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+/*
+ * Copyright 2022 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_support_parent_index_iterator_h
+#define wasm_support_parent_index_iterator_h
+
+#include <cstddef>
+#include <iterator>
+
+namespace wasm {
+
+// A helper for defining iterators that contain references to some parent object
+// and indices into that parent object. Provides operator implementations for
+// equality, inequality, index updates, and index differences, but not for
+// dereferencing.
+//
+// Users of this utility should subclass ParentIndexIterator<...> and define the
+// following members in the subclass:
+//
+//   - using value_type = ...
+//   - using pointer = ...
+//   - using reference = ...
+//   - reference operator*() const
+//
+// `Parent` is the parent type that the iterator contains and is being indexed
+// into and `Iterator` is the subclass of ParentIndexIterator<...> that is being
+// defined.
+template<typename Parent, typename Iterator> struct ParentIndexIterator {
+  using iterator_category = std::random_access_iterator_tag;
+  using difference_type = std::ptrdiff_t;
+
+  Parent parent;
+  size_t index;
+
+  const Iterator& self() const { return *static_cast<const Iterator*>(this); }
+
+  Iterator& self() { return *static_cast<Iterator*>(this); }
+
+  bool operator==(const Iterator& other) const {
+    return index == other.index && parent == other.parent;
+  }
+  bool operator!=(const Iterator& other) const { return !(*this == other); }
+  Iterator& operator++() {
+    ++index;
+    return self();
+  }
+  Iterator& operator--() {
+    --index;
+    return self();
+  }
+  Iterator operator++(int) {
+    auto it = self();
+    index++;
+    return it;
+  }
+  Iterator operator--(int) {
+    auto it = self();
+    index--;
+    return it;
+  }
+  Iterator& operator+=(difference_type off) {
+    index += off;
+    return self();
+  }
+  Iterator operator+(difference_type off) const {
+    return Iterator(self()) += off;
+  }
+  Iterator& operator-=(difference_type off) {
+    index -= off;
+    return self();
+  }
+  Iterator operator-(difference_type off) const {
+    return Iterator(self()) -= off;
+  }
+  difference_type operator-(const Iterator& other) const {
+    assert(parent == other.parent);
+    return index - other.index;
+  }
+};
+
+} // namespace wasm
+
+#endif // wasm_support_parent_index_iterator_h