retrieveUsers.js

retrieveUsers.js

const users = {}


function node(key, value) {
	this.next = null
	this.prev = null
	this.key = key
	this.value = value
}

let head = null
let tail = null

function saveUserLogin(userId) {
	let userNode = users[userId]
	if (userNode) {
		userNode.value = new Date()
		if (userNode === head && userNode === tail) {
			return // we only update the login date
		}

		// we only care about maintaining order from the tail to the head
		// so we have to move the userNode always to the top
		if (userNode === tail) {
			tail = tail.next;
			tail.prev = null;
		} else {
			userNode.prev.next = userNode.next
			userNode.next.prev = userNode.prev
		}
	} else {
		userNode = new node(userId, new Date())
		users[userId] = userNode

		if (head === null) {
			head = userNode
			tail = userNode
		}
	}

	// always add the user to the top/head
	userNode.next = null
	userNode.prev = head;
	head.next = userNode;
	head = userNode
}

function retrieveUsersByLastLogin(days = 10) {
	if (!tail) return []

	let tailNode = tail
	const result = []
	const today = new Date()
	// now we know that by traversing the list from the tail
	// we will get an ordered list of dates in ascendent mode
	while (tailNode !== null && daysDiff(tailNode.value, today) >= days) {
		result.push(tailNode.key)
		tailNode = tailNode.next
	}

	return result
}

Yep that's the implementation. What i will improve is to extract the double linked list to a class and create a method to delete node from a reference.

👍