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 30 namespace libtextclassifier2 { 31 32 class 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 61 extern FastMathClass FastMathInstance; 62 63 inline float VeryFastExp2(float f) { return FastMathInstance.VeryFastExp2(f); } 64 inline float VeryFastExp(float f) { return FastMathInstance.VeryFastExp(f); } 65 66 } // namespace libtextclassifier2 67 68 #endif // LIBTEXTCLASSIFIER_UTIL_MATH_FASTEXP_H_ 69
