long (long x, long y) { if (x < 0 || y < 0) { // Use technique from section 8-2 of Henry S. Warren, Jr., // Hacker's Delight (2nd ed.) (Addison Wesley, 2013), 173-174. long x1 = x >> 32; long x2 = x & 0xFFFFFFFFL; long y1 = y >> 32; long y2 = y & 0xFFFFFFFFL; long z2 = x2 * y2; long t = x1 * y2 + (z2 >>> 32); long z1 = t & 0xFFFFFFFFL; long z0 = t >> 32; z1 += x2 * y1; return x1 * y1 + z0 + (z1 >> 32); } else { // Use Karatsuba technique with two base 2^32 digits. long x1 = x >>> 32; long y1 = y >>> 32; long x2 = x & 0xFFFFFFFFL; long y2 = y & 0xFFFFFFFFL; long A = x1 * y1; long B = x2 * y2; long C = (x1 + x2) * (y1 + y2); long K = C - A - B; return (((B >>> 32) + K) >>> 32) + A; } }