Function finiteFieldMultiply

modules/data/qrcode/qrcodegen.c:308–316

Returns the product of the two given field elements modulo GF(2^8/0x11D). All inputs are valid. This could be implemented as a 256*256 lookup table.

// Returns the product of the two given field elements modulo GF(2^8/0x11D).
// All inputs are valid. This could be implemented as a 256*256 lookup table.
testable uint8_t finiteFieldMultiply(uint8_t x, uint8_t y) {
	// Russian peasant multiplication
	uint8_t z = 0;
	for (int i = 7; i >= 0; i--) {
		z = (z << 1) ^ ((z >> 7) * 0x11D);
		z ^= ((y >> i) & 1) * x;
	}
	return z;
}