runtime/base/bit_vector.h

/*
 * Copyright (C) 2013 The Android Open Source Project
 *
 * 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 ART_RUNTIME_BASE_BIT_VECTOR_H_
#define ART_RUNTIME_BASE_BIT_VECTOR_H_

#include <stdint.h>
#include <iterator>

#include "base/bit_utils.h"
#include "globals.h"

namespace art {

class Allocator;

/*
 * Expanding bitmap, used for tracking resources.  Bits are numbered starting
 * from zero.  All operations on a BitVector are unsynchronized.
 */
class BitVector {
 public:
  class IndexContainer;

  /**
   * @brief Convenient iterator across the indexes of the BitVector's set bits.
   *
   * @details IndexIterator is a Forward iterator (C++11: 24.2.5) from the lowest
   * to the highest index of the BitVector's set bits. Instances can be retrieved
   * only through BitVector::Indexes() which returns an IndexContainer wrapper
   * object with begin() and end() suitable for range-based loops:
   *   for (uint32_t idx : bit_vector.Indexes()) {
   *     // Use idx.
   *   }
   */
  class IndexIterator :
      std::iterator<std::forward_iterator_tag, uint32_t, ptrdiff_t, void, uint32_t> {
   public:
    bool operator==(const IndexIterator& other) const;

    bool operator!=(const IndexIterator& other) const {
      return !(*this == other);
    }

    uint32_t operator*() const;

    IndexIterator& operator++();

    IndexIterator operator++(int);

    // Helper function to check for end without comparing with bit_vector.Indexes().end().
    bool Done() const {
      return bit_index_ == BitSize();
    }

   private:
    struct begin_tag { };
    struct end_tag { };

    IndexIterator(const BitVector* bit_vector, begin_tag)
      : bit_storage_(bit_vector->GetRawStorage()),
        storage_size_(bit_vector->storage_size_),
        bit_index_(FindIndex(0u)) { }

    IndexIterator(const BitVector* bit_vector, end_tag)
      : bit_storage_(bit_vector->GetRawStorage()),
        storage_size_(bit_vector->storage_size_),
        bit_index_(BitSize()) { }

    uint32_t BitSize() const {
      return storage_size_ * kWordBits;
    }

    uint32_t FindIndex(uint32_t start_index) const;
    const uint32_t* const bit_storage_;
    const uint32_t storage_size_;  // Size of vector in words.
    uint32_t bit_index_;           // Current index (size in bits).

    friend class BitVector::IndexContainer;
  };

  /**
   * @brief BitVector wrapper class for iteration across indexes of set bits.
   */
  class IndexContainer {
   public:
    explicit IndexContainer(const BitVector* bit_vector) : bit_vector_(bit_vector) { }

    IndexIterator begin() const {
      return IndexIterator(bit_vector_, IndexIterator::begin_tag());
    }

    IndexIterator end() const {
      return IndexIterator(bit_vector_, IndexIterator::end_tag());
    }

   private:
    const BitVector* const bit_vector_;
  };

  BitVector(uint32_t start_bits,
            bool expandable,
            Allocator* allocator,
            uint32_t storage_size = 0,
            uint32_t* storage = nullptr);

  virtual ~BitVector();

  // The number of words necessary to encode bits.
  static constexpr uint32_t BitsToWords(uint32_t bits) {
    return RoundUp(bits, kWordBits) / kWordBits;
  }

  // Mark the specified bit as "set".
  void SetBit(uint32_t idx) {
    /*
     * TUNING: this could have pathologically bad growth/expand behavior.  Make sure we're
     * not using it badly or change resize mechanism.
     */
    if (idx >= storage_size_ * kWordBits) {
      EnsureSize(idx);
    }
    storage_[WordIndex(idx)] |= BitMask(idx);
  }

  // Mark the specified bit as "unset".
  void ClearBit(uint32_t idx) {
    // If the index is over the size, we don't have to do anything, it is cleared.
    if (idx < storage_size_ * kWordBits) {
      // Otherwise, go ahead and clear it.
      storage_[WordIndex(idx)] &= ~BitMask(idx);
    }
  }

  // Determine whether or not the specified bit is set.
  bool IsBitSet(uint32_t idx) const {
    // If the index is over the size, whether it is expandable or not, this bit does not exist:
    // thus it is not set.
    return (idx < (storage_size_ * kWordBits)) && IsBitSet(storage_, idx);
  }

  // Mark all bits bit as "clear".
  void ClearAllBits();

  // Mark specified number of bits as "set". Cannot set all bits like ClearAll since there might
  // be unused bits - setting those to one will confuse the iterator.
  void SetInitialBits(uint32_t num_bits);

  void Copy(const BitVector* src);

  // Intersect with another bit vector.
  void Intersect(const BitVector* src2);

  // Union with another bit vector.
  bool Union(const BitVector* src);

  // Set bits of union_with that are not in not_in.
  bool UnionIfNotIn(const BitVector* union_with, const BitVector* not_in);

  void Subtract(const BitVector* src);

  // Are we equal to another bit vector?  Note: expandability attributes must also match.
  bool Equal(const BitVector* src) const;

  /**
   * @brief Are all the bits set the same?
   * @details expandability and size can differ as long as the same bits are set.
   */
  bool SameBitsSet(const BitVector *src) const;

  bool IsSubsetOf(const BitVector *other) const;

  // Count the number of bits that are set.
  uint32_t NumSetBits() const;

  // Count the number of bits that are set in range [0, end).
  uint32_t NumSetBits(uint32_t end) const;

  IndexContainer Indexes() const {
    return IndexContainer(this);
  }

  uint32_t GetStorageSize() const {
    return storage_size_;
  }

  bool IsExpandable() const {
    return expandable_;
  }

  uint32_t GetRawStorageWord(size_t idx) const {
    return storage_[idx];
  }

  uint32_t* GetRawStorage() {
    return storage_;
  }

  const uint32_t* GetRawStorage() const {
    return storage_;
  }

  size_t GetSizeOf() const {
    return storage_size_ * kWordBytes;
  }

  /**
   * @return the highest bit set, -1 if none are set
   */
  int GetHighestBitSet() const;

  // Is bit set in storage. (No range check.)
  static bool IsBitSet(const uint32_t* storage, uint32_t idx) {
    return (storage[WordIndex(idx)] & BitMask(idx)) != 0;
  }

  // Number of bits set in range [0, end) in storage. (No range check.)
  static uint32_t NumSetBits(const uint32_t* storage, uint32_t end);

  // Fill given memory region with the contents of the vector and zero padding.
  void CopyTo(void* dst, size_t len) const {
    DCHECK_LE(static_cast<size_t>(GetHighestBitSet() + 1), len * kBitsPerByte);
    size_t vec_len = GetSizeOf();
    if (vec_len < len) {
      void* dst_padding = reinterpret_cast<uint8_t*>(dst) + vec_len;
      memcpy(dst, storage_, vec_len);
      memset(dst_padding, 0, len - vec_len);
    } else {
      memcpy(dst, storage_, len);
    }
  }

  void Dump(std::ostream& os, const char* prefix) const;

 private:
  /**
   * @brief Dump the bitvector into buffer in a 00101..01 format.
   * @param buffer the ostringstream used to dump the bitvector into.
   */
  void DumpHelper(const char* prefix, std::ostringstream& buffer) const;

  // Ensure there is space for a bit at idx.
  void EnsureSize(uint32_t idx);

  // The index of the word within storage.
  static constexpr uint32_t WordIndex(uint32_t idx) {
    return idx >> 5;
  }

  // A bit mask to extract the bit for the given index.
  static constexpr uint32_t BitMask(uint32_t idx) {
    return 1 << (idx & 0x1f);
  }

  static constexpr uint32_t kWordBytes = sizeof(uint32_t);
  static constexpr uint32_t kWordBits = kWordBytes * 8;

  uint32_t*  storage_;            // The storage for the bit vector.
  uint32_t   storage_size_;       // Current size, in 32-bit words.
  Allocator* const allocator_;    // Allocator if expandable.
  const bool expandable_;         // Should the bitmap expand if too small?
};


}  // namespace art

#endif  // ART_RUNTIME_BASE_BIT_VECTOR_H_