Class ChecksumFletcher32

Fletcher-32 checksum algorithm implementation. Produces a 32-bit checksum by maintaining two running sums over 16-bit words derived from the input data, using a modulus of 65535 (0xFFFF).

Inheritance
object
ChecksumFletcher32
Implements
Inherited Members
Namespace: Scylla.Core.Util.Checksum
Assembly: ScyllaCore.dll
Syntax
public sealed class ChecksumFletcher32 : IChecksum
Remarks

Algorithm details:

  • Input word width: 16 bits (byte pairs in little-endian order).
  • Both accumulators (c0 and c1) are kept modulo 0xFFFF (65535).
  • Initial values: c0 = 0, c1 = 0.
  • Output: 32-bit value (c1 << 16) | c0.
  • Odd-length inputs: the trailing byte is treated as the low byte of a 16-bit word padded with zero. Padding is deferred until Value is read, so streaming updates correctly handle boundaries that split a word across chunks.

Like ChecksumAdler32, Fletcher-32 defers modulo reduction to block boundaries for throughput. The block size for Fletcher-32 is 360 16-bit words (720 bytes), which is the largest block that keeps both accumulators safely within 32-bit unsigned arithmetic.

Comparison with Adler-32: Both algorithms share a similar two-accumulator structure and similar performance. Fletcher-32 uses a modulus of 65535 (not a prime), while Adler-32 uses the prime 65521. Adler-32 is typically preferred for zlib compatibility; Fletcher-32 is preferred in embedded systems and network protocols (e.g., ATM AAL5, SCTP) where its particular error-detection properties are specified by the standard.

Security note: Fletcher-32 is NOT cryptographically secure. For security-sensitive hashing use the Scylla Crypto module instead.

Properties

Value

Gets the current Fletcher-32 checksum value as the standard 32-bit composite (c1 << 16) | c0.

Declaration
public long Value { get; }
Property Value
Type Description
long

A non-negative long in the range [0, 0xFFFE_FFFE]. The upper 16 bits contain c1 and the lower 16 bits contain c0 (both mod 0xFFFF). Returns 0 when no data has been processed. If the last Update(byte[], int, int) call provided an odd number of bytes, the pending byte is incorporated into the value (padded with zero) without mutating the internal accumulators, so subsequent Update(byte[]) calls remain valid.

Methods

Reset()

Resets the Fletcher-32 checksum to its initial state (c0 = 0, c1 = 0, no pending byte), discarding any previously processed data.

Declaration
public void Reset()
Remarks

After calling Reset, Value returns 0 and any pending odd byte from the previous computation is cleared. Use this to reuse the same instance for multiple independent checksum operations without allocating a new object.

Update(byte[])

Updates the Fletcher-32 checksum by incorporating all bytes in buffer.

Declaration
public void Update(byte[] buffer)
Parameters
Type Name Description
byte[] buffer

The byte array to process. If null or empty, the call is a no-op and the checksum state is unchanged.

Update(byte[], int, int)

Updates the Fletcher-32 checksum by incorporating count bytes from buffer starting at offset.

Declaration
public void Update(byte[] buffer, int offset, int count)
Parameters
Type Name Description
byte[] buffer

The source byte array. If null, or if count is zero, the call is a no-op.
int offset

Zero-based index of the first byte to read in buffer.
int count

Number of bytes to process. Must not cause offset + count to exceed buffer.Length.
Remarks

Bytes are combined into 16-bit little-endian words before accumulation. If a previous call left a pending odd byte (stored in _pendingByte), the first byte of this call is paired with it to form a complete word before continuing with the rest of the input.

Words are processed in blocks of up to NMAX (360) words, with modulo reduction applied once per block. If the final count is odd, the trailing byte is stored in _pendingByte for pairing on the next call or for zero-padding when Value is read.

Multiple calls accumulate into the same state; the final result is equivalent to a single call with the fully concatenated input.

Implements

Extension Methods

See Also