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https://github.com/MoleskiCoder/EightBit.git
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92 lines
1.8 KiB
C++
92 lines
1.8 KiB
C++
#pragma once
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#ifdef _MSC_VER
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# include <intrin.h>
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#endif
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#ifdef __GNUG__
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# include <x86intrin.h>
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#endif
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namespace EightBit {
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int countBits(uint8_t value);
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bool oddParity(uint8_t value);
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int findFirstSet(int value);
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void assume(int expression);
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}
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/*
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Published in 1988, the C Programming Language 2nd Ed.
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(by Brian W.Kernighan and Dennis M.Ritchie) mentions
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this in exercise 2 - 9. On April 19, 2006 Don Knuth pointed
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out to me that this method "was first published by Peter
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Wegner in CACM 3 (1960), 322.
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(Also discovered independently by Derrick Lehmer and published
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in 1964 in a book edited by Beckenbach.)"
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*/
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inline int EightBit::countBits(uint8_t value) {
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int count; // c accumulates the total bits set in value
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for (count = 0; value; ++count)
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value &= value - 1; // clear the least significant bit set
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return count;
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}
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inline bool EightBit::oddParity(uint8_t value) {
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return countBits(value) % 2;
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}
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inline int EightBit::findFirstSet(int value) {
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#ifdef _MSC_VER
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unsigned long index;
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if (_BitScanForward(&index, value))
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return index + 1;
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return 0;
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#elif defined(__GNUG__)
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return __builtin_ffs(value);
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#else
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# error Find first set not implemented
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#endif
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}
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inline void EightBit::assume(int expression) {
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#ifdef _MSC_VER
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__assume(expression);
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#elif defined(__GNUG__)
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if (!expression)
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__builtin_unreachable();
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#else
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assert(expression);
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#endif
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}
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#define ASSUME(x) EightBit::assume(x)
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#ifdef _MSC_VER
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# define LIKELY(x) (x)
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# define UNLIKELY(x) (x)
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# define PARITY(x) (__popcnt(x) % 2)
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# define UNREACHABLE ASSUME(0)
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#elif defined(__GNUG__)
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# define LIKELY(x) __builtin_expect(!!(x), 1)
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# define UNLIKELY(x) __builtin_expect(!!(x), 0)
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# define PARITY(x) __builtin_parity(x)
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# define UNREACHABLE __builtin_unreachable();
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#else
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# define LIKELY(x) (x)
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# define UNLIKELY(x) (x)
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# define PARITY(x) EightBit::oddParity(x)
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# define UNREACHABLE ASSUME(0)
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#endif
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