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[cubemap] / metacube2.cpp

diff --git a/metacube2.cpp b/metacube2.cpp
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+/*
+ * 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;
+}