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+
+// 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: sorenj@google.com (Jeffrey Sorensen)
+
+#ifndef FST_EXTENSIONS_NGRAM_BITMAP_INDEX_H_
+#define FST_EXTENSIONS_NGRAM_BITMAP_INDEX_H_
+
+#include <vector>
+using std::vector;
+
+#include <fst/compat.h>
+
+// This class is a bitstring storage class with an index that allows
+// seeking to the Nth set or clear bit in time O(Log(N)) where N is
+// the length of the bit vector.  In addition, it allows counting set or
+// clear bits over ranges in constant time.
+//
+// This is accomplished by maintaining an "secondary" index of limited
+// size in bits that maintains a running count of the number of bits set
+// in each block of bitmap data.  A block is defined as the number of
+// uint64 values that can fit in the secondary index before an overflow
+// occurs.
+//
+// To handle overflows, a "primary" index containing a running count of
+// bits set in each block is created using the type uint64.
+
+namespace fst {
+
+class BitmapIndex {
+ public:
+  static size_t StorageSize(size_t size) {
+    return ((size + kStorageBlockMask) >> kStorageLogBitSize);
+  }
+
+  BitmapIndex() : bits_(NULL), size_(0) { }
+
+  bool Get(size_t index) const {
+    return (bits_[index >> kStorageLogBitSize] &
+            (kOne << (index & kStorageBlockMask))) != 0;
+  }
+
+  static void Set(uint64* bits, size_t index) {
+    bits[index >> kStorageLogBitSize] |= (kOne << (index & kStorageBlockMask));
+  }
+
+  static void Clear(uint64* bits, size_t index) {
+    bits[index >> kStorageLogBitSize] &= ~(kOne << (index & kStorageBlockMask));
+  }
+
+  size_t Bits() const {
+    return size_;
+  }
+
+  size_t ArraySize() const {
+    return StorageSize(size_);
+  }
+
+  // Returns the number of one bits in the bitmap
+  size_t GetOnesCount() const {
+    return primary_index_[primary_index_size() - 1];
+  }
+
+  // Returns the number of one bits in positions 0 to limit - 1.
+  // REQUIRES: limit <= Bits()
+  size_t Rank1(size_t end) const;
+
+  // Returns the number of one bits in the range start to end - 1.
+  // REQUIRES: limit <= Bits()
+  size_t GetOnesCountInRange(size_t start, size_t end) const {
+    return Rank1(end) - Rank1(start);
+  }
+
+  // Returns the number of zero bits in positions 0 to limit - 1.
+  // REQUIRES: limit <= Bits()
+  size_t Rank0(size_t end) const {
+    return end - Rank1(end);
+  }
+
+  // Returns the number of zero bits in the range start to end - 1.
+  // REQUIRES: limit <= Bits()
+  size_t GetZeroesCountInRange(size_t start, size_t end) const {
+    return end - start - GetOnesCountInRange(start, end);
+  }
+
+  // Return true if any bit between begin inclusive and end exclusive
+  // is set.  0 <= begin <= end <= Bits() is required.
+  //
+  bool TestRange(size_t start, size_t end) const {
+    return Rank1(end) > Rank1(start);
+  }
+
+  // Returns the offset to the nth set bit (zero based)
+  // or Bits() if index >= number of ones
+  size_t Select1(size_t bit_index) const;
+
+  // Returns the offset to the nth clear bit (zero based)
+  // or Bits() if index > number of
+  size_t Select0(size_t bit_index) const;
+
+  // Rebuilds from index for the associated Bitmap, should be called
+  // whenever changes have been made to the Bitmap or else behavior
+  // of the indexed bitmap methods will be undefined.
+  void BuildIndex(const uint64 *bits, size_t size);
+
+  // the secondary index accumulates counts until it can possibly overflow
+  // this constant computes the number of uint64 units that can fit into
+  // units the size of uint16.
+  static const uint64 kOne = 1;
+  static const uint32 kStorageBitSize = 64;
+  static const uint32 kStorageLogBitSize = 6;
+  static const uint32 kSecondaryBlockSize = ((1 << 16) - 1)
+      >> kStorageLogBitSize;
+
+ private:
+  static const uint32 kStorageBlockMask = kStorageBitSize - 1;
+
+  // returns, from the index, the count of ones up to array_index
+  size_t get_index_ones_count(size_t array_index) const;
+
+  // because the indexes, both primary and secondary, contain a running
+  // count of the population of one bits contained in [0,i), there is
+  // no reason to have an element in the zeroth position as this value would
+  // necessarily be zero.  (The bits are indexed in a zero based way.)  Thus
+  // we don't store the 0th element in either index.  Both of the following
+  // functions, if greater than 0, must be decremented by one before retreiving
+  // the value from the corresponding array.
+  // returns the 1 + the block that contains the bitindex in question
+  // the inverted version works the same but looks for zeros using an inverted
+  // view of the index
+  size_t find_primary_block(size_t bit_index) const;
+
+  size_t find_inverted_primary_block(size_t bit_index) const;
+
+  // similarly, the secondary index (which resets its count to zero at
+  // the end of every kSecondaryBlockSize entries) does not store the element
+  // at 0.  Note that the rem_bit_index parameter is the number of bits
+  // within the secondary block, after the bits accounted for by the primary
+  // block have been removed (i.e. the remaining bits)  And, because we
+  // reset to zero with each new block, there is no need to store those
+  // actual zeros.
+  // returns 1 + the secondary block that contains the bitindex in question
+  size_t find_secondary_block(size_t block, size_t rem_bit_index) const;
+
+  size_t find_inverted_secondary_block(size_t block, size_t rem_bit_index)
+      const;
+
+  // We create a primary index based upon the number of secondary index
+  // blocks.  The primary index uses fields wide enough to accomodate any
+  // index of the bitarray so cannot overflow
+  // The primary index is the actual running
+  // count of one bits set for all blocks (and, thus, all uint64s).
+  size_t primary_index_size() const {
+    return (ArraySize() + kSecondaryBlockSize - 1) / kSecondaryBlockSize;
+  }
+
+  const uint64* bits_;
+  size_t size_;
+
+  // The primary index contains the running popcount of all blocks
+  // which means the nth value contains the popcounts of
+  // [0,n*kSecondaryBlockSize], however, the 0th element is omitted.
+  vector<uint32> primary_index_;
+  // The secondary index contains the running popcount of the associated
+  // bitmap.  It is the same length (in units of uint16) as the
+  // bitmap's map is in units of uint64s.
+  vector<uint16> secondary_index_;
+};
+
+}  // end namespace fst
+
+#endif  // FST_EXTENSIONS_NGRAM_BITMAP_INDEX_H_