RWC: Previous Fibonacci Number

previous-fibonacci.js

/**
 * Gets the previous number in the Fibonacci sequence, or -1 if this number isn't in the sequence
 * @param {number} num
 * @returns {number} previous Fibonacci number (or -1)
 */
function getPreviousFibonacciNumber(num) {
	// Handle negatives
	if (num <= 0) {
		return -1;
	}

	// 1 is an odd case, because it appears twice, so solution could be either 0 or 1
	if (num === 1) {
		return 0;
	}

	const mostRecentNumbersInSequence = [1, 1];
	let sum = add(mostRecentNumbersInSequence);
	while (sum <= num) {
		mostRecentNumbersInSequence.push(sum); // Add sum to sequence
		mostRecentNumbersInSequence.shift(); // Demote n[k-2] from sequence
		sum = add(mostRecentNumbersInSequence);
	}

	if (mostRecentNumbersInSequence[1] === num) {
		return mostRecentNumbersInSequence[0];
	} else {
		return -1;
	}
}

/**
 * Sums up an array
 * @param {number[]} nums array of numbers
 * @returns {number} sum of all numbers in array
 */
function add(nums) {
	const sum = nums.reduce((total, addend) => total + addend, 0);
	return sum;
}

console.log(
	getPreviousFibonacciNumber(-30),
	getPreviousFibonacciNumber(1),
	getPreviousFibonacciNumber(13),
	getPreviousFibonacciNumber(144),
	getPreviousFibonacciNumber(2584),
	getPreviousFibonacciNumber(2589),
);