[ot] Simplify form_clusters()

src/hb-buffer-private.hh

diff --git a/src/hb-buffer-private.hh b/src/hb-buffer-private.hh
index ced748f..32d5a5f 100644
--- a/src/hb-buffer-private.hh
+++ b/src/hb-buffer-private.hh
@@ -189,7 +189,14 @@
 			      unsigned int cluster_end);
 
   HB_INTERNAL void merge_clusters (unsigned int start,
-				   unsigned int end);
+				   unsigned int end)
+  {
+    if (end - start < 2)
+      return;
+    merge_clusters_impl (start, end);
+  }
+  HB_INTERNAL void merge_clusters_impl (unsigned int start,
+					unsigned int end);
   HB_INTERNAL void merge_out_clusters (unsigned int start,
 				       unsigned int end);
   /* Merge clusters for deleting current glyph, and skip it. */

src/hb-buffer.cc

diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 4f953f0..0a11fec 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -504,16 +504,13 @@
 }
 
 void
-hb_buffer_t::merge_clusters (unsigned int start,
-			     unsigned int end)
+hb_buffer_t::merge_clusters_impl (unsigned int start,
+				  unsigned int end)
 {
 #ifdef HB_NO_MERGE_CLUSTERS
   return;
 #endif
 
-  if (unlikely (end - start < 2))
-    return;
-
   unsigned int cluster = info[start].cluster;
 
   for (unsigned int i = start + 1; i < end; i++)

src/hb-ot-shape.cc

diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index 8d6bb3d..bbd044f 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -264,11 +264,18 @@
 static void
 hb_form_clusters (hb_buffer_t *buffer)
 {
+  unsigned int base = 0;
   unsigned int count = buffer->len;
   hb_glyph_info_t *info = buffer->info;
   for (unsigned int i = 1; i < count; i++)
-    if (HB_UNICODE_GENERAL_CATEGORY_IS_MARK (_hb_glyph_info_get_general_category (&info[i])))
-      buffer->merge_clusters (i - 1, i + 1);
+  {
+    if (likely (!HB_UNICODE_GENERAL_CATEGORY_IS_MARK (_hb_glyph_info_get_general_category (&info[i]))))
+    {
+      buffer->merge_clusters (base, i);
+      base = i;
+    }
+  }
+  buffer->merge_clusters (base, count);
 }
 
 static void