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/**
* File: voicecrc.c
* Description: CRC check used by the Voice library functions in libultra.
*
* For general information about CRC, see the crc.c file (that's a lot of c's!).
*/
#include "ultra64.h"
#include "stddef.h"
#define VOICE_CRC_LENGTH 8
#define VOICE_CRC_GENERATOR 0x85
/**
* This function is essentially the same as __osContDataCrc, but allows for a variable message length, specified by
* `numBytes`.
*/
u8 __osVoiceContDataCrc(u8* data, size_t numBytes) {
s32 ret = 0;
u32 bit;
size_t byte;
for (byte = numBytes; byte != 0; byte--, data++) {
// Loop over each bit in the byte starting with most significant
for (bit = (1 << (VOICE_CRC_LENGTH - 1)); bit != 0; bit >>= 1) {
ret <<= 1;
if (*data & bit) {
if (ret & (1 << VOICE_CRC_LENGTH)) {
// Same as ret++; ret ^= 0x85 since last bit always 0 after the shift
ret ^= VOICE_CRC_GENERATOR - 1;
} else {
ret++;
}
} else if (ret & (1 << VOICE_CRC_LENGTH)) {
ret ^= VOICE_CRC_GENERATOR;
}
}
}
// Act like a byte of zeros is appended to data
do {
ret <<= 1;
if (ret & (1 << VOICE_CRC_LENGTH)) {
ret ^= VOICE_CRC_GENERATOR;
}
byte++;
} while (byte < VOICE_CRC_LENGTH);
// Discarding the excess is done automatically by the return type
return ret;
}
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