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using System;
using System.Text;
namespace Firebase.Database
{
/// <summary>
/// Offline key generator which mimics the official Firebase generators.
/// Credit: https://github.com/bubbafat/FirebaseSharp/blob/master/src/FirebaseSharp.Portable/FireBasePushIdGenerator.cs
/// </summary>
public class FirebaseKeyGenerator
{
// Modeled after base64 web-safe chars, but ordered by ASCII.
private const string PushCharsString = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
private static readonly char[] PushChars;
private static readonly DateTimeOffset Epoch = new DateTimeOffset(1970, 1, 1, 0, 0, 0, 0, TimeSpan.Zero);
private static readonly Random random = new Random();
private static readonly byte[] lastRandChars = new byte[12];
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
private static long lastPushTime;
static FirebaseKeyGenerator()
{
PushChars = Encoding.UTF8.GetChars(Encoding.UTF8.GetBytes(PushCharsString));
}
/// <summary>
/// Returns next firebase key based on current time.
/// </summary>
/// <returns>
/// The <see cref="string" />.
/// </returns>
public static string Next()
{
// 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.
var id = new StringBuilder(20);
var now = (long) (DateTimeOffset.Now - Epoch).TotalMilliseconds;
var duplicateTime = now == lastPushTime;
lastPushTime = now;
var timeStampChars = new char[8];
for (var i = 7; i >= 0; i--)
{
var index = (int) (now % PushChars.Length);
timeStampChars[i] = PushChars[index];
now = (long) Math.Floor((double) now / PushChars.Length);
}
if (now != 0) throw new Exception("We should have converted the entire timestamp.");
id.Append(timeStampChars);
if (!duplicateTime)
{
for (var i = 0; i < 12; i++) lastRandChars[i] = (byte) random.Next(0, PushChars.Length);
}
else
{
// If the timestamp hasn't changed since last push, use the same random number,
// except incremented by 1.
var lastIndex = 11;
for (; lastIndex >= 0 && lastRandChars[lastIndex] == PushChars.Length - 1; lastIndex--)
lastRandChars[lastIndex] = 0;
lastRandChars[lastIndex]++;
}
for (var i = 0; i < 12; i++) id.Append(PushChars[lastRandChars[i]]);
if (id.Length != 20) throw new Exception("Length should be 20.");
return id.ToString();
}
}
}
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