/*
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* Copyright (C) 2018 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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// Fast approximation for exp.
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#ifndef LIBTEXTCLASSIFIER_UTILS_MATH_FASTEXP_H_
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#define LIBTEXTCLASSIFIER_UTILS_MATH_FASTEXP_H_
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#include <cassert>
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#include <cmath>
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#include <limits>
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#include "utils/base/casts.h"
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#include "utils/base/integral_types.h"
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#include "utils/base/logging.h"
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namespace libtextclassifier3 {
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class FastMathClass {
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private:
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static const int kBits = 7;
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static const int kMask1 = (1 << kBits) - 1;
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static const int kMask2 = 0xFF << kBits;
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static constexpr float kLogBase2OfE = 1.44269504088896340736f;
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struct Table {
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int32 exp1[1 << kBits];
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};
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public:
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float VeryFastExp2(float f) const {
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TC3_DCHECK_LE(fabs(f), 126);
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const float g = f + (127 + (1 << (23 - kBits)));
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const int32 x = bit_cast<int32>(g);
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int32 ret = ((x & kMask2) << (23 - kBits))
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| cache_.exp1[x & kMask1];
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return bit_cast<float>(ret);
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}
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float VeryFastExp(float f) const {
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return VeryFastExp2(f * kLogBase2OfE);
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}
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private:
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static const Table cache_;
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};
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extern FastMathClass FastMathInstance;
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inline float VeryFastExp(float f) { return FastMathInstance.VeryFastExp(f); }
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} // namespace libtextclassifier3
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#endif // LIBTEXTCLASSIFIER_UTILS_MATH_FASTEXP_H_
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