// ntt maps a ringElement to its nttElement representation.
//
// It implements NTT, according to FIPS 203 (DRAFT), Algorithm 8.
func ntt(f ringElement) nttElement {
	k := 1
	for len := 128; len >= 2; len /= 2 {
		for start := 0; start < 256; start += 2 * len {
			zeta := zetas[k]