/blenlib/intern/edgehash.c b/source/blender/blenlib/intern/edgehash.c
index aa571bbc1c5..b4b3f0dbda3 100644
--- a/source/blender/blenlib/intern/edgehash.c
+++ b/source/blender/blenlib/intern/edgehash.c
@@ -623,3 +623,38 @@ void BLI_edgeset_free(EdgeSet *es)
 }
 
 /** \} */
+
+/** \name Debugging & Introspection
+ * \{ */
+#ifdef DEBUG
+
+/**
+ * Measure how well the hash function performs
+ * (1.0 is approx as good as random distribution).
+ */
+double BLI_edgehash_calc_quality(EdgeHash *eh)
+{
+	uint64_t sum = 0;
+	unsigned int i;
+
+	if (eh->nentries == 0)
+		return -1.0;
+
+	for (i = 0; i < eh->nbuckets; i++) {
+		uint64_t count = 0;
+		EdgeEntry *e;
+		for (e = eh->buckets[i]; e; e = e->next) {
+			count += 1;
+		}
+		sum += count * (count + 1);
+	}
+	return ((double)sum * (double)eh->nbuckets /
+	        ((double)eh->nentries * (eh->nentries + 2 * eh->nbuckets - 1)));
+}
+double BLI_edgeset_calc_quality(EdgeSet *es)
+{
+	return BLI_edgehash_calc_quality((EdgeHash *)es);
+}
+
+#endif
+/** \} */