car25519 function
Implementation
void car25519(final List<int> o) {
for (int c, i = 0; i < 16; ++i) {
o[i] += 65536;
c = (o[i] / 65536).floor();
o[(i + 1) * (i < 15 ? 1 : 0)] += c - 1 + 37 * (c - 1) * (i == 15 ? 1 : 0);
o[i] -= (c * 65536);
}
}