Multi reader/writer cache/buffer

media/blink/rangemap.h

+// Copyright 2015 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.
+
+#ifndef MEDIA_BLINK_RANGEMAP_H_
+#define MEDIA_BLINK_RANGEMAP_H_
+
+#include <limits>
+#include <map>
+
+#include "base/logging.h"
+
+namespace media {
+
+// A RangeMap<KeyType, ValueType> is similar to a std::map<KeyType, ValueType>,
+// where incrementing, decrementing and setting ranges of values has been
+// optimized. Internally, a RangeMap<> keeps a list KeyType ranges and the
+// associated ValueType. Set/Increment operations should generally take
+// O(log(N)) + O(M) time where N is the number of ranges in the map and
+// M is the number of modified ranges. Adjacent ranges which have the same
+// value are automatically merged
+//
+// Internally, RangeMap<> uses an std::map, where the beginning of each range
+// is stored along with the value for that range.
+
+// Simple range class.
+// Range ends are always non-inclusive.
+template <typename T>
+struct Range {
+ public:
+  Range(const T& begin_, const T& end_) : begin(begin_), end(end_) {}
+  T begin;
+  T end;
+
+  Range Intersect(const Range& other) const {
+    return Range(std::max(begin, other.begin), std::min<T>(end, other.end));
+  }
+
+  bool Empty() const { return begin >= end; }
+};
+
+template <typename KeyType,
+          typename ValueType,
+          class Compare = std::less<KeyType>,
+          class Minmax = std::numeric_limits<KeyType>>
+class RangeMapConstIterator {
+ public:
+  typedef std::map<KeyType, ValueType, Compare> MapType;
+  RangeMapConstIterator() {}
+  RangeMapConstIterator(const MapType* map,
+                        typename MapType::const_iterator first,
+                        typename MapType::const_iterator second)
+      : map_(map), first_(first), second_(second) {}
+
+  bool operator==(const RangeMapConstIterator& other) const {
+    return first_ == other.first_ && second_ == other.second_;
+  }
+
+  bool operator!=(const RangeMapConstIterator& other) const {
+    return first_ != other.first_ || second_ != other.second_;
+  }
+
+  // Returns the beginning of the current range.
+  KeyType range_begin() const {
+    if (first_ == map_->end()) {
+      return Minmax::min();
+    } else {
+      return first_->first;
+    }
+  }
+
+  // Returns the end of the current range, non-inclusive.
+  KeyType range_end() const {
+    if (second_ == map_->end()) {
+      return Minmax::max();
+    } else {
+      return second_->first;
+    }
+  }
+
+  // Returns the current range.
+  Range<KeyType> range() const {
+    return Range<KeyType>(range_begin(), range_end());
+  }
+
+  // Returns the value associated with the current range.
+  ValueType value() const {
+    if (first_ == map_->end())
+      return 0;
+    return first_->second;
+  }
+
+  // Needed to make the following construct work:
+  // for (const auto& range_value_pair : rangemap)
+  // Note however that this will skip the "end" range, which
+  // is usually ok since it generally has the default value.
+  std::pair<Range<KeyType>, ValueType> operator*() const {
+    return std::make_pair(range(), value());
+  }
+
+  // Go to the next range.
+  // The beginning of the next range always matches the end of the current
+  // range.
+  void operator++() {
+    first_ = second_;
+    second_++;
+  }
+
+  // Go to the previous range.
+  void operator--() {
+    second_ = first_;
+    if (first_ == map_->begin()) {
+      first_ = map_->end();
+    } else {
+      first_--;
+    }
+  }
+
+ private:
+  const MapType* map_;
+  typename MapType::const_iterator first_;
+  typename MapType::const_iterator second_;
+};
+
+template <typename KeyType,
+          typename ValueType,
+          class Compare = std::less<KeyType>,
+          class Minmax = std::numeric_limits<KeyType>>
+class RangeMap {
+ public:
+  typedef std::map<KeyType, ValueType, Compare> MapType;
+  typedef RangeMapConstIterator<KeyType, ValueType, Compare, Minmax>
+      const_iterator;
+
+  // Returns the value at a particular point.
+  // Defaults to ValueType().
+  ValueType operator[](KeyType k) const {
+    typename MapType::const_iterator i = map_.upper_bound(k);
+    if (i == map_.begin()) {
+      return 0;
+    } else {
+      --i;
+      return i->second;
+    }
+  }
+
+  // Increase [from..to) by |how_much|.
+  void IncrementRange(KeyType from, KeyType to, ValueType how_much) {
+    DCHECK_GE(to, from);
+    if (from == to || how_much == 0)
+      return;
+    typename MapType::iterator a = MakeEntry(from);
+    typename MapType::iterator b = MakeEntry(to);
+    for (typename MapType::iterator i = a; i != b; ++i) {
+      i->second += how_much;
+    }
+    RemoveDuplicates(a);
+    // b may be invalid
+    RemoveDuplicates(map_.lower_bound(to));
+  }
+
+  // Set [from..to) to |how_much|.
+  void SetRange(KeyType from, KeyType to, ValueType how_much) {
+    DCHECK_GE(to, from);
+    if (from == to)
+      return;
+    typename MapType::iterator a = MakeEntry(from);
+    typename MapType::iterator b = MakeEntry(to);
+    a->second = how_much;
+    while (true) {
+      typename MapType::iterator c = a;
+      ++c;
+      if (c == b) {
+        break;
+      } else {
+        map_.erase(c);
+      }
+    }
+    RemoveDuplicates(a);
+    // b may be invalid
+    RemoveDuplicates(map_.lower_bound(to));
+  }
+
+  // Returns an iterator to the first range.
+  // Note, there is always at least one range.
+  const_iterator begin() const {
+    return const_iterator(&map(), map_.end(), map_.begin());
+  }
+
+  // Returns an iterator to the last range.
+  // Note that this works slightly different than most containers
+  // as end() returns a valid, but mostly uninteresting iterator.
+  // For example, if you have you do:
+  //   RangeMap<int, int> tmp;
+  //   tmp.IncrementRange(-5, 5, 1);
+  // Then end will return the range [6, maxint) with a value of 0.
+  const_iterator end() const {
+    if (map_.empty()) {
+      return const_iterator(&map(), map_.end(), map_.end());
+    }
+    typename MapType::const_iterator i = map_.end();
+    --i;
+    return const_iterator(&map(), i, map_.end());
+  }
+
+  // Returns an iterator to the range containing |k|.
+  // Always returns a valid iterator.
+  const_iterator find(KeyType k) const {
+    if (map_.empty()) {
+      return const_iterator(&map(), map_.end(), map_.end());
+    }
+    typename MapType::const_iterator i = map_.upper_bound(k);
+    typename MapType::const_iterator j = i;
+    if (j == map_.begin()) {
+      j = map_.end();
+    } else {
+      --j;
+    }
+    return const_iterator(&map(), j, i);
+  }
+
+ private:
+  const MapType& map() const { return map_; }
+
+  // Make an entry in map_ with the key |k| and return it's iterator.
+  // If such an entry already exists, just re-use it.
+  // If a new entry is created, it's value will be set to the same
+  // as the preceeding entry, or ValueType() if no preceeding entry exists.
+  // After calling this function, we'll need to call RemoveDuplicates()
+  // to clean up any duplicates that we made.
+  typename MapType::iterator MakeEntry(KeyType k) {
+    typename MapType::value_type tmp(k, 0);
+    std::pair<typename MapType::iterator, bool> insert_result;
+    insert_result = map_.insert(tmp);
+    if (insert_result.second) {
+      if (insert_result.first != map_.begin()) {
+        typename MapType::iterator i = insert_result.first;
+        --i;
+        insert_result.first->second = i->second;
+      }
+    }
+    return insert_result.first;
+  }
+
+  // Remove duplicates before and after |i|.
+  void RemoveDuplicates(typename MapType::iterator i) {
+    if (i == map_.end())
+      return;
+    if (i == map_.begin() && i->second == 0) {
+      typename MapType::iterator j = i;
+      ++i;
+      map_.erase(j);
+      if (i == map_.end())
+        return;
+    }
+    if (i != map_.begin()) {
+      typename MapType::iterator j = i;
+      --i;
+      if (i->second == j->second) {
+        map_.erase(j);
+      }
+    }
+    typename MapType::iterator j = i;
+    ++j;
+    if (j != map_.end() && i->second == j->second) {
+      map_.erase(j);
+    }
+  }
+
+  MapType map_;
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BLINK_RANGEMAP_H_