src/extensions/ngram/bitmap-index.cc

// 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.
//
// Copyright 2005-2010 Google, Inc.
// Author: Jeffrey Soresnen (sorenj@google.com)

#include <fst/extensions/ngram/bitmap-index.h>

#include <algorithm>
#include <iterator>

#include <fst/extensions/ngram/nthbit.h>

namespace fst {

// These two internal classes implemented inverted views of the
// primary and secondary indexes.  That is, they provide iterators
// that have operator*'s that return the number zeros rather than
// the number of ones.

class primary_index_inverted : public vector<uint32>::const_iterator {
 public:
  primary_index_inverted() {}
  primary_index_inverted(vector<uint32>::const_iterator loc,
                         vector<uint32>::const_iterator begin) :
    vector<uint32>::const_iterator(loc), begin_(begin) {}
  uint32 operator*() {
    return BitmapIndex::kStorageBitSize * BitmapIndex::kSecondaryBlockSize *
           (1 + std::distance<vector<uint32>::const_iterator>(begin_, *this)) -
           vector<uint32>::const_iterator::operator*();
  }
 private:
  vector<uint32>::const_iterator begin_;
};

class secondary_index_inverted : public vector<uint16>::const_iterator {
 public:
  secondary_index_inverted() : vector<uint16>::const_iterator() {}
  secondary_index_inverted(vector<uint16>::const_iterator loc,
                           vector<uint16>::const_iterator block_begin) :
    vector<uint16>::const_iterator(loc), block_begin_(block_begin) {}
  uint16 operator*() {
    return ((1 + std::distance<vector<uint16>::const_iterator>(
        block_begin_, *this)) << BitmapIndex::kStorageLogBitSize) -
        vector<uint16>::const_iterator::operator*();
  }
 private:
  vector<uint16>::const_iterator block_begin_;
};

size_t BitmapIndex::Rank1(size_t end) const {
  if (end == 0) return 0;
  CHECK_LE(end, Bits());
  const uint32 end_word = (end - 1) >> BitmapIndex::kStorageLogBitSize;
  const uint32 sum = get_index_ones_count(end_word);
  const uint64 zero = 0;
  const uint64 ones = ~zero;
  return sum + __builtin_popcountll(bits_[end_word] &
      (ones >> (kStorageBitSize - (end & kStorageBlockMask))));
}

size_t BitmapIndex::Select1(size_t bit_index) const {
  if (bit_index >= GetOnesCount()) return Bits();
  // search primary index for the relevant block
  uint32 rembits = bit_index + 1;
  const uint32 block = find_primary_block(bit_index + 1);
  uint32 offset = 0;
  if (block > 0) {
    rembits -= primary_index_[block - 1];
    offset += block * kSecondaryBlockSize;
  }
  // search the secondary index
  uint32 word = find_secondary_block(offset, rembits);
  if (word > 0) {
    rembits -= secondary_index_[offset + word - 1];
    offset += word;
  }
  int nth = nth_bit(bits_[offset], rembits);
  return (offset << BitmapIndex::kStorageLogBitSize) + nth;
}

size_t BitmapIndex::Select0(size_t bit_index) const {
  if (bit_index >= Bits() - GetOnesCount()) return Bits();
  // search inverted primary index for relevant block
  uint32 remzeros = bit_index + 1;
  uint32 offset = 0;
  const uint32 block = find_inverted_primary_block(bit_index + 1);
  if (block > 0) {
    remzeros -= *primary_index_inverted(primary_index_.begin() + block - 1,
                                        primary_index_.begin());
    offset += block * kSecondaryBlockSize;
  }
  // search the inverted secondary index
  uint32 word = find_inverted_secondary_block(offset, remzeros);
  if (word > 0) {
    vector<uint16>::const_iterator block_begin =
        secondary_index_.begin() + offset;
    remzeros -= *secondary_index_inverted(block_begin + word - 1, block_begin);
    offset += word;
  }
  int nth = nth_bit(~bits_[offset], remzeros);
  return (offset << BitmapIndex::kStorageLogBitSize) + nth;
}

size_t BitmapIndex::get_index_ones_count(size_t array_index) const {
  uint32 sum = 0;
  if (array_index > 0) {
    sum += secondary_index_[array_index-1];
    uint32 end_block = (array_index - 1) / kSecondaryBlockSize;
    if (end_block > 0) sum += primary_index_[end_block-1];
  }
  return sum;
}

void BitmapIndex::BuildIndex(const uint64 *bits, size_t size) {
  bits_ = bits;
  size_ = size;
  secondary_index_.clear();
  secondary_index_.reserve(ArraySize());
  primary_index_.clear();
  primary_index_.reserve(primary_index_size());
  const uint64 zero = 0;
  const uint64 ones = ~zero;
  uint32 popcount = 0;
  for (uint32 block_begin = 0; block_begin < ArraySize();
       block_begin += kSecondaryBlockSize) {
    uint32 block_popcount = 0;
    uint32 block_end = block_begin + kSecondaryBlockSize;
    if (block_end > ArraySize()) block_end = ArraySize();
    for (uint32 j = block_begin; j < block_end; ++j) {
      uint64 mask = ones;
      if (j == ArraySize() - 1) {
        mask = ones >> (-size_ & BitmapIndex::kStorageBlockMask);
      }
      block_popcount += __builtin_popcountll(bits_[j] & mask);
      secondary_index_.push_back(block_popcount);
    }
    popcount += block_popcount;
    primary_index_.push_back(popcount);
  }
}

size_t BitmapIndex::find_secondary_block(
    size_t block_begin, size_t rem_bit_index) const {
  size_t block_end = block_begin + kSecondaryBlockSize;
  if (block_end > secondary_index_.size()) block_end = secondary_index_.size();
  return std::distance(secondary_index_.begin() + block_begin,
                       std::lower_bound(secondary_index_.begin() + block_begin,
                                        secondary_index_.begin() + block_end,
                                        rem_bit_index));
}

size_t BitmapIndex::find_inverted_secondary_block(
    size_t block_begin, size_t rem_bit_index) const {
  size_t block_end = block_begin + kSecondaryBlockSize;
  if (block_end > secondary_index_.size()) block_end = secondary_index_.size();
  secondary_index_inverted start(secondary_index_.begin() + block_begin,
                                 secondary_index_.begin() + block_begin);
  secondary_index_inverted end(secondary_index_.begin() + block_end,
                               secondary_index_.begin() + block_begin);
  return std::distance(start,
                       std::lower_bound(start, end, rem_bit_index));
}

inline size_t BitmapIndex::find_primary_block(size_t bit_index) const {
  return std::distance(primary_index_.begin(),
                       std::lower_bound(primary_index_.begin(),
                                        primary_index_.end(), bit_index));
}

size_t BitmapIndex::find_inverted_primary_block(size_t bit_index) const {
  primary_index_inverted start(primary_index_.begin(), primary_index_.begin());
  primary_index_inverted end(primary_index_.end(), primary_index_.begin());
  return std::distance(start, std::lower_bound(start, end, bit_index));
}

}  // end namespace fst