text: // Copyright 2017 The Dawn Authors
  //
  // 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 SRC_DAWN_COMMON_BITSETITERATOR_H_
  #define SRC_DAWN_COMMON_BITSETITERATOR_H_
  
  #include <bitset>
  #include <limits>
  
  #include "dawn/common/Assert.h"
  #include "dawn/common/Math.h"
  #include "dawn/common/UnderlyingType.h"
  
  // This is ANGLE's BitSetIterator class with a customizable return type.
  // Types have been updated to be more specific.
  // TODO(crbug.com/dawn/306): it could be optimized, in particular when N <= 64
  
  template <typename T>
  T roundUp(const T value, const T alignment) {
      auto temp = value + alignment - static_cast<T>(1);
      return temp - temp % alignment;
  }
  
  template <size_t N, typename T>
  class BitSetIterator final {
    public:
      explicit BitSetIterator(const std::bitset<N>& bitset);
      BitSetIterator(const BitSetIterator& other);
      BitSetIterator& operator=(const BitSetIterator& other);
  
      class Iterator final {
        public:
          explicit Iterator(const std::bitset<N>& bits);
          Iterator& operator++();
  
          bool operator==(const Iterator& other) const;
          bool operator!=(const Iterator& other) const;
  
          T operator*() const {
              using U = UnderlyingType<T>;
              ASSERT(static_cast<U>(mCurrentBit) <= std::numeric_limits<U>::max());
              return static_cast<T>(static_cast<U>(mCurrentBit));
          }
  
        private:
          uint32_t getNextBit();
  
          static constexpr size_t kBitsPerWord = sizeof(uint32_t) * 8;
          std::bitset<N> mBits;
          uint32_t mCurrentBit;
          uint32_t mOffset;
      };
  
      Iterator begin() const {
          return Iterator(mBits);
      }
      Iterator end() const {
          return Iterator(std::bitset<N>(0));
      }
  
    private:
      const std::bitset<N> mBits;
  };
  
  template <size_t N, typename T>
  BitSetIterator<N, T>::BitSetIterator(const std::bitset<N>& bitset) : mBits(bitset) {
  }
  
  template <size_t N, typename T>
  BitSetIterator<N, T>::BitSetIterator(const BitSetIterator& other) : mBits(other.mBits) {
  }
  
  template <size_t N, typename T>
  BitSetIterator<N, T>& BitSetIterator<N, T>::operator=(const BitSetIterator& other) {
      mBits = other.mBits;
      return *this;
  }
  
  template <size_t N, typename T>
  BitSetIterator<N, T>::Iterator::Iterator(const std::bitset<N>& bits)
      : mBits(bits), mCurrentBit(0), mOffset(0) {
      if (bits.any()) {
          mCurrentBit = getNextBit();
      } else {
          mOffset = static_cast<uint32_t>(roundUp(N, kBitsPerWord));
      }
  }
  
  template <size_t N, typename T>
  typename BitSetIterator<N, T>::Iterator& BitSetIterator<N, T>::Iterator::operator++() {
      DAWN_ASSERT(mBits.any());
      mBits.set(mCurrentBit - mOffset, 0);
      mCurrentBit = getNextBit();
      return *this;
  }
  
  template <size_t N, typename T>
  bool BitSetIterator<N, T>::Iterator::operator==(const Iterator& other) const {
      return mOffset == other.mOffset && mBits == other.mBits;
  }
  
  template <size_t N, typename T>
  bool BitSetIterator<N, T>::Iterator::operator!=(const Iterator& other) const {
      return !(*this == other);
  }
  
  template <size_t N, typename T>
  uint32_t BitSetIterator<N, T>::Iterator::getNextBit() {
      static std::bitset<N> wordMask(std::numeric_limits<uint32_t>::max());
  
      while (mOffset < N) {
          uint32_t wordBits = static_cast<uint32_t>((mBits & wordMask).to_ulong());
          if (wordBits != 0ul) {
              return ScanForward(wordBits) + mOffset;
          }
  
          mBits >>= kBitsPerWord;
          mOffset += kBitsPerWord;
      }
      return 0;
  }
  
  // Helper to avoid needing to specify the template parameter size
  template <size_t N>
  BitSetIterator<N, uint32_t> IterateBitSet(const std::bitset<N>& bitset) {
      return BitSetIterator<N, uint32_t>(bitset);
  }
  
  #endif  // SRC_DAWN_COMMON_BITSETITERATOR_H_
  