move Iterator, Enumerate, EvictingCacheMap, Foreach, Merge, and

[folly.git] / folly / container / Enumerate.h

diff --git a/folly/container/Enumerate.h b/folly/container/Enumerate.h
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+/*
+ * Copyright 2017 Facebook, Inc.
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <iterator>
+#include <memory>
+
+#include <folly/portability/SysTypes.h>
+
+/**
+ * Similar to Python's enumerate(), folly::enumerate() can be used to
+ * iterate a range with a for-range loop, and it also allows to
+ * retrieve the count of iterations so far.
+ *
+ * For example:
+ *
+ * for (auto it : folly::enumerate(vec)) {
+ *   // *it is a reference to the current element. Const if vec is const.
+ *   // it->member can be used as well.
+ *   // it.index contains the iteration count.
+ * }
+ *
+ * If the iteration variable is const, the reference is too.
+ *
+ * for (const auto it : folly::enumerate(vec)) {
+ *   // *it is always a const reference.
+ * }
+ *
+ * @author Giuseppe Ottaviano <ott@fb.com>
+ */
+
+namespace folly {
+
+namespace detail {
+
+template <class T>
+struct MakeConst {
+  using type = const T;
+};
+template <class T>
+struct MakeConst<T&> {
+  using type = const T&;
+};
+template <class T>
+struct MakeConst<T*> {
+  using type = const T*;
+};
+
+// Raw pointers don't have an operator->() member function, so the
+// second overload will be SFINAEd out in that case. Otherwise, the
+// second is preferred in the partial order for getPointer(_, 0).
+template <class Iterator>
+auto getPointer(const Iterator& it, long) -> decltype(std::addressof(*it)) {
+  return std::addressof(*it);
+}
+template <class Iterator>
+auto getPointer(const Iterator& it, int) -> decltype(it.operator->()) {
+  return it.operator->();
+}
+
+template <class Iterator>
+class Enumerator {
+ public:
+  explicit Enumerator(Iterator it) : it_(std::move(it)) {}
+
+  class Proxy {
+   public:
+    using difference_type = ssize_t;
+    using value_type = typename std::iterator_traits<Iterator>::value_type;
+    using reference = typename std::iterator_traits<Iterator>::reference;
+    using pointer = typename std::iterator_traits<Iterator>::pointer;
+    using iterator_category = std::input_iterator_tag;
+
+    explicit Proxy(const Enumerator* e) : it_(e->it_), index(e->idx_) {}
+
+    // Non-const Proxy: Forward constness from Iterator.
+    reference operator*() {
+      return *it_;
+    }
+    pointer operator->() {
+      return getPointer(it_, 0);
+    }
+
+    // Const Proxy: Force const references.
+    typename MakeConst<reference>::type operator*() const {
+      return *it_;
+    }
+    typename MakeConst<pointer>::type operator->() const {
+      return getPointer(it_, 0);
+    }
+
+   private:
+    const Iterator& it_;
+
+   public:
+    const size_t index;
+  };
+
+  Proxy operator*() const {
+    return Proxy(this);
+  }
+
+  Enumerator& operator++() {
+    ++it_;
+    ++idx_;
+    return *this;
+  }
+
+  template <typename OtherIterator>
+  bool operator==(const Enumerator<OtherIterator>& rhs) {
+    return it_ == rhs.it_;
+  }
+
+  template <typename OtherIterator>
+  bool operator!=(const Enumerator<OtherIterator>& rhs) {
+    return !(*this == rhs);
+  }
+
+ private:
+  template <typename OtherIterator>
+  friend class Enumerator;
+
+  Iterator it_;
+  size_t idx_ = 0;
+};
+
+template <class Range>
+class RangeEnumerator {
+  Range r_;
+  using BeginIteratorType = decltype(std::declval<Range>().begin());
+  using EndIteratorType = decltype(std::declval<Range>().end());
+
+ public:
+  explicit RangeEnumerator(Range&& r) : r_(std::forward<Range>(r)) {}
+
+  Enumerator<BeginIteratorType> begin() {
+    return Enumerator<BeginIteratorType>(r_.begin());
+  }
+  Enumerator<EndIteratorType> end() {
+    return Enumerator<EndIteratorType>(r_.end());
+  }
+};
+
+} // namespace detail
+
+template <class Range>
+detail::RangeEnumerator<Range> enumerate(Range&& r) {
+  return detail::RangeEnumerator<Range>(std::forward<Range>(r));
+}
+
+} // namespace folly