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