utils/math/fastexp.h

// Copyright 2020 Google LLC
//
// 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
//
//     https://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.
//

// Fast approximation for exp.

#ifndef LIBTEXTCLASSIFIER_UTILS_MATH_FASTEXP_H_
#define LIBTEXTCLASSIFIER_UTILS_MATH_FASTEXP_H_

#include <cassert>
#include <cmath>
#include <limits>

#include "utils/base/casts.h"
#include "utils/base/integral_types.h"
#include "utils/base/logging.h"

namespace libtextclassifier3 {

class FastMathClass {
 private:
  static constexpr int kBits = 7;
  static constexpr int kMask1 = (1 << kBits) - 1;
  static constexpr int kMask2 = 0xFF << kBits;
  static constexpr float kLogBase2OfE = 1.44269504088896340736f;

  struct Table {
    int32 exp1[1 << kBits];
  };

 public:
  float VeryFastExp2(float f) const {
    TC3_DCHECK_LE(fabs(f), 126);
    const float g = f + (127 + (1 << (23 - kBits)));
    const int32 x = bit_cast<int32>(g);
    int32 ret = ((x & kMask2) << (23 - kBits))
      | cache_.exp1[x & kMask1];
    return bit_cast<float>(ret);
  }

  float VeryFastExp(float f) const {
    return VeryFastExp2(f * kLogBase2OfE);
  }

 private:
  static const Table cache_;
};

extern FastMathClass FastMathInstance;

inline float VeryFastExp(float f) { return FastMathInstance.VeryFastExp(f); }

}  // namespace libtextclassifier3

#endif  // LIBTEXTCLASSIFIER_UTILS_MATH_FASTEXP_H_