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/*
* Copyright 2013 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkTFitsIn_DEFINED
#define SkTFitsIn_DEFINED
#include <cassert>
#include <limits>
#include <type_traits>
/**
* In C++ an unsigned to signed cast where the source value cannot be represented in the destination
* type results in an implementation defined destination value. Unlike C, C++ does not allow a trap.
* This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is
* incorrect:
*
* when testing if a value of a smaller signed type can be represented in a larger unsigned type
* (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1
*
* when testing if a value of a larger unsigned type can be represented in a smaller signed type
* (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true.
*
* Consider the cases:
* u = unsigned, less digits
* U = unsigned, more digits
* s = signed, less digits
* S = signed, more digits
* v is the value we're considering.
*
* u -> U: (u)(U)v == v, trivially true
* U -> u: (U)(u)v == v, both casts well defined, test works
* s -> S: (s)(S)v == v, trivially true
* S -> s: (S)(s)v == v, first cast implementation value, second cast defined, test works
* s -> U: (s)(U)v == v, *this is bad*, the second cast results in implementation defined value
* S -> u: (S)(u)v == v, the second cast is required to prevent promotion of rhs to unsigned
* u -> S: (u)(S)v == v, trivially true
* U -> s: (U)(s)v == v, *this is bad*,
* first cast results in implementation defined value,
* second cast is defined. However, this creates false positives
* uint16_t x = 0xFFFF
* (uint16_t)(int8_t)x == x
* => (uint16_t)-1 == x
* => 0xFFFF == x
* => true
*
* So for the eight cases three are trivially true, three more are valid casts, and two are special.
* The two 'full' checks which otherwise require two comparisons are valid cast checks.
* The two remaining checks s -> U [v >= 0] and U -> s [v <= max(s)] can be done with one op.
*/
template <typename D, typename S>
static constexpr inline bool SkTFitsIn(S src) {
// SkTFitsIn() is used in public headers, so needs to be written targeting at most C++11.
return assert(std::is_integral<S>::value || std::is_enum<S>::value),
assert(std::is_integral<D>::value || std::is_enum<D>::value),
// E.g. (int8_t)(uint8_t) int8_t(-1) == -1, but the uint8_t == 255, not -1.
(std::is_signed<S>::value && std::is_unsigned<D>::value && sizeof(S) <= sizeof(D)) ?
(S)0 <= src :
// E.g. (uint8_t)(int8_t) uint8_t(255) == 255, but the int8_t == -1.
(std::is_signed<D>::value && std::is_unsigned<S>::value && sizeof(D) <= sizeof(S)) ?
src <= (S)std::numeric_limits<D>::max() :
// else
(S)(D)src == src;
}
#endif
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