Simple non-cryptographic hash with decent performance. More...

#include "ndebug.hpp"
#include <stddef.h>
#include <stdint.h>
#include <span>
#include "debug_end.hpp"

Functions
constexpr uint32_t	Fnv1a::feed (uint32_t hash, uint8_t byte)
	Update the hash with the given byte.
template<size_t Size>
constexpr uint32_t	Fnv1a::checksum (std::span< uint8_t, Size > span, uint32_t hash=initialHash)
	Compute the hash over the given span.
template<size_t Size>
constexpr void	Fnv1a::putAtEnd (std::span< uint8_t, Size > span, uint32_t hash=initialHash)
template<size_t Size>
constexpr bool	Fnv1a::checkAtEnd (std::span< uint8_t, Size > span, uint32_t hash=initialHash)

Variables
constexpr uint32_t	Fnv1a::initialHash = 0x811c9dc5
constexpr size_t	Fnv1a::size = sizeof(initialHash)

Detailed Description

Simple non-cryptographic hash with decent performance.

FNV-1A hash

To verify message integrity, you need some kind of a checksum or hash. I have chosen the FNV-1A hash because it is extremely simple to implement in software (only marginally more complicated than the "add every byte" hash sometimes known as lose-lose because of its terrible performance).

There are faster hashes (they tend to be faster because they work more than a byte at a time) but when the input is slowly coming over UART this works perfectly fine.

This isn't meant to be a cryptographic hash (FNV-1A was never intended to be that). So it isn't used for signing, only to detect errors.

For a simple comparison of other hashes, see test/hash_comparison.py but for a much better one see https://softwareengineering.stackexchange.com/a/145633

Function Documentation

◆ checkAtEnd()

template<size_t Size>

bool Fnv1a::checkAtEnd	(	std::span< uint8_t, Size >	span,
		uint32_t	hash = initialHash )

constexpr

Checks that the span[:-4] sums to the same has as is stored at span[-4:].

◆ putAtEnd()