1 files changed, 118 insertions, 0 deletions
diff --git a/dom/quota/Flatten.h b/dom/quota/Flatten.h
new file mode 100644
index 0000000000..2eb29c6010
--- /dev/null
+++ b/dom/quota/Flatten.h
@@ -0,0 +1,118 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_QUOTA_FLATTEN_H_
+#define DOM_QUOTA_FLATTEN_H_
+
+#include <iterator>
+#include <type_traits>
+#include <utility>
+
+// XXX This should be moved to MFBT.
+
+namespace mozilla::dom::quota {
+
+namespace detail {
+
+using std::begin;
+using std::end;
+
+template <typename T, typename NestedRange>
+auto Flatten(NestedRange&& aRange) -> std::enable_if_t<
+    std::is_same_v<T, std::decay_t<typename decltype(begin(
+                          std::declval<const NestedRange&>()))::value_type>>,
+    std::conditional_t<std::is_rvalue_reference_v<NestedRange>,
+                       std::decay_t<NestedRange>, NestedRange>> {
+  return std::forward<NestedRange>(aRange);
+}
+
+template <typename T, typename NestedRange>
+struct FlatIter {
+  using OuterIterator =
+      decltype(begin(std::declval<const std::decay_t<NestedRange>&>()));
+  using InnerIterator =
+      decltype(begin(*begin(std::declval<const std::decay_t<NestedRange>&>())));
+
+  explicit FlatIter(const NestedRange& aRange, OuterIterator aIter)
+      : mOuterIter{std::move(aIter)}, mOuterEnd{end(aRange)} {
+    InitInner();
+  }
+
+  const T& operator*() const { return *mInnerIter; }
+
+  FlatIter& operator++() {
+    ++mInnerIter;
+    if (mInnerIter == mInnerEnd) {
+      ++mOuterIter;
+      InitInner();
+    }
+    return *this;
+  }
+
+  bool operator!=(const FlatIter& aOther) const {
+    return mOuterIter != aOther.mOuterIter ||
+           (mOuterIter != mOuterEnd && mInnerIter != aOther.mInnerIter);
+  }
+
+ private:
+  void InitInner() {
+    while (mOuterIter != mOuterEnd) {
+      const typename OuterIterator::value_type& innerRange = *mOuterIter;
+
+      mInnerIter = begin(innerRange);
+      mInnerEnd = end(innerRange);
+
+      if (mInnerIter != mInnerEnd) {
+        break;
+      }
+
+      ++mOuterIter;
+    }
+  }
+
+  OuterIterator mOuterIter;
+  const OuterIterator mOuterEnd;
+
+  InnerIterator mInnerIter;
+  InnerIterator mInnerEnd;
+};
+
+template <typename T, typename NestedRange>
+struct FlatRange {
+  explicit FlatRange(NestedRange aRange) : mRange{std::move(aRange)} {}
+
+  auto begin() const {
+    using std::begin;
+    return FlatIter<T, NestedRange>{mRange, begin(mRange)};
+  }
+  auto end() const {
+    using std::end;
+    return FlatIter<T, NestedRange>{mRange, end(mRange)};
+  }
+
+ private:
+  NestedRange mRange;
+};
+
+template <typename T, typename NestedRange>
+auto Flatten(NestedRange&& aRange) -> std::enable_if_t<
+    !std::is_same_v<
+        T, std::decay_t<typename decltype(begin(
+               std::declval<const std::decay_t<NestedRange>&>()))::value_type>>,
+    FlatRange<T, NestedRange>> {
+  return FlatRange<T, NestedRange>{std::forward<NestedRange>(aRange)};
+}
+
+}  // namespace detail
+
+template <typename T, typename NestedRange>
+auto Flatten(NestedRange&& aRange) -> decltype(auto) {
+  return detail::Flatten<T>(std::forward<NestedRange>(aRange));
+}
+
+}  // namespace mozilla::dom::quota
+
+#endif