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ssffcsum — 16-bit Fletcher Checksum

SSF | EDC

16-bit Fletcher checksum for lightweight data integrity verification.

The checksum has error-detection capability comparable to a 16-bit CRC at a fraction of the code size (approximately 88 bytes on MSP430 at -O3) and requires no lookup table. A single call covers a contiguous buffer; successive calls with the previous return value as initial accumulate across non-contiguous or streaming chunks and produce the same result as one call over the whole dataset.

Dependencies | Notes | Configuration | API Summary | Function Reference

↑ Dependencies

↑ Notes

  • Always pass SSF_FCSUM_INITIAL as initial for the first call in a sequence; pass the return value of the previous call for each subsequent chunk.
  • Requires no lookup table; approximately 88 bytes of program memory on MSP430 with -O3.
  • Error-detection capability is comparable to a 16-bit CRC but with simpler, smaller code.
  • For stronger error detection see ssfcrc16 or ssfcrc32.

↑ Configuration

This module has no compile-time configuration options in ssfoptions.h.

↑ API Summary

Definitions

Symbol Kind Description
SSF_FCSUM_INITIAL Constant 0 — initial checksum state; pass as initial to begin a fresh computation

Functions

Function Description
e.g. uint16_t SSFFCSum16(in, inLen, initial) Compute or accumulate a 16-bit Fletcher checksum over a byte buffer

↑ Function Reference

↑ uint16_t SSFFCSum16()

uint16_t SSFFCSum16(const uint8_t *in, size_t inLen, uint16_t initial);

Computes or accumulates a 16-bit Fletcher checksum over inLen bytes starting at in. May be called repeatedly on successive chunks; passing the return value of one call as initial to the next produces the same result as a single call over all chunks concatenated. When inLen is 0 the value of initial is returned unchanged.

Parameter Direction Type Description
in in const uint8_t * Pointer to the input bytes. Must not be NULL when inLen > 0.
inLen in size_t Number of bytes to process. May be 0, in which case initial is returned unchanged.
initial in uint16_t Starting checksum state. Pass SSF_FCSUM_INITIAL to begin a new computation; pass the return value of the previous call to continue an incremental computation.

Returns: Updated 16-bit Fletcher checksum state. Pass this value as initial to the next call when processing data in multiple chunks, or compare it against an expected checksum for verification.

Example:

uint16_t fc;

/* Single-buffer computation */
fc = SSFFCSum16((uint8_t *)"abcde", 5, SSF_FCSUM_INITIAL);
/* fc == 0xC8F0 */

/* Incremental computation over chunks — same result */
fc = SSFFCSum16((uint8_t *)"a",   1, SSF_FCSUM_INITIAL);
fc = SSFFCSum16((uint8_t *)"bcd", 3, fc);
fc = SSFFCSum16((uint8_t *)"e",   1, fc);
/* fc == 0xC8F0 */

/* Verifying received data against a known checksum */
uint8_t  packet[]  = {0x01u, 0x02u, 0x03u, 0x04u};
uint16_t expected  = 0x140Au;
if (SSFFCSum16(packet, sizeof(packet), SSF_FCSUM_INITIAL) == expected)
{
    /* Packet integrity confirmed */
}