Added SDL_crc16() to be used in joystick GUIDs after 2.24.0

src/stdlib/SDL_crc16.c

@@ -0,0 +1,54 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 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.
+*/
+#include "../SDL_internal.h"
+
+#include "SDL_stdinc.h"
+
+
+/* Public domain CRC implementation adapted from:
+   http://home.thep.lu.se/~bjorn/crc/crc32_simple.c
+*/
+/* NOTE: DO NOT CHANGE THIS ALGORITHM
+   There is code that relies on this in the joystick code
+*/
+
+static Uint16 crc16_for_byte(Uint8 r)
+{
+    Uint16 crc = 0;
+    int i;
+    for (i = 0; i < 8; ++i) {
+        crc = ((crc ^ r) & 1? 0xA001 : 0) ^ crc >> 1;
+        r >>= 1;
+    }
+    return crc;
+}
+
+Uint16 SDL_crc16(Uint16 crc, const void *data, size_t len)
+{
+    /* As an optimization we can precalculate a 256 entry table for each byte */
+    size_t i;
+    for(i = 0; i < len; ++i) {
+        crc = crc16_for_byte((Uint8)crc ^ ((const Uint8*)data)[i]) ^ crc >> 8;
+    }
+    return crc;
+}
+
+/* vi: set ts=4 sw=4 expandtab: */