strauss-shamir.js

multiplyAndAddUnsafe(Q: ProjectivePoint, a: bigint, b: bigint): ProjectivePoint | undefined {
  // R=R*2+P   //bits from a,b = 1,0
  // R=R*2     //bits from a,b = 0,0
  // R=R*2+Q   //bits from a,b = 0,1
  // R=R*2+P+Q //bits from a,b = 1,1
  const G: ProjectivePoint = this;
  const spl = (a: bigint) => a.toString(2).padStart(max, '0').split('').map(i => Number(i));
  const max = Math.max(ut.bitLen(a), ut.bitLen(b));
  const aBits = spl(a);
  const bBits = spl(b);
  const choices = [ProjectivePoint.ZERO, G, Q, G.add(Q)]; // 0, G, Q, G+Q
  const select = (bitNumber: number) => choices[aBits[bitNumber] + (bBits[bitNumber] * 2)];
  let R = select(0);
  for (let i = 1; i < max; i++) {
    R = R.double()
    R = R.add(select(i));
  }
  return R.is0() ? undefined : R;
}