[map] Add is_equal() / towards making hb_map_t hashable

New API:
+ hb_map_is_equal()

docs/harfbuzz-sections.txt

@@ -459,6 +459,7 @@ hb_map_destroy
 hb_map_get
 hb_map_get_empty
 hb_map_get_population
+hb_map_is_equal
 hb_map_get_user_data
 hb_map_has
 hb_map_is_empty

src/hb-map.cc

@@ -289,3 +289,23 @@ hb_map_get_population (const hb_map_t *map)
 {
   return map->get_population ();
 }
+
+/**
+ * hb_map_is_equal:
+ * @map: A map
+ * @other: Another map
+ *
+ * Tests whether @map and @other are equal (contain the same
+ * elements).
+ *
+ * Return value: %true if the two maps are equal, %false otherwise.
+ *
+ * Since: REPLACEME
+ **/
+hb_bool_t
+hb_map_is_equal (const hb_map_t *map,
+		 const hb_map_t *other)
+{
+  return map->is_equal (*other);
+}
+

src/hb-map.h

@@ -91,6 +91,10 @@ hb_map_is_empty (const hb_map_t *map);
 HB_EXTERN unsigned int
 hb_map_get_population (const hb_map_t *map);
 
+HB_EXTERN hb_bool_t
+hb_map_is_equal (const hb_map_t *map,
+		 const hb_map_t *other);
+
 HB_EXTERN void
 hb_map_set (hb_map_t       *map,
 	    hb_codepoint_t  key,

src/hb-map.hh

@@ -42,6 +42,7 @@ template <typename K, typename V,
 struct hb_hashmap_t
 {
   hb_hashmap_t ()  { init (); }
+  hb_hashmap_t (std::nullptr_t) : hb_hashmap_t () {}
   ~hb_hashmap_t () { fini (); }
 
   hb_hashmap_t (const hb_hashmap_t& o) : hb_hashmap_t () { resize (population); hb_copy (o, *this); }
@@ -238,6 +239,19 @@ struct hb_hashmap_t
   bool is_empty () const { return population == 0; }
   explicit operator bool () const { return !is_empty (); }
 
+  bool is_equal (const hb_hashmap_t &other) const
+  {
+    if (population != other.population) return false;
+
+    for (auto pair : iter ())
+      if (get (pair.first) != pair.second)
+        return false;
+
+    return true;
+  }
+  bool operator == (const hb_hashmap_t &other) const { return is_equal (other); }
+  bool operator != (const hb_hashmap_t &other) const { return !is_equal (other); }
+
   unsigned int get_population () const { return population; }
 
   /*
@@ -394,6 +408,7 @@ struct hb_map_t : hb_hashmap_t<hb_codepoint_t,
 
   ~hb_map_t () = default;
   hb_map_t () : hashmap () {}
+  hb_map_t (std::nullptr_t) : hb_map_t () {}
   hb_map_t (const hb_map_t &o) : hashmap ((hashmap &) o) {}
   hb_map_t (hb_map_t &&o) : hashmap (std::move ((hashmap &) o)) {}
   hb_map_t& operator= (const hb_map_t&) = default;

src/test-map.cc

@@ -152,6 +152,30 @@ main (int argc, char **argv)
     }
   }
 
+#if 0
+  /* Test hashing maps. */
+  {
+    hb_hashmap_t<hb_map_t, hb_map_t, const hb_map_t *, const hb_map_t *, &invalid_map, &invalid_map> m1;
+    hb_hashmap_t<hb_map_t, hb_map_t, std::nullptr_t, std::nullptr_t, nullptr, nullptr> m2;
+
+    m1.map (hb_map_t (), hb_map_t ());
+    m2.map (hb_map_t (), hb_map_t ());
+
+ //   m1.map (hb_map_t (), hb_map_t {1});
+ //   m2.map (hb_map_t (), hb_map_t {1});
+
+ //   m1.map (hb_map_t {1}, hb_map_t {2});
+ //   m2.map (hb_map_t {1}, hb_map_t {2});
+
+    /* Cannot override empty map. */
+    assert (m1.get (hb_map_t ()) == hb_map_t ());
+    assert (m2.get (hb_map_t ()) == hb_map_t ());
+
+ //   assert (m1.get (hb_map_t {1}) == hb_map_t {2});
+ //   assert (m2.get (hb_map_t {1}) == hb_map_t {2});
+  }
+#endif
+
   /* Test hashing sets. */
   {
     hb_hashmap_t<hb_set_t, hb_set_t, const hb_set_t *, const hb_set_t *, &invalid_set, &invalid_set> m1;