1
2// Licensed under the Apache License, Version 2.0 (the "License");
3// you may not use this file except in compliance with the License.
4// You may obtain a copy of the License at
5//
6//     http://www.apache.org/licenses/LICENSE-2.0
7//
8// Unless required by applicable law or agreed to in writing, software
9// distributed under the License is distributed on an "AS IS" BASIS,
10// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
11// See the License for the specific language governing permissions and
12// limitations under the License.
13//
14// Copyright 2005-2010 Google, Inc.
15// Author: sorenj@google.com (Jeffrey Sorensen)
16//         dr@google.com (Doug Rohde)
17
18#ifndef FST_EXTENSIONS_NGRAM_NTHBIT_H_
19#define FST_EXTENSIONS_NGRAM_NTHBIT_H_
20
21#include <fst/types.h>
22
23extern uint32 nth_bit_bit_offset[];
24
25inline uint32 nth_bit(uint64 v, uint32 r) {
26  uint32 shift = 0;
27  uint32 c = __builtin_popcount(v & 0xffffffff);
28  uint32 mask = -(r > c);
29  r -= c & mask;
30  shift += (32 & mask);
31
32  c = __builtin_popcount((v >> shift) & 0xffff);
33  mask = -(r > c);
34  r -= c & mask;
35  shift += (16 & mask);
36
37  c = __builtin_popcount((v >> shift) & 0xff);
38  mask = -(r > c);
39  r -= c & mask;
40  shift += (8 & mask);
41
42  return shift + ((nth_bit_bit_offset[(v >> shift) & 0xff] >>
43                   ((r - 1) << 2)) & 0xf);
44}
45
46#endif  // FST_EXTENSIONS_NGRAM_NTHBIT_H_
47