/**
* Copyright 2017 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import { assert } from '@firebase/util';
/**
* 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).
*/
export const nextPushId = (function() {
// 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: number[] = [];
return function(now: number) {
const duplicateTime = now === lastPushTime;
lastPushTime = now;
let i;
const timeStampChars = new Array(8);
for (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);
}
assert(now === 0, 'Cannot push at time == 0');
let id = timeStampChars.join('');
if (!duplicateTime) {
for (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.
for (i = 11; i >= 0 && lastRandChars[i] === 63; i--) {
lastRandChars[i] = 0;
}
lastRandChars[i]++;
}
for (i = 0; i < 12; i++) {
id += PUSH_CHARS.charAt(lastRandChars[i]);
}
assert(id.length === 20, 'nextPushId: Length should be 20.');
return id;
};
})();
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