index.js

function fibonacciLike (input) {
  const sequences = [];
  input.forEach((number, i) => {
    // Append to the all sequences where this is a legitimate continuation.
    // We need to consider all of them because any values of a, b where a + b = c could form
    // part of the solution depending on what comes afterwards.
    sequences.forEach(sequence => {
      const [a, b] = sequence;
      if (a + b === number) sequence.unshift(number);
    });

    // Need to add this as a combo with every other number so far.
    // We shouldn't be doubling up because the input in increasing monotonically.
    for (let j = 0; j < i; j++) {
      sequences.unshift([number, input[j]]);
    }
  });
  const max = Math.max(...sequences.map(sequence => sequence.length));
  return max >= 3 ? max : 0;
}

module.exports = fibonacciLike;