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