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56 lines
1.4 KiB
56 lines
1.4 KiB
/*
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* Copyright 2013 Google Inc.
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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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class Log2 {
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public:
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Log2();
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static void init();
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// Q24 in, Q24 out
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static int32_t lookup(uint32_t x);
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};
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#define LOG2_LG_N_SAMPLES 9
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#define LOG2_N_SAMPLES (1 << LOG2_LG_N_SAMPLES)
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#define LOG2_INLINE
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extern int32_t log2tab[LOG2_N_SAMPLES << 1];
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#ifdef __GNUC__
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static int clz(unsigned int x) {
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return __builtin_clz(x);
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}
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// TODO: other implementations (including ANSI C)
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#endif
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#ifdef LOG2_INLINE
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inline
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int32_t Log2::lookup(uint32_t x) {
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int exp = clz(x | 1);
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unsigned int y = x << exp;
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const int SHIFT = 31 - LOG2_LG_N_SAMPLES;
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int lowbits = y & ((1 << SHIFT) - 1);
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int y_int = (y >> (SHIFT - 1)) & ((LOG2_N_SAMPLES - 1) << 1);
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int dz = log2tab[y_int];
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int z0 = log2tab[y_int + 1];
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int z = z0 + (((int64_t)dz * (int64_t)lowbits) >> SHIFT);
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return z - (exp << 24);
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}
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#endif
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