SDL_bits.h

00001 /*
00002   Simple DirectMedia Layer
00003   Copyright (C) 1997-2014 Sam Lantinga <slouken@libsdl.org>
00004 
00005   This software is provided 'as-is', without any express or implied
00006   warranty.  In no event will the authors be held liable for any damages
00007   arising from the use of this software.
00008 
00009   Permission is granted to anyone to use this software for any purpose,
00010   including commercial applications, and to alter it and redistribute it
00011   freely, subject to the following restrictions:
00012 
00013   1. The origin of this software must not be misrepresented; you must not
00014      claim that you wrote the original software. If you use this software
00015      in a product, an acknowledgment in the product documentation would be
00016      appreciated but is not required.
00017   2. Altered source versions must be plainly marked as such, and must not be
00018      misrepresented as being the original software.
00019   3. This notice may not be removed or altered from any source distribution.
00020 */
00021 
00022 /**
00023  *  \file SDL_bits.h
00024  *
00025  *  Functions for fiddling with bits and bitmasks.
00026  */
00027 
00028 #ifndef _SDL_bits_h
00029 #define _SDL_bits_h
00030 
00031 #include "SDL_stdinc.h"
00032 
00033 #include "begin_code.h"
00034 /* Set up for C function definitions, even when using C++ */
00035 #ifdef __cplusplus
00036 extern "C" {
00037 #endif
00038 
00039 /**
00040  *  \file SDL_bits.h
00041  */
00042 
00043 /**
00044  *  Get the index of the most significant bit. Result is undefined when called
00045  *  with 0. This operation can also be stated as "count leading zeroes" and
00046  *  "log base 2".
00047  *
00048  *  \return Index of the most significant bit, or -1 if the value is 0.
00049  */
00050 SDL_FORCE_INLINE int
00051 SDL_MostSignificantBitIndex32(Uint32 x)
00052 {
00053 #if defined(__GNUC__) && __GNUC__ >= 4
00054     /* Count Leading Zeroes builtin in GCC.
00055      * http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Other-Builtins.html
00056      */
00057     if (x == 0) {
00058         return -1;
00059     }
00060     return 31 - __builtin_clz(x);
00061 #else
00062     /* Based off of Bit Twiddling Hacks by Sean Eron Anderson
00063      * <seander@cs.stanford.edu>, released in the public domain.
00064      * http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
00065      */
00066     const Uint32 b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000};
00067     const int    S[] = {1, 2, 4, 8, 16};
00068 
00069     int msbIndex = 0;
00070     int i;
00071 
00072     if (x == 0) {
00073         return -1;
00074     }
00075 
00076     for (i = 4; i >= 0; i--)
00077     {
00078         if (x & b[i])
00079         {
00080             x >>= S[i];
00081             msbIndex |= S[i];
00082         }
00083     }
00084 
00085     return msbIndex;
00086 #endif
00087 }
00088 
00089 /* Ends C function definitions when using C++ */
00090 #ifdef __cplusplus
00091 }
00092 #endif
00093 #include "close_code.h"
00094 
00095 #endif /* _SDL_bits_h */
00096 
00097 /* vi: set ts=4 sw=4 expandtab: */