C bindings: Implement _Validate(), and some pre-requisites

mojo/public/c/bindings/lib/map.c

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/public/c/bindings/map.h"
+
+struct MojomMapLayout {
+  struct MojomStructHeader header_;
+  union MojomArrayHeaderPtr keys;
+  union MojomArrayHeaderPtr values;
+};
+MOJO_STATIC_ASSERT(sizeof(struct MojomMapLayout) == 24u,
+                   "MojomMapLayout is an invalid size.");
+
+MojomValidationResult MojomMap_Validate(
+    const struct MojomTypeDescriptorStruct* in_type_desc,
+    const struct MojomStructHeader* in_struct,
+    uint32_t in_buf_size,
+    uint32_t in_num_handles,
+    struct MojomValidationContext* inout_context) {
+  // A mojom map consists of 2 arrays (pointers), both of equal size.
+  const struct MojomMapLayout* map = (const struct MojomMapLayout*)in_struct;
+  struct MojomArrayHeader* keys_arr =
+      (struct MojomArrayHeader*)((char*)map +
+                                   (offsetof(struct MojomMapLayout, keys) +
+                                    map->keys.offset));
+  struct MojomArrayHeader* values_arr =
+      (struct MojomArrayHeader*)((char*)map +
+                                   (offsetof(struct MojomMapLayout, values) +
+                                    map->values.offset));
+
+  if (keys_arr->num_elements != values_arr->num_elements)
+    return MOJOM_VALIDATION_DIFFERENT_SIZED_ARRAYS_IN_MAP;
+
+  return MOJOM_VALIDATION_ERROR_NONE;
+}