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/*
* Copyright (C) 2022 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef SkSafeMath_DEFINED
#define SkSafeMath_DEFINED
#include <cstddef>
#include <cstdint>
#include <limits>
// Copy of Skia's SafeMath API used to validate Mesh parameters to support
// deferred creation of SkMesh instances on RenderThread.
// SafeMath always check that a series of operations do not overflow.
// This must be correct for all platforms, because this is a check for safety at runtime.
class SafeMath {
public:
SafeMath() = default;
bool ok() const { return fOK; }
explicit operator bool() const { return fOK; }
size_t mul(size_t x, size_t y) {
return sizeof(size_t) == sizeof(uint64_t) ? mul64(x, y) : mul32(x, y);
}
size_t add(size_t x, size_t y) {
size_t result = x + y;
fOK &= result >= x;
return result;
}
/**
* Return a + b, unless this result is an overflow/underflow. In those cases, fOK will
* be set to false, and it is undefined what this returns.
*/
int addInt(int a, int b) {
if (b < 0 && a < std::numeric_limits<int>::min() - b) {
fOK = false;
return a;
} else if (b > 0 && a > std::numeric_limits<int>::max() - b) {
fOK = false;
return a;
}
return a + b;
}
// These saturate to their results
static size_t Add(size_t x, size_t y) {
SafeMath tmp;
size_t sum = tmp.add(x, y);
return tmp.ok() ? sum : SIZE_MAX;
}
static size_t Mul(size_t x, size_t y) {
SafeMath tmp;
size_t prod = tmp.mul(x, y);
return tmp.ok() ? prod : SIZE_MAX;
}
private:
uint32_t mul32(uint32_t x, uint32_t y) {
uint64_t bx = x;
uint64_t by = y;
uint64_t result = bx * by;
fOK &= result >> 32 == 0;
// Overflow information is capture in fOK. Return the result modulo 2^32.
return (uint32_t)result;
}
uint64_t mul64(uint64_t x, uint64_t y) {
if (x <= std::numeric_limits<uint64_t>::max() >> 32 &&
y <= std::numeric_limits<uint64_t>::max() >> 32) {
return x * y;
} else {
auto hi = [](uint64_t x) { return x >> 32; };
auto lo = [](uint64_t x) { return x & 0xFFFFFFFF; };
uint64_t lx_ly = lo(x) * lo(y);
uint64_t hx_ly = hi(x) * lo(y);
uint64_t lx_hy = lo(x) * hi(y);
uint64_t hx_hy = hi(x) * hi(y);
uint64_t result = 0;
result = this->add(lx_ly, (hx_ly << 32));
result = this->add(result, (lx_hy << 32));
fOK &= (hx_hy + (hx_ly >> 32) + (lx_hy >> 32)) == 0;
return result;
}
}
bool fOK = true;
};
#endif // SkSafeMath_DEFINED