partition: Reduce partition size range

With our existing heuristics, we would on average produce partitions
with size ~15% over the target. This is not entirely desirable and skews
the comparison metrics somewhat; it's especially problematic when the
target partition size is a power of two, since it creates an easy path
to get to the max partition size by incrementally doubling until the
target, and then merging partitions with target and target/2 together.

For now just reducing the extra size allowance a little to 3/8 of target
maintains reasonable partitioning properties with closer-to-target
average size.

Author: zeux

src/partition.cpp

@@ -307,6 +307,8 @@ size_t meshopt_partitionClusters(unsigned int* destination, const unsigned int*
 
 	assert(target_partition_size > 0);
 
+	size_t max_partition_size = target_partition_size + target_partition_size * 3 / 8;
+
 	meshopt_Allocator allocator;
 
 	unsigned char* used = allocator.allocate<unsigned char>(vertex_count);
@@ -318,6 +320,8 @@ size_t meshopt_partitionClusters(unsigned int* destination, const unsigned int*
 
 	for (size_t i = 0; i < cluster_count; ++i)
 	{
+		assert(cluster_index_counts[i] > 0);
+
 		cluster_offsets[i] = cluster_nextoffset;
 		cluster_nextoffset += cluster_index_counts[i];
 	}
@@ -369,7 +373,7 @@ size_t meshopt_partitionClusters(unsigned int* destination, const unsigned int*
 		if (groups[top.id].size >= target_partition_size)
 			continue;
 
-		int best_group = pickGroupToMerge(groups, top.id, adjacency, target_partition_size + target_partition_size / 2);
+		int best_group = pickGroupToMerge(groups, top.id, adjacency, max_partition_size);
 
 		// we can't grow the group any more, emit as is
 		if (best_group == -1)