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Problem 16: Power digit sum

curriculum/challenges/english/blocks/project-euler-problems-1-to-100/5900f37d1000cf542c50fe8f.md

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--description--

2<sup>15</sup> = 32768 and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26.

What is the sum of the digits of the number 2<sup><code>exponent</code></sup>?

--hints--

powerDigitSum(15) should return a number.

js
assert.isNumber(powerDigitSum(15));

powerDigitSum(15) should return 26.

js
assert.strictEqual(powerDigitSum(15), 26);

powerDigitSum(128) should return 166.

js
assert.strictEqual(powerDigitSum(128), 166);

powerDigitSum(1000) should return 1366.

js
assert.strictEqual(powerDigitSum(1000), 1366);

--seed--

--seed-contents--

js
function powerDigitSum(exponent) {

  return true;
}

powerDigitSum(15);

--solutions--

js
function powerDigitSum(exponent) {
  const bigNum = [1];
  let sum = 0;

  for (let i = 1; i <= exponent; i++) {
    let count = bigNum.length + 1;
    let overflow = 0;
    for (let j = 0; j < count; j++) {
      let digit = bigNum[j] || 0;
      digit = 2 * digit + overflow;

      if (digit > 9) {
        digit -= 10;
        overflow = 1;
      } else {
        overflow = 0;
      }

      bigNum[j] = digit;
    }
  }

  bigNum.forEach(function(num) {
    return sum += num;
  });

  return sum;
}