[subset] Optimize set iteration by caching the last found page map index.

Reuse it if possible on consecutive calls to next(). Will signifcantly speed up cases where the entire set is iterated. In local testing iterating a very large set was 10x faster.

src/hb-set.hh

diff --git a/src/hb-set.hh b/src/hb-set.hh
index 4bbca90..18412e3 100644
--- a/src/hb-set.hh
+++ b/src/hb-set.hh
@@ -220,6 +220,7 @@
   hb_object_header_t header;
   bool successful; /* Allocations successful */
   mutable unsigned int population;
+  mutable unsigned int last_page_lookup;
   hb_sorted_vector_t<page_map_t> page_map;
   hb_vector_t<page_t> pages;
 
@@ -227,6 +228,7 @@
   {
     successful = true;
     population = 0;
+    last_page_lookup = 0;
     page_map.init ();
     pages.init ();
   }
@@ -238,6 +240,7 @@
   void fini_shallow ()
   {
     population = 0;
+    last_page_lookup = 0;
     page_map.fini ();
     pages.fini ();
   }
@@ -742,27 +745,43 @@
       return *codepoint != INVALID;
     }
 
-    page_map_t map = {get_major (*codepoint), 0};
-    unsigned int i;
-    page_map.bfind (map, &i, HB_BFIND_NOT_FOUND_STORE_CLOSEST);
-    if (i < page_map.length && page_map[i].major == map.major)
+    unsigned int major = get_major (*codepoint);
+    unsigned int i = last_page_lookup;
+
+    if (unlikely (i >= page_map.length || (page_map.arrayZ[i]).major != major))
     {
-      if (pages[page_map[i].index].next (codepoint))
+      page_map_t map = {major, 0};
+      page_map.bfind (map, &i, HB_BFIND_NOT_FOUND_STORE_CLOSEST);
+      if (i >= page_map.length) {
+        *codepoint = INVALID;
+        return false;
+      }
+    }
+
+    page_map_t &current = page_map.arrayZ[i];
+    if (likely (current.major == major))
+    {
+      if (pages.arrayZ[current.index].next (codepoint))
       {
-	*codepoint += page_map[i].major * page_t::PAGE_BITS;
-	return true;
+        *codepoint += current.major * page_t::PAGE_BITS;
+        last_page_lookup = i;
+        return true;
       }
       i++;
     }
+
     for (; i < page_map.length; i++)
     {
-      hb_codepoint_t m = pages[page_map[i].index].get_min ();
+      const page_map_t &current = page_map.arrayZ[i];
+      hb_codepoint_t m = pages.arrayZ[current.index].get_min ();
       if (m != INVALID)
       {
-	*codepoint = page_map[i].major * page_t::PAGE_BITS + m;
+	*codepoint = current.major * page_t::PAGE_BITS + m;
+        last_page_lookup = i;
 	return true;
       }
     }
+    last_page_lookup = 0;
     *codepoint = INVALID;
     return false;
   }