1 files changed, 92 insertions, 0 deletions
diff --git a/FireBase/FirebaseKeyGenerator.cs b/FireBase/FirebaseKeyGenerator.cs
new file mode 100644
index 0000000..acad399
--- /dev/null
+++ b/FireBase/FirebaseKeyGenerator.cs
@@ -0,0 +1,92 @@
+namespace Firebase.Database
+{
+    using System;
+    using System.Text;
+
+    /// <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 (int 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 (int 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 (int i = 0; i < 12; i++)
+            {
+                id.Append(PushChars[lastRandChars[i]]);
+            }
+
+            if (id.Length != 20)
+            {
+                throw new Exception("Length should be 20.");
+            }
+
+            return id.ToString();
+        }
+    }
+}