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      1 /*
      2   Simple DirectMedia Layer
      3   Copyright (C) 1997-2014 Sam Lantinga <slouken (at) 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, or -1 if the value is 0.
     49  */
     50 SDL_FORCE_INLINE int
     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     if (x == 0) {
     58         return -1;
     59     }
     60     return 31 - __builtin_clz(x);
     61 #else
     62     /* Based off of Bit Twiddling Hacks by Sean Eron Anderson
     63      * <seander (at) cs.stanford.edu>, released in the public domain.
     64      * http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
     65      */
     66     const Uint32 b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000};
     67     const int    S[] = {1, 2, 4, 8, 16};
     68 
     69     int msbIndex = 0;
     70     int i;
     71 
     72     if (x == 0) {
     73         return -1;
     74     }
     75 
     76     for (i = 4; i >= 0; i--)
     77     {
     78         if (x & b[i])
     79         {
     80             x >>= S[i];
     81             msbIndex |= S[i];
     82         }
     83     }
     84 
     85     return msbIndex;
     86 #endif
     87 }
     88 
     89 /* Ends C function definitions when using C++ */
     90 #ifdef __cplusplus
     91 }
     92 #endif
     93 #include "close_code.h"
     94 
     95 #endif /* _SDL_bits_h */
     96 
     97 /* vi: set ts=4 sw=4 expandtab: */
     98