Atlas - SDL_bits.h
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1/*
2  Simple DirectMedia Layer
3  Copyright (C) 1997-2025 Sam Lantinga <[email protected]>
4
5  This software is provided 'as-is', without any express or implied
6  warranty.  In no event will the authors be held liable for any damages
7  arising from the use of this software.
8
9  Permission is granted to anyone to use this software for any purpose,
10  including commercial applications, and to alter it and redistribute it
11  freely, subject to the following restrictions:
12
13  1. The origin of this software must not be misrepresented; you must not
14     claim that you wrote the original software. If you use this software
15     in a product, an acknowledgment in the product documentation would be
16     appreciated but is not required.
17  2. Altered source versions must be plainly marked as such, and must not be
18     misrepresented as being the original software.
19  3. This notice may not be removed or altered from any source distribution.
20*/
21
22/**
23 * # CategoryBits
24 *
25 * Functions for fiddling with bits and bitmasks.
26 */
27
28#ifndef SDL_bits_h_
29#define SDL_bits_h_
30
31#include <SDL3/SDL_stdinc.h>
32
33#include <SDL3/SDL_begin_code.h>
34/* Set up for C function definitions, even when using C++ */
35#ifdef __cplusplus
36extern "C" {
37#endif
38
39#if defined(__WATCOMC__) && defined(__386__)
40extern __inline int _SDL_bsr_watcom(Uint32);
41#pragma aux _SDL_bsr_watcom = \
42    "bsr eax, eax" \
43    parm [eax] nomemory \
44    value [eax] \
45    modify exact [eax] nomemory;
46#endif
47
48/**
49 * Get the index of the most significant (set) bit in a 32-bit number.
50 *
51 * Result is undefined when called with 0. This operation can also be stated
52 * as "count leading zeroes" and "log base 2".
53 *
54 * Note that this is a forced-inline function in a header, and not a public
55 * API function available in the SDL library (which is to say, the code is
56 * embedded in the calling program and the linker and dynamic loader will not
57 * be able to find this function inside SDL itself).
58 *
59 * \param x the 32-bit value to examine.
60 * \returns the index of the most significant bit, or -1 if the value is 0.
61 *
62 * \threadsafety It is safe to call this function from any thread.
63 *
64 * \since This function is available since SDL 3.2.0.
65 */
66SDL_FORCE_INLINE int SDL_MostSignificantBitIndex32(Uint32 x)
67{
68#if defined(__GNUC__) && (__GNUC__ >= 4 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4))
69    /* Count Leading Zeroes builtin in GCC.
70     * http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Other-Builtins.html
71     */
72    if (x == 0) {
73        return -1;
74    }
75    return 31 - __builtin_clz(x);
76#elif defined(__WATCOMC__) && defined(__386__)
77    if (x == 0) {
78        return -1;
79    }
80    return _SDL_bsr_watcom(x);
81#elif defined(_MSC_VER) && _MSC_VER >= 1400
82    unsigned long index;
83    if (_BitScanReverse(&index, x)) {
84        return (int)index;
85    }
86    return -1;
87#else
88    /* Based off of Bit Twiddling Hacks by Sean Eron Anderson
89     * <[email protected]>, released in the public domain.
90     * http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
91     */
92    const Uint32 b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000};
93    const int    S[] = {1, 2, 4, 8, 16};
94
95    int msbIndex = 0;
96    int i;
97
98    if (x == 0) {
99        return -1;
100    }
101
102    for (i = 4; i >= 0; i--)
103    {
104        if (x & b[i])
105        {
106            x >>= S[i];
107            msbIndex |= S[i];
108        }
109    }
110
111    return msbIndex;
112#endif
113}
114
115/**
116 * Determine if a unsigned 32-bit value has exactly one bit set.
117 *
118 * If there are no bits set (`x` is zero), or more than one bit set, this
119 * returns false. If any one bit is exclusively set, this returns true.
120 *
121 * Note that this is a forced-inline function in a header, and not a public
122 * API function available in the SDL library (which is to say, the code is
123 * embedded in the calling program and the linker and dynamic loader will not
124 * be able to find this function inside SDL itself).
125 *
126 * \param x the 32-bit value to examine.
127 * \returns true if exactly one bit is set in `x`, false otherwise.
128 *
129 * \threadsafety It is safe to call this function from any thread.
130 *
131 * \since This function is available since SDL 3.2.0.
132 */
133SDL_FORCE_INLINE bool SDL_HasExactlyOneBitSet32(Uint32 x)
134{
135    if (x && !(x & (x - 1))) {
136        return true;
137    }
138    return false;
139}
140
141/* Ends C function definitions when using C++ */
142#ifdef __cplusplus
143}
144#endif
145#include <SDL3/SDL_close_code.h>
146
147#endif /* SDL_bits_h_ */
148
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