cowbell/splittypie

/**
 * Fancy ID generator that creates 20-character string identifiers with the following properties:
 *
 * 1. They're based on timestamp so that they sort *after* any existing ids.
 * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other
 *    clients' IDs.
 * 3. They sort *lexicographically* (so the timestamp is converted to characters that will
 *    sort properly).
 * 4. They're monotonically increasing.  Even if you generate more than one in the same
 *    timestamp, the
 *    latter ones will sort after the former ones.  We do this by using the previous random bits
 *    but "incrementing" them by 1 (only in the case of a timestamp collision).
 */
// Modeled after base64 web-safe chars, but ordered by ASCII.
const PUSH_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
let lastPushTime = 0;
// We generate 72-bits of randomness which get turned into 12 characters and appended to the
// timestamp to prevent collisions with other clients.  We store the last characters we
// generated because in the event of a collision, we"ll use those same characters except
// "incremented" by one.
const lastRandChars = [];

export default function generateUniqueId() {
    let now = new Date().getTime();
    const duplicateTime = (now === lastPushTime);
    lastPushTime = now;

    const timeStampChars = new Array(8);
    for (let i = 7; i >= 0; i--) {
        timeStampChars[i] = PUSH_CHARS.charAt(now % 64);
        // NOTE: Can"t use << here because javascript will convert to int and lose the upper bits.
        now = Math.floor(now / 64);
    }
    if (now !== 0) throw new Error("We should have converted the entire timestamp.");

    let id = timeStampChars.join("");

    if (!duplicateTime) {
        for (let i = 0; i < 12; i++) {
            lastRandChars[i] = Math.floor(Math.random() * 64);
        }
    } else {
        // If the timestamp hasn"t changed since last push, use the same random number,
        // except incremented by 1.
        let i;
        for (i = 11; i >= 0 && lastRandChars[i] === 63; i--) {
            lastRandChars[i] = 0;
        }
        lastRandChars[i]++;
    }
    for (let i = 0; i < 12; i++) {
        id += PUSH_CHARS.charAt(lastRandChars[i]);
    }

    if (id.length !== 20) {
        throw new Error("Length should be 20.");
    }

    return id;
}