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void Main()
{
var pushId = FirebasePushIDGenerator.GeneratePushID();
Console.WriteLine(pushId);
Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
Thread.Sleep(5);
Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
Thread.Sleep(5);
Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
Thread.Sleep(5);
Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
Console.WriteLine("Convert '{0}' to unix time ms", pushId);
Console.WriteLine(FirebasePushIDGenerator.ConvertPushID(pushId));
}
public class FirebasePushIDGenerator
{
/**
* 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 string PUSH_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
private static long 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.
private static char[] lastRandChars = new char[12];
// Random number generator
private static Random rng = new Random();
public static string GeneratePushID()
{
var now = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
Console.WriteLine("now=" + now);
var duplicateTime = (now == lastPushTime);
lastPushTime = now;
var timeStampChars = new char[8];
for (var i = 7; i >= 0; i--)
{
timeStampChars[i] = PUSH_CHARS[(int)(now % 64)];
now = now >> 6;
}
if (now != 0) throw new Exception("We should have converted the entire timestamp.");
var id = string.Join(string.Empty, timeStampChars);
if (!duplicateTime)
{
for (var i = 0; i < 12; i++)
{
lastRandChars[i] = (char)rng.Next(0, 63);
}
}
else
{
// If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
int i;
for (i = 11; i >= 0 && lastRandChars[i] == 63; i--)
{
lastRandChars[i] = (char)0;
}
lastRandChars[i]++;
}
for (var i = 0; i < 12; i++)
{
id += PUSH_CHARS[lastRandChars[i]];
}
if (id.Length != 20) throw new Exception("Length should be 20.");
return id;
}
public static long ConvertPushID(string id)
{
var timestamp = 0L;
for (var i = 0; i < 8; i++)
{
var n = PUSH_CHARS.IndexOf(id[i]);
timestamp = (timestamp << 6) + n;
}
return timestamp;
}
}
Output. Note the first 3 timestamps had identical milliseconds, so last digit of random bits was incremented by 1.
Also converts first PushID back in to unix timestamp. Verify that they are the same.
now=1476984292596
-KUY6wIoxwYFJbKhYuxA
now=1476984292596
-KUY6wIoxwYFJbKhYuxB
now=1476984292596
-KUY6wIoxwYFJbKhYuxC
now=1476984292602
-KUY6wIugap8n5ZN1UPx
now=1476984292608
-KUY6wJ-uMBurx137YK7
now=1476984292613
-KUY6wJ4amFnKQlcEgiJ
Convert '-KUY6wIoxwYFJbKhYuxA' to unix time ms
1476984292596
@0xFireball

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0xFireball commented Dec 18, 2016

Thanks!

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