X-Git-Url: https://git.sesse.net/?p=cubemap;a=blobdiff_plain;f=metacube2.cpp;fp=metacube2.cpp;h=f7e4ce20c9468c572b97390a8a320008474fda39;hp=0000000000000000000000000000000000000000;hb=979a284b4039b0ea74525b700b9f1089b8c4248d;hpb=70c0baf4bcec3a77f0626d5a7bfde87fc7339698

diff --git a/metacube2.cpp b/metacube2.cpp
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+++ b/metacube2.cpp
@@ -0,0 +1,50 @@
+/*
+ * Implementation of Metacube2 utility functions.
+ *
+ * Note: This file is meant to compile as both C and C++, for easier inclusion
+ * in other projects.
+ */
+
+#include <stdlib.h>
+
+#include "metacube2.h"
+
+/*
+ * https://www.ece.cmu.edu/~koopman/pubs/KoopmanCRCWebinar9May2012.pdf
+ * recommends this for messages as short as ours (see table at page 34).
+ */
+#define METACUBE2_CRC_POLYNOMIAL 0x8FDB
+
+/* Semi-random starting value to make sure all-zero won't pass. */
+#define METACUBE2_CRC_START 0x1234
+
+/* This code is based on code generated by pycrc. */
+uint16_t metacube2_compute_crc(const struct metacube2_block_header *hdr)
+{
+	static const int data_len = sizeof(hdr->size) + sizeof(hdr->flags);
+	const uint8_t *data = (uint8_t *)&hdr->size;
+	uint16_t crc = METACUBE2_CRC_START;
+	int i, j;
+
+	for (i = 0; i < data_len; ++i) {	
+		uint8_t c = data[i];
+		for (j = 0; j < 8; j++) {
+			int bit = crc & 0x8000;
+			crc = (crc << 1) | ((c >> (7 - j)) & 0x01);
+			if (bit) {
+				crc ^= METACUBE2_CRC_POLYNOMIAL;
+			}
+		}
+	}
+
+	/* Finalize. */
+	for (i = 0; i < 16; i++) {
+		int bit = crc & 0x8000;
+		crc = crc << 1;
+		if (bit) {
+			crc ^= METACUBE2_CRC_POLYNOMIAL;
+		}
+	}
+
+	return crc;
+}