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code/SDL2/include/SDL_bits.h

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+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2014 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/**
+ *  \file SDL_bits.h
+ *
+ *  Functions for fiddling with bits and bitmasks.
+ */
+
+#ifndef _SDL_bits_h
+#define _SDL_bits_h
+
+#include "SDL_stdinc.h"
+
+#include "begin_code.h"
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ *  \file SDL_bits.h
+ */
+
+/**
+ *  Get the index of the most significant bit. Result is undefined when called
+ *  with 0. This operation can also be stated as "count leading zeroes" and
+ *  "log base 2".
+ *
+ *  \return Index of the most significant bit, or -1 if the value is 0.
+ */
+SDL_FORCE_INLINE int
+SDL_MostSignificantBitIndex32(Uint32 x)
+{
+#if defined(__GNUC__) && __GNUC__ >= 4
+    /* Count Leading Zeroes builtin in GCC.
+     * http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Other-Builtins.html
+     */
+    if (x == 0) {
+        return -1;
+    }
+    return 31 - __builtin_clz(x);
+#else
+    /* Based off of Bit Twiddling Hacks by Sean Eron Anderson
+     * <seander@cs.stanford.edu>, released in the public domain.
+     * http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
+     */
+    const Uint32 b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000};
+    const int    S[] = {1, 2, 4, 8, 16};
+
+    int msbIndex = 0;
+    int i;
+
+    if (x == 0) {
+        return -1;
+    }
+
+    for (i = 4; i >= 0; i--)
+    {
+        if (x & b[i])
+        {
+            x >>= S[i];
+            msbIndex |= S[i];
+        }
+    }
+
+    return msbIndex;
+#endif
+}
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include "close_code.h"
+
+#endif /* _SDL_bits_h */
+
+/* vi: set ts=4 sw=4 expandtab: */