#include "PearsonHash.h"



// Permutation function for Pearson hashing (uses less space than a lookup table)
// Needs to be bijective, which it is
uint8_t PearsonHash::_pearsonPermutation(uint8_t n) {
	return (n*121) % 256;
}


uint16_t PearsonHash::calcHash16(const void* data, uint16_t len) {
	uint8_t h[2] = {  _pearsonPermutation((((uint8_t*)data)[0]+1)%256), _pearsonPermutation(((uint8_t*)data)[0]) };
	for (uint16_t i = 0; i < len; i++) {
		auto _tmp = h[0];
		h[0] = _pearsonPermutation(h[1] ^ ((uint8_t*)data)[i]);
		h[1] = _pearsonPermutation(_tmp ^ ((uint8_t*)data)[i]);
	}
	void* _tmp = h; // To avoid the type punning warning
	return *((uint16_t*)_tmp);
}


bool PearsonHash::checkHash16(uint16_t crc, const void* data, uint16_t len) {
	return crc == calcHash16(data, len);
}