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();
        }
    }
}