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Merged
ckormanyos merged 1 commit into from
Aug 14, 2025
Merged

Added comments for better readability #672

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29 changes: 21 additions & 8 deletions code_snippets/chapter17/chapter17_02-001_crc32_mpeg2.c
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Original file line number Diff line number Diff line change
Expand Up @@ -33,12 +33,13 @@ uint32_t Crc32_Mpeg2(const uint8_t* DataIn,
/* Set the initial value. */
uint32_t CrcResult = UINT32_C(0xFFFFFFFF);

/* Loop counter for iterating through input data */
size_t LoopCnt;

/* Loop through the input data stream */
for(LoopCnt = 0U; LoopCnt < DataLength; ++LoopCnt)
{
/* CRC32/MPEG2 Table based on nibbles. */
/* Lookup table for CRC32/MPEG-2 based on 4-bit nibbles */
static const uint32_t Crc32_Mpeg2_Table[16U] =
{
UINT32_C(0x00000000), UINT32_C(0x04C11DB7),
Expand All @@ -50,44 +51,55 @@ uint32_t Crc32_Mpeg2(const uint8_t* DataIn,
UINT32_C(0x350C9B64), UINT32_C(0x31CD86D3),
UINT32_C(0x3C8EA00A), UINT32_C(0x384FBDBD)
};


/*current byte from the input */
const uint8_t DataByte = DataIn[LoopCnt];

/* Perform the CRC32/MPEG2 algorithm. */

/* Index for first nibble lookup */
uint_fast8_t DataIndex =
( ((uint_fast8_t) (CrcResult >> 28U))
^ ((uint_fast8_t) (DataByte >> 4U)))
& UINT8_C(0x0F);
( ((uint_fast8_t) (CrcResult >> 28U))
^ ((uint_fast8_t) (DataByte >> 4U)))
& UINT8_C(0x0F);


CrcResult =

CrcResult =
(uint32_t) ( (uint32_t) (CrcResult << 4U)
& UINT32_C(0xFFFFFFF0))
^ Crc32_Mpeg2_Table[DataIndex];

DataIndex =

DataIndex =
(((uint_fast8_t) (CrcResult >> 28U))
^ ((uint_fast8_t) (DataByte))) & UINT8_C(0x0F);

CrcResult =
CrcResult =
(uint32_t) ( (uint32_t) (CrcResult << 4U)
& UINT32_C(0xFFFFFFF0))
^ Crc32_Mpeg2_Table[DataIndex];

}

return CrcResult;
}

int main(void)
{
/* Test data: ASCII values for characters '1' to '9' */
static const uint8_t TestData[9U] =
{
0x31U, 0x32U, 0x33U, 0x34U, 0x35U, 0x36U, 0x37U, 0x38U, 0x39U
};

/* Expected CRC32 result for the test data */
static const uint32_t ControlValue = UINT32_C(0x0376E6E7);

/* Compute CRC32 result for the test data */
const uint32_t TestResult = Crc32_Mpeg2(TestData, 9U);

/* Check if computed result matches expected value */
const bool TestResultIsOk =
(bool) ((TestResult == ControlValue) ? true : false);

Expand All @@ -100,5 +112,6 @@ int main(void)
printf("CRC32 test result is: Not OK\n");
}

/* Return 0 if test passed, -1 otherwise */
return ((TestResultIsOk == true) ? 0 : -1);
}
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