diff --git a/pack-bitmap.c b/pack-bitmap.c
index 35c5ef9d3c..3519edb896 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -129,17 +129,13 @@ static struct ewah_bitmap *lookup_stored_bitmap(struct stored_bitmap *st)
 	return composed;
 }
 
-/*
- * Read a bitmap from the current read position on the mmaped
- * index, and increase the read position accordingly
- */
-static struct ewah_bitmap *read_bitmap_1(struct bitmap_index *index)
+struct ewah_bitmap *read_bitmap(const unsigned char *map,
+				size_t map_size, size_t *map_pos)
 {
 	struct ewah_bitmap *b = ewah_pool_new();
 
-	ssize_t bitmap_size = ewah_read_mmap(b,
-		index->map + index->map_pos,
-		index->map_size - index->map_pos);
+	ssize_t bitmap_size = ewah_read_mmap(b, map + *map_pos,
+					     map_size - *map_pos);
 
 	if (bitmap_size < 0) {
 		error(_("failed to load bitmap index (corrupted?)"));
@@ -147,10 +143,20 @@ static struct ewah_bitmap *read_bitmap_1(struct bitmap_index *index)
 		return NULL;
 	}
 
-	index->map_pos += bitmap_size;
+	*map_pos += bitmap_size;
+
 	return b;
 }
 
+/*
+ * Read a bitmap from the current read position on the mmaped
+ * index, and increase the read position accordingly
+ */
+static struct ewah_bitmap *read_bitmap_1(struct bitmap_index *index)
+{
+	return read_bitmap(index->map, index->map_size, &index->map_pos);
+}
+
 static uint32_t bitmap_num_objects(struct bitmap_index *index)
 {
 	if (index->midx)
diff --git a/pack-bitmap.h b/pack-bitmap.h
index db9ae554fa..21aabf805e 100644
--- a/pack-bitmap.h
+++ b/pack-bitmap.h
@@ -160,4 +160,6 @@ int bitmap_is_preferred_refname(struct repository *r, const char *refname);
 
 int verify_bitmap_files(struct repository *r);
 
+struct ewah_bitmap *read_bitmap(const unsigned char *map,
+				size_t map_size, size_t *map_pos);
 #endif