import JSBI from "jsbi";
import invariant from "tiny-invariant";
import { ZERO, MaxUint256 } from "../internalConstants";

const TWO = JSBI.BigInt(2);
const POWERS_OF_2 = [128, 64, 32, 16, 8, 4, 2, 1].map(
  (pow: number): [number, JSBI] => [
    pow,
    JSBI.exponentiate(TWO, JSBI.BigInt(pow)),
  ]
);

export function mostSignificantBit(x: JSBI): number {
  invariant(JSBI.greaterThan(x, ZERO), "ZERO");
  invariant(JSBI.lessThanOrEqual(x, MaxUint256), "MAX");

  let msb: number = 0;
  for (const [power, min] of POWERS_OF_2) {
    if (JSBI.greaterThanOrEqual(x, min)) {
      x = JSBI.signedRightShift(x, JSBI.BigInt(power));
      msb += power;
    }
  }
  return msb;
}