field element") } return fieldElement(a), nil } // fieldReduceOnce reduces a value a < 2q. func fieldReduceOnce(a uint16) fieldElement { x := a - q // If x underflowed, then x >= 2¹⁶ - q > 2¹⁵, so the top bit is set. x += (x >> 15) * q return fieldElement(x) } func fieldAdd(a, b fieldElement) fieldElement { x := uint16(a + b) return fieldReduceOnce(x) } func fieldSub(a, b fieldElement) fieldElement { x := uint16(a - b + q) return fieldReduceOnce(x) } const ( barrettMultiplier = 5039 // 2¹² * 2¹²...