Atlas - SDL_murmur3.c
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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#include "SDL_internal.h"
22
23// Public domain murmur3 32-bit hash algorithm
24//
25// Adapted from: https://en.wikipedia.org/wiki/MurmurHash
26
27static SDL_INLINE Uint32 murmur_32_scramble(Uint32 k)
28{
29    k *= 0xcc9e2d51;
30    k = (k << 15) | (k >> 17);
31    k *= 0x1b873593;
32    return k;
33}
34
35Uint32 SDLCALL SDL_murmur3_32(const void *data, size_t len, Uint32 seed)
36{
37    const Uint8 *bytes = (const Uint8 *)data;
38    Uint32 hash = seed;
39    Uint32 k;
40
41    // Read in groups of 4.
42    if ((((uintptr_t)bytes) & 3) == 0) {
43        // We can do aligned 32-bit reads
44        for (size_t i = len >> 2; i--; ) {
45            k = *(const Uint32 *)bytes;
46            k = SDL_Swap32LE(k);
47            bytes += sizeof(Uint32);
48            hash ^= murmur_32_scramble(k);
49            hash = (hash << 13) | (hash >> 19);
50            hash = hash * 5 + 0xe6546b64;
51        }
52    } else {
53        for (size_t i = len >> 2; i--; ) {
54            SDL_memcpy(&k, bytes, sizeof(Uint32));
55            k = SDL_Swap32LE(k);
56            bytes += sizeof(Uint32);
57            hash ^= murmur_32_scramble(k);
58            hash = (hash << 13) | (hash >> 19);
59            hash = hash * 5 + 0xe6546b64;
60        }
61    }
62
63    // Read the rest.
64    size_t left = (len & 3);
65    if (left) {
66        k = 0;
67        for (size_t i = left; i--; ) {
68            k <<= 8;
69            k |= bytes[i];
70        }
71
72        // A swap is *not* necessary here because the preceding loop already
73        // places the low bytes in the low places according to whatever endianness
74        // we use. Swaps only apply when the memory is copied in a chunk.
75        hash ^= murmur_32_scramble(k);
76    }
77
78    /* Finalize. */
79    hash ^= len;
80    hash ^= hash >> 16;
81    hash *= 0x85ebca6b;
82    hash ^= hash >> 13;
83    hash *= 0xc2b2ae35;
84    hash ^= hash >> 16;
85
86    return hash;
87}
88
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