include/SDL_bits.h
author Sam Lantinga <slouken@libsdl.org>
Sat, 18 May 2013 14:17:52 -0700
changeset 7191 75360622e65f
parent 7033 3b12bae22f9e
child 7597 b7219f6cf67a
permissions -rw-r--r--
File style cleanup for the SDL 2.0 release
     1 /*
     2   Simple DirectMedia Layer
     3   Copyright (C) 1997-2013 Sam Lantinga <slouken@libsdl.org>
     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  *  \file SDL_bits.h
    24  *
    25  *  Functions for fiddling with bits and bitmasks.
    26  */
    27 
    28 #ifndef _SDL_bits_h
    29 #define _SDL_bits_h
    30 
    31 #include "SDL_stdinc.h"
    32 
    33 #include "begin_code.h"
    34 /* Set up for C function definitions, even when using C++ */
    35 #ifdef __cplusplus
    36 extern "C" {
    37 #endif
    38 
    39 /**
    40  *  \file SDL_bits.h
    41  */
    42 
    43 /**
    44  *  Get the index of the most significant bit. Result is undefined when called
    45  *  with 0. This operation can also be stated as "count leading zeroes" and
    46  *  "log base 2".
    47  *
    48  *  \return Index of the most significant bit.
    49  */
    50 SDL_FORCE_INLINE Sint8
    51 SDL_MostSignificantBitIndex32(Uint32 x)
    52 {
    53 #if defined(__GNUC__) && __GNUC__ >= 4
    54     /* Count Leading Zeroes builtin in GCC.
    55      * http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Other-Builtins.html
    56      */
    57     return 31 - __builtin_clz(x);
    58 #else
    59     /* Based off of Bit Twiddling Hacks by Sean Eron Anderson
    60      * <seander@cs.stanford.edu>, released in the public domain.
    61      * http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
    62      */
    63     const Uint32 b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000};
    64     const Uint8  S[] = {1, 2, 4, 8, 16};
    65 
    66     Uint8 msbIndex = 0;
    67     int i;
    68 
    69     for (i = 4; i >= 0; i--)
    70     {
    71         if (x & b[i])
    72         {
    73             x >>= S[i];
    74             msbIndex |= S[i];
    75         }
    76     }
    77 
    78     return msbIndex;
    79 #endif
    80 }
    81 
    82 /* Ends C function definitions when using C++ */
    83 #ifdef __cplusplus
    84 }
    85 #endif
    86 #include "close_code.h"
    87 
    88 #endif /* _SDL_bits_h */
    89 
    90 /* vi: set ts=4 sw=4 expandtab: */