1 files changed, 64 insertions, 0 deletions
diff --git a/src/lib/crc16.c b/src/lib/crc16.c
new file mode 100644
index 00000000..55af8647
--- /dev/null
+++ b/src/lib/crc16.c
@@ -0,0 +1,64 @@
+/*
+ * Ouroboros - Copyright (C) 2016 - 2026
+ *
+ * 16-bit Cyclic Redundancy Check (CCITT-FALSE variant)
+ *
+ *    Dimitri Staessens <dimitri@ouroboros.rocks>
+ *    Sander Vrijders   <sander@ouroboros.rocks>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * version 2.1 as published by the Free Software Foundation.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., http://www.fsf.org/about/contact/.
+ */
+
+/*
+ * CRC-16/CCITT-FALSE (reveng catalog, alias CRC-16/IBM-3740):
+ *   poly   = 0x1021
+ *   init   = 0xffff
+ *   refin  = false
+ *   refout = false
+ *   xorout = 0x0000
+ *   check  = crc16_ccitt_false("123456789") == 0x29b1
+ */
+
+#include "config.h"
+
+#include <ouroboros/crc16.h>
+
+/* Bit-by-bit MSB-first CRC.  Header-check use case rarely exceeds a
+ * few hundred bytes; a 256-entry uint16_t table costs 512 B of
+ * .rodata and is easy to add later if profiling demands it.
+ */
+void crc16_ccitt_false(uint16_t *   crc,
+                       const void * buf,
+                       size_t       len)
+{
+        const uint8_t * p;
+        uint16_t        c;
+        size_t          n;
+        int             i;
+
+        p = (const uint8_t *) buf;
+        c = *crc ^ 0xffff;
+
+        for (n = 0; n < len; n++) {
+                c ^= ((uint16_t) p[n]) << 8;
+                for (i = 0; i < 8; i++) {
+                        if (c & 0x8000)
+                                c = (uint16_t) ((c << 1) ^ 0x1021);
+                        else
+                                c = (uint16_t) (c << 1);
+                }
+        }
+
+        *crc = c;
+}