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59 lines
1.5 KiB
59 lines
1.5 KiB
/*
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* Copyright 2021 Google LLC
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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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#pragma once
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#include <assert.h>
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#include <bit>
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#include <cstdint>
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#include <limits>
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class BitsUtil {
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public:
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// ⌊log2(n)⌋, resp. numbers of bits needed to represent n in binary encoding
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// minus one. Input cannot be zero.
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// Same as util/bits/Bits::Log2FloorNonZero64(..).
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static uint8_t Log2FloorNonZero64(uint64_t n);
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private:
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static uint8_t CountLeadingZeros64(uint64_t x);
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};
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inline uint8_t BitsUtil::Log2FloorNonZero64(uint64_t n) {
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assert(n != 0);
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return std::numeric_limits<uint64_t>::digits - CountLeadingZeros64(n) - 1;
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}
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inline uint8_t BitsUtil::CountLeadingZeros64(uint64_t x) {
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uint8_t zeroes = 60;
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if (x >> 32) {
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zeroes -= 32;
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x >>= 32;
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}
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if (x >> 16) {
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zeroes -= 16;
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x >>= 16;
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}
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if (x >> 8) {
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zeroes -= 8;
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x >>= 8;
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}
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if (x >> 4) {
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zeroes -= 4;
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x >>= 4;
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}
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return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[x] + zeroes;
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}
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