main.js

const isZiinaUser = (phoneNumber) => {
    return false;
};

const contactsMap = new Map(); // key - useId, val -> set of phone numbers
const phoneToUserMap = new Map(); // key - phone number, val -> set of userIds

const syncContacts = (userId, phoneNumbers) => {
    const prevPhoneNumbersSet = contactsMap.get(userId) ?? new Set();
    const phoneNumbersSet = new Set(phoneNumbers);
    const phoneNumberToDelArr = [];
    for (const phoneNumber of prevPhoneNumbersSet) {
        if (!phoneNumbersSet.has(phoneNumber)) {
            phoneNumberToDelArr.push(phoneNumber);
        }
    }

    for (const phoneNumber of phoneNumberToDelArr) {
        phoneToUserMap.get(phoneNumber)?.delete(userId);
    }

    contactsMap.set(userId, phoneNumbersSet);
    for (const phoneNumber of phoneNumbersSet) {
        if (phoneToUserMap.has(phoneNumber)) {
            phoneToUserMap.get(phoneNumber).add(userId)
            continue;
        }
        phoneToUserMap.set(phoneNumber, new Set([userId]))
    }
};

/*
type ProspectiveUser {
	phoneNumber: string,
	friendsOnZiina: int
}
*/

const prospectiveUsers = (userId) => {
    console.log(contactsMap)
    console.log(phoneToUserMap)
    let prospectiveUsersArr = [];
    for (const phoneNumber of (contactsMap.get(userId) ?? [])) {
        if (isZiinaUser(phoneNumber)) {
            continue;
        }
        prospectiveUsersArr.push({
            phoneNumber,
            friendsOnZiina: phoneToUserMap.get(phoneNumber)?.size ?? 0
        })
    }
    return prospectiveUsersArr
};

const testFn = () => {
    syncContacts(1, ["12345", "54321", "56788", "123123"]);
    syncContacts(2, ["12345", "54321"]);

    console.log(prospectiveUsers(1));
    console.log(prospectiveUsers(2));
    console.log('====')

    syncContacts(1, ["54321", "56788", "123123"]);
    console.log(prospectiveUsers(1));
    console.log(prospectiveUsers(2));
};

testFn();