1/*
2 * Copyright (C) 2017 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17// Fast approximation for exp.
18
19#ifndef LIBTEXTCLASSIFIER_UTIL_MATH_FASTEXP_H_
20#define LIBTEXTCLASSIFIER_UTIL_MATH_FASTEXP_H_
21
22#include <cassert>
23#include <cmath>
24#include <limits>
25
26#include "util/base/casts.h"
27#include "util/base/integral_types.h"
28#include "util/base/logging.h"
29
30namespace libtextclassifier2 {
31
32class FastMathClass {
33 private:
34  static const int kBits = 7;
35  static const int kMask1 = (1 << kBits) - 1;
36  static const int kMask2 = 0xFF << kBits;
37  static constexpr float kLogBase2OfE = 1.44269504088896340736f;
38
39  struct Table {
40    int32 exp1[1 << kBits];
41  };
42
43 public:
44  float VeryFastExp2(float f) const {
45    TC_DCHECK_LE(fabs(f), 126);
46    const float g = f + (127 + (1 << (23 - kBits)));
47    const int32 x = bit_cast<int32>(g);
48    int32 ret = ((x & kMask2) << (23 - kBits))
49      | cache_.exp1[x & kMask1];
50    return bit_cast<float>(ret);
51  }
52
53  float VeryFastExp(float f) const {
54    return VeryFastExp2(f * kLogBase2OfE);
55  }
56
57 private:
58  static const Table cache_;
59};
60
61extern FastMathClass FastMathInstance;
62
63inline float VeryFastExp2(float f) { return FastMathInstance.VeryFastExp2(f); }
64inline float VeryFastExp(float f) { return FastMathInstance.VeryFastExp(f); }
65
66}  // namespace libtextclassifier2
67
68#endif  // LIBTEXTCLASSIFIER_UTIL_MATH_FASTEXP_H_
69