1/*
2 * Copyright 2013 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8#ifndef SkTFitsIn_DEFINED
9#define SkTFitsIn_DEFINED
10
11#include "include/private/base/SkDebug.h"
12
13#include <cstdint>
14#include <limits>
15#include <type_traits>
16
17/**
18 * std::underlying_type is only defined for enums. For integral types, we just want the type.
19 */
20template <typename T, class Enable = void>
21struct sk_strip_enum {
22 typedef T type;
23};
24
25template <typename T>
26struct sk_strip_enum<T, typename std::enable_if<std::is_enum<T>::value>::type> {
27 typedef typename std::underlying_type<T>::type type;
28};
29
30
31/**
32 * In C++ an unsigned to signed cast where the source value cannot be represented in the destination
33 * type results in an implementation defined destination value. Unlike C, C++ does not allow a trap.
34 * This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is
35 * incorrect:
36 *
37 * when testing if a value of a smaller signed type can be represented in a larger unsigned type
38 * (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1
39 *
40 * when testing if a value of a larger unsigned type can be represented in a smaller signed type
41 * (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true.
42 *
43 * Consider the cases:
44 * u = unsigned, less digits
45 * U = unsigned, more digits
46 * s = signed, less digits
47 * S = signed, more digits
48 * v is the value we're considering.
49 *
50 * u -> U: (u)(U)v == v, trivially true
51 * U -> u: (U)(u)v == v, both casts well defined, test works
52 * s -> S: (s)(S)v == v, trivially true
53 * S -> s: (S)(s)v == v, first cast implementation value, second cast defined, test works
54 * s -> U: (s)(U)v == v, *this is bad*, the second cast results in implementation defined value
55 * S -> u: (S)(u)v == v, the second cast is required to prevent promotion of rhs to unsigned
56 * u -> S: (u)(S)v == v, trivially true
57 * U -> s: (U)(s)v == v, *this is bad*,
58 * first cast results in implementation defined value,
59 * second cast is defined. However, this creates false positives
60 * uint16_t x = 0xFFFF
61 * (uint16_t)(int8_t)x == x
62 * => (uint16_t)-1 == x
63 * => 0xFFFF == x
64 * => true
65 *
66 * So for the eight cases three are trivially true, three more are valid casts, and two are special.
67 * The two 'full' checks which otherwise require two comparisons are valid cast checks.
68 * The two remaining checks s -> U [v >= 0] and U -> s [v <= max(s)] can be done with one op.
69 */
70
71template <typename D, typename S>
72static constexpr inline
73typename std::enable_if<(std::is_integral<S>::value || std::is_enum<S>::value) &&
74 (std::is_integral<D>::value || std::is_enum<D>::value), bool>::type
75/*bool*/ SkTFitsIn(S src) {
76 // Ensure that is_signed and is_unsigned are passed the arithmetic underlyng types of enums.
77 using Sa = typename sk_strip_enum<S>::type;
78 using Da = typename sk_strip_enum<D>::type;
79
80 // SkTFitsIn() is used in public headers, so needs to be written targeting at most C++11.
81 return
82
83 // E.g. (int8_t)(uint8_t) int8_t(-1) == -1, but the uint8_t == 255, not -1.
84 (std::is_signed<Sa>::value && std::is_unsigned<Da>::value && sizeof(Sa) <= sizeof(Da)) ?
85 (S)0 <= src :
86
87 // E.g. (uint8_t)(int8_t) uint8_t(255) == 255, but the int8_t == -1.
88 (std::is_signed<Da>::value && std::is_unsigned<Sa>::value && sizeof(Da) <= sizeof(Sa)) ?
89 src <= (S)std::numeric_limits<Da>::max() :
90
91#if !defined(SK_DEBUG) && !defined(__MSVC_RUNTIME_CHECKS )
92 // Correct (simple) version. This trips up MSVC's /RTCc run-time checking.
93 (S)(D)src == src;
94#else
95 // More complex version that's safe with /RTCc. Used in all debug builds, for coverage.
96 (std::is_signed<Sa>::value) ?
97 (intmax_t)src >= (intmax_t)std::numeric_limits<Da>::min() &&
98 (intmax_t)src <= (intmax_t)std::numeric_limits<Da>::max() :
99
100 // std::is_unsigned<S> ?
101 (uintmax_t)src <= (uintmax_t)std::numeric_limits<Da>::max();
102#endif
103}
104
105#endif
106

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source code of flutter_engine/third_party/skia/include/private/base/SkTFitsIn.h