function multiply(a, b) {
  // If a is zero or b is zero or if both a and b are zeros then the production is also zero.
  if (b === 0 || a === 0) {
    return 0;
  }

  // Otherwise we will have four different cases that are described above.
  const multiplyByOddPositive = () => multiply(multiplyByTwo(a), divideByTwo(b - 1)) + a;
  const multiplyByOddNegative = () => multiply(multiplyByTwo(a), divideByTwo(b + 1)) - a;

  const multiplyByEven = () => multiply(multiplyByTwo(a), divideByTwo(b));
  const multiplyByOdd = () => (isPositive(b) ? multiplyByOddPositive() : multiplyByOddNegative());

  return isEven(b) ? multiplyByEven() : multiplyByOdd();
}
Multiply two signed numbers using bitwise operations.

If a is zero or b is zero or if both a and b are zeros:
multiply(a, b) = 0

If b is even:
multiply(a, b) = multiply(2a, b/2)

If b is odd and b is positive:
multiply(a, b) = multiply(2a, (b-1)/2) + a

If b is odd and b is negative:
multiply(a, b) = multiply(2a, (b+1)/2) - a

Time complexity: O(log b)

@param {number} a
@param {number} b
@return {number}