зеркало из https://github.com/mozilla/moz-skia.git
87 строки
2.7 KiB
C++
87 строки
2.7 KiB
C++
/*
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* Copyright 2012 Google Inc.
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*
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* Use of this source code is governed by a BSD-style license that can be
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* found in the LICENSE file.
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*/
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#ifndef SkChecksum_DEFINED
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#define SkChecksum_DEFINED
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#include "SkTypes.h"
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class SkChecksum : SkNoncopyable {
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private:
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/*
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* Our Rotate and Mash helpers are meant to automatically do the right
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* thing depending if sizeof(uintptr_t) is 4 or 8.
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*/
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enum {
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ROTR = 17,
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ROTL = sizeof(uintptr_t) * 8 - ROTR,
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HALFBITS = sizeof(uintptr_t) * 4
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};
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static inline uintptr_t Mash(uintptr_t total, uintptr_t value) {
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return ((total >> ROTR) | (total << ROTL)) ^ value;
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}
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public:
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/**
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* Compute a 32-bit checksum for a given data block
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*
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* @param data Memory address of the data block to be processed. Must be
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* 32-bit aligned.
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* @param size Size of the data block in bytes. Must be a multiple of 4.
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* @return checksum result
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*/
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static uint32_t Compute(const uint32_t* data, size_t size) {
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SkASSERT(SkIsAlign4(size));
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/*
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* We want to let the compiler use 32bit or 64bit addressing and math
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* so we use uintptr_t as our magic type. This makes the code a little
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* more obscure (we can't hard-code 32 or 64 anywhere, but have to use
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* sizeof()).
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*/
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uintptr_t result = 0;
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const uintptr_t* ptr = reinterpret_cast<const uintptr_t*>(data);
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/*
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* count the number of quad element chunks. This takes into account
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* if we're on a 32bit or 64bit arch, since we use sizeof(uintptr_t)
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* to compute how much to shift-down the size.
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*/
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size_t n4 = size / (sizeof(uintptr_t) << 2);
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for (size_t i = 0; i < n4; ++i) {
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result = Mash(result, *ptr++);
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result = Mash(result, *ptr++);
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result = Mash(result, *ptr++);
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result = Mash(result, *ptr++);
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}
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size &= ((sizeof(uintptr_t) << 2) - 1);
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data = reinterpret_cast<const uint32_t*>(ptr);
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const uint32_t* stop = data + (size >> 2);
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while (data < stop) {
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result = Mash(result, *data++);
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}
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/*
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* smash us down to 32bits if we were 64. Note that when uintptr_t is
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* 32bits, this code-path should go away, but I still got a warning
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* when I wrote
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* result ^= result >> 32;
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* since >>32 is undefined for 32bit ints, hence the wacky HALFBITS
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* define.
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*/
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if (8 == sizeof(result)) {
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result ^= result >> HALFBITS;
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}
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return static_cast<uint32_t>(result);
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}
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};
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#endif
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