/* Copyright 2010-2014 MongoDB 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.
*/

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Cryptography;
using System.Text;
using System.Threading;

namespace MongoDB.Bson
{
    /// <summary>
    /// Represents an ObjectId (see also BsonObjectId).
    /// </summary>
    [Serializable]
    public struct ObjectId : IComparable<ObjectId>, IEquatable<ObjectId>, IConvertible
    {
        // private static fields
        private static ObjectId __emptyInstance = default(ObjectId);
        private static int __staticMachine;
        private static short __staticPid;
        private static int __staticIncrement; // high byte will be masked out when generating new ObjectId

        // private fields
        // we're using 14 bytes instead of 12 to hold the ObjectId in memory but unlike a byte[] there is no additional object on the heap
        // the extra two bytes are not visible to anyone outside of this class and they buy us considerable simplification
        // an additional advantage of this representation is that it will serialize to JSON without any 64 bit overflow problems
        private int _timestamp;
        private int _machine;
        private short _pid;
        private int _increment;

        // static constructor
        static ObjectId()
        {
            __staticMachine = (GetMachineHash() + AppDomain.CurrentDomain.Id) & 0x00ffffff; // add AppDomain Id to ensure uniqueness across AppDomains
            __staticIncrement = (new Random()).Next();

            try
            {
                __staticPid = (short)GetCurrentProcessId(); // use low order two bytes only
            }
            catch (SecurityException)
            {
                __staticPid = 0;
            }
        }

        // constructors
        /// <summary>
        /// Initializes a new instance of the ObjectId class.
        /// </summary>
        /// <param name="bytes">The bytes.</param>
        public ObjectId(byte[] bytes)
        {
            if (bytes == null)
            {
                throw new ArgumentNullException("bytes");
            }
            Unpack(bytes, out _timestamp, out _machine, out _pid, out _increment);
        }

        /// <summary>
        /// Initializes a new instance of the ObjectId class.
        /// </summary>
        /// <param name="timestamp">The timestamp (expressed as a DateTime).</param>
        /// <param name="machine">The machine hash.</param>
        /// <param name="pid">The PID.</param>
        /// <param name="increment">The increment.</param>
        public ObjectId(DateTime timestamp, int machine, short pid, int increment)
            : this(GetTimestampFromDateTime(timestamp), machine, pid, increment)
        {
        }

        /// <summary>
        /// Initializes a new instance of the ObjectId class.
        /// </summary>
        /// <param name="timestamp">The timestamp.</param>
        /// <param name="machine">The machine hash.</param>
        /// <param name="pid">The PID.</param>
        /// <param name="increment">The increment.</param>
        public ObjectId(int timestamp, int machine, short pid, int increment)
        {
            if ((machine & 0xff000000) != 0)
            {
                throw new ArgumentOutOfRangeException("machine", "The machine value must be between 0 and 16777215 (it must fit in 3 bytes).");
            }
            if ((increment & 0xff000000) != 0)
            {
                throw new ArgumentOutOfRangeException("increment", "The increment value must be between 0 and 16777215 (it must fit in 3 bytes).");
            }

            _timestamp = timestamp;
            _machine = machine;
            _pid = pid;
            _increment = increment;
        }

        /// <summary>
        /// Initializes a new instance of the ObjectId class.
        /// </summary>
        /// <param name="value">The value.</param>
        public ObjectId(string value)
        {
            if (value == null)
            {
                throw new ArgumentNullException("value");
            }
            Unpack(BsonUtils.ParseHexString(value), out _timestamp, out _machine, out _pid, out _increment);
        }

        // public static properties
        /// <summary>
        /// Gets an instance of ObjectId where the value is empty.
        /// </summary>
        public static ObjectId Empty
        {
            get { return __emptyInstance; }
        }

        // public properties
        /// <summary>
        /// Gets the timestamp.
        /// </summary>
        public int Timestamp
        {
            get { return _timestamp; }
        }

        /// <summary>
        /// Gets the machine.
        /// </summary>
        public int Machine
        {
            get { return _machine; }
        }

        /// <summary>
        /// Gets the PID.
        /// </summary>
        public short Pid
        {
            get { return _pid; }
        }

        /// <summary>
        /// Gets the increment.
        /// </summary>
        public int Increment
        {
            get { return _increment; }
        }

        /// <summary>
        /// Gets the creation time (derived from the timestamp).
        /// </summary>
        public DateTime CreationTime
        {
            get { return BsonConstants.UnixEpoch.AddSeconds(_timestamp); }
        }

        // public operators
        /// <summary>
        /// Compares two ObjectIds.
        /// </summary>
        /// <param name="lhs">The first ObjectId.</param>
        /// <param name="rhs">The other ObjectId</param>
        /// <returns>True if the first ObjectId is less than the second ObjectId.</returns>
        public static bool operator <(ObjectId lhs, ObjectId rhs)
        {
            return lhs.CompareTo(rhs) < 0;
        }

        /// <summary>
        /// Compares two ObjectIds.
        /// </summary>
        /// <param name="lhs">The first ObjectId.</param>
        /// <param name="rhs">The other ObjectId</param>
        /// <returns>True if the first ObjectId is less than or equal to the second ObjectId.</returns>
        public static bool operator <=(ObjectId lhs, ObjectId rhs)
        {
            return lhs.CompareTo(rhs) <= 0;
        }

        /// <summary>
        /// Compares two ObjectIds.
        /// </summary>
        /// <param name="lhs">The first ObjectId.</param>
        /// <param name="rhs">The other ObjectId.</param>
        /// <returns>True if the two ObjectIds are equal.</returns>
        public static bool operator ==(ObjectId lhs, ObjectId rhs)
        {
            return lhs.Equals(rhs);
        }

        /// <summary>
        /// Compares two ObjectIds.
        /// </summary>
        /// <param name="lhs">The first ObjectId.</param>
        /// <param name="rhs">The other ObjectId.</param>
        /// <returns>True if the two ObjectIds are not equal.</returns>
        public static bool operator !=(ObjectId lhs, ObjectId rhs)
        {
            return !(lhs == rhs);
        }

        /// <summary>
        /// Compares two ObjectIds.
        /// </summary>
        /// <param name="lhs">The first ObjectId.</param>
        /// <param name="rhs">The other ObjectId</param>
        /// <returns>True if the first ObjectId is greather than or equal to the second ObjectId.</returns>
        public static bool operator >=(ObjectId lhs, ObjectId rhs)
        {
            return lhs.CompareTo(rhs) >= 0;
        }

        /// <summary>
        /// Compares two ObjectIds.
        /// </summary>
        /// <param name="lhs">The first ObjectId.</param>
        /// <param name="rhs">The other ObjectId</param>
        /// <returns>True if the first ObjectId is greather than the second ObjectId.</returns>
        public static bool operator >(ObjectId lhs, ObjectId rhs)
        {
            return lhs.CompareTo(rhs) > 0;
        }

        // public static methods
        /// <summary>
        /// Generates a new ObjectId with a unique value.
        /// </summary>
        /// <returns>An ObjectId.</returns>
        public static ObjectId GenerateNewId()
        {
            return GenerateNewId(GetTimestampFromDateTime(DateTime.UtcNow));
        }

        /// <summary>
        /// Generates a new ObjectId with a unique value (with the timestamp component based on a given DateTime).
        /// </summary>
        /// <param name="timestamp">The timestamp component (expressed as a DateTime).</param>
        /// <returns>An ObjectId.</returns>
        public static ObjectId GenerateNewId(DateTime timestamp)
        {
            return GenerateNewId(GetTimestampFromDateTime(timestamp));
        }

        /// <summary>
        /// Generates a new ObjectId with a unique value (with the given timestamp).
        /// </summary>
        /// <param name="timestamp">The timestamp component.</param>
        /// <returns>An ObjectId.</returns>
        public static ObjectId GenerateNewId(int timestamp)
        {
            int increment = Interlocked.Increment(ref __staticIncrement) & 0x00ffffff; // only use low order 3 bytes
            return new ObjectId(timestamp, __staticMachine, __staticPid, increment);
        }

        /// <summary>
        /// Packs the components of an ObjectId into a byte array.
        /// </summary>
        /// <param name="timestamp">The timestamp.</param>
        /// <param name="machine">The machine hash.</param>
        /// <param name="pid">The PID.</param>
        /// <param name="increment">The increment.</param>
        /// <returns>A byte array.</returns>
        public static byte[] Pack(int timestamp, int machine, short pid, int increment)
        {
            if ((machine & 0xff000000) != 0)
            {
                throw new ArgumentOutOfRangeException("machine", "The machine value must be between 0 and 16777215 (it must fit in 3 bytes).");
            }
            if ((increment & 0xff000000) != 0)
            {
                throw new ArgumentOutOfRangeException("increment", "The increment value must be between 0 and 16777215 (it must fit in 3 bytes).");
            }

            byte[] bytes = new byte[12];
            bytes[0] = (byte)(timestamp >> 24);
            bytes[1] = (byte)(timestamp >> 16);
            bytes[2] = (byte)(timestamp >> 8);
            bytes[3] = (byte)(timestamp);
            bytes[4] = (byte)(machine >> 16);
            bytes[5] = (byte)(machine >> 8);
            bytes[6] = (byte)(machine);
            bytes[7] = (byte)(pid >> 8);
            bytes[8] = (byte)(pid);
            bytes[9] = (byte)(increment >> 16);
            bytes[10] = (byte)(increment >> 8);
            bytes[11] = (byte)(increment);
            return bytes;
        }

        /// <summary>
        /// Parses a string and creates a new ObjectId.
        /// </summary>
        /// <param name="s">The string value.</param>
        /// <returns>A ObjectId.</returns>
        public static ObjectId Parse(string s)
        {
            if (s == null)
            {
                throw new ArgumentNullException("s");
            }
            ObjectId objectId;
            if (TryParse(s, out objectId))
            {
                return objectId;
            }
            else
            {
                var message = string.Format("'{0}' is not a valid 24 digit hex string.", s);
                throw new FormatException(message);
            }
        }

        /// <summary>
        /// Tries to parse a string and create a new ObjectId.
        /// </summary>
        /// <param name="s">The string value.</param>
        /// <param name="objectId">The new ObjectId.</param>
        /// <returns>True if the string was parsed successfully.</returns>
        public static bool TryParse(string s, out ObjectId objectId)
        {
            // don't throw ArgumentNullException if s is null
            if (s != null && s.Length == 24)
            {
                byte[] bytes;
                if (BsonUtils.TryParseHexString(s, out bytes))
                {
                    objectId = new ObjectId(bytes);
                    return true;
                }
            }

            objectId = default(ObjectId);
            return false;
        }

        /// <summary>
        /// Unpacks a byte array into the components of an ObjectId.
        /// </summary>
        /// <param name="bytes">A byte array.</param>
        /// <param name="timestamp">The timestamp.</param>
        /// <param name="machine">The machine hash.</param>
        /// <param name="pid">The PID.</param>
        /// <param name="increment">The increment.</param>
        public static void Unpack(byte[] bytes, out int timestamp, out int machine, out short pid, out int increment)
        {
            if (bytes == null)
            {
                throw new ArgumentNullException("bytes");
            }
            if (bytes.Length != 12)
            {
                throw new ArgumentOutOfRangeException("bytes", "Byte array must be 12 bytes long.");
            }
            timestamp = (bytes[0] << 24) + (bytes[1] << 16) + (bytes[2] << 8) + bytes[3];
            machine = (bytes[4] << 16) + (bytes[5] << 8) + bytes[6];
            pid = (short)((bytes[7] << 8) + bytes[8]);
            increment = (bytes[9] << 16) + (bytes[10] << 8) + bytes[11];
        }

        // private static methods
        /// <summary>
        /// Gets the current process id.  This method exists because of how CAS operates on the call stack, checking
        /// for permissions before executing the method.  Hence, if we inlined this call, the calling method would not execute
        /// before throwing an exception requiring the try/catch at an even higher level that we don't necessarily control.
        /// </summary>
        [MethodImpl(MethodImplOptions.NoInlining)]
        private static int GetCurrentProcessId()
        {
            return Process.GetCurrentProcess().Id;
        }

        private static int GetMachineHash()
        {
            var hostName = Environment.MachineName; // use instead of Dns.HostName so it will work offline
            var md5 = MD5.Create();
            var hash = md5.ComputeHash(Encoding.UTF8.GetBytes(hostName));
            return (hash[0] << 16) + (hash[1] << 8) + hash[2]; // use first 3 bytes of hash
        }

        private static int GetTimestampFromDateTime(DateTime timestamp)
        {
            return (int)Math.Floor((BsonUtils.ToUniversalTime(timestamp) - BsonConstants.UnixEpoch).TotalSeconds);
        }

        // public methods
        /// <summary>
        /// Compares this ObjectId to another ObjectId.
        /// </summary>
        /// <param name="other">The other ObjectId.</param>
        /// <returns>A 32-bit signed integer that indicates whether this ObjectId is less than, equal to, or greather than the other.</returns>
        public int CompareTo(ObjectId other)
        {
            int r = _timestamp.CompareTo(other._timestamp);
            if (r != 0) { return r; }
            r = _machine.CompareTo(other._machine);
            if (r != 0) { return r; }
            r = _pid.CompareTo(other._pid);
            if (r != 0) { return r; }
            return _increment.CompareTo(other._increment);
        }

        /// <summary>
        /// Compares this ObjectId to another ObjectId.
        /// </summary>
        /// <param name="rhs">The other ObjectId.</param>
        /// <returns>True if the two ObjectIds are equal.</returns>
        public bool Equals(ObjectId rhs)
        {
            return
                _timestamp == rhs._timestamp &&
                _machine == rhs._machine &&
                _pid == rhs._pid &&
                _increment == rhs._increment;
        }

        /// <summary>
        /// Compares this ObjectId to another object.
        /// </summary>
        /// <param name="obj">The other object.</param>
        /// <returns>True if the other object is an ObjectId and equal to this one.</returns>
        public override bool Equals(object obj)
        {
            if (obj is ObjectId)
            {
                return Equals((ObjectId)obj);
            }
            else
            {
                return false;
            }
        }

        /// <summary>
        /// Gets the hash code.
        /// </summary>
        /// <returns>The hash code.</returns>
        public override int GetHashCode()
        {
            int hash = 17;
            hash = 37 * hash + _timestamp.GetHashCode();
            hash = 37 * hash + _machine.GetHashCode();
            hash = 37 * hash + _pid.GetHashCode();
            hash = 37 * hash + _increment.GetHashCode();
            return hash;
        }

        /// <summary>
        /// Converts the ObjectId to a byte array.
        /// </summary>
        /// <returns>A byte array.</returns>
        public byte[] ToByteArray()
        {
            return Pack(_timestamp, _machine, _pid, _increment);
        }

        /// <summary>
        /// Returns a string representation of the value.
        /// </summary>
        /// <returns>A string representation of the value.</returns>
        public override string ToString()
        {
            return BsonUtils.ToHexString(Pack(_timestamp, _machine, _pid, _increment));
        }

        // explicit IConvertible implementation
        TypeCode IConvertible.GetTypeCode()
        {
            return TypeCode.Object;
        }

        bool IConvertible.ToBoolean(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        byte IConvertible.ToByte(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        char IConvertible.ToChar(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        DateTime IConvertible.ToDateTime(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        decimal IConvertible.ToDecimal(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        double IConvertible.ToDouble(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        short IConvertible.ToInt16(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        int IConvertible.ToInt32(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        long IConvertible.ToInt64(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        sbyte IConvertible.ToSByte(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        float IConvertible.ToSingle(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        string IConvertible.ToString(IFormatProvider provider)
        {
            return ToString();
        }

        object IConvertible.ToType(Type conversionType, IFormatProvider provider)
        {
            switch (Type.GetTypeCode(conversionType))
            {
                case TypeCode.String:
                    return ((IConvertible)this).ToString(provider);
                case TypeCode.Object:
                    if (conversionType == typeof(object) || conversionType == typeof(ObjectId))
                    {
                        return this;
                    }
                    if (conversionType == typeof(BsonObjectId))
                    {
                        return new BsonObjectId(this);
                    }
                    if (conversionType == typeof(BsonString))
                    {
                        return new BsonString(((IConvertible)this).ToString(provider));
                    }
                    break;
            }

            throw new InvalidCastException();
        }

        ushort IConvertible.ToUInt16(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        uint IConvertible.ToUInt32(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }

        ulong IConvertible.ToUInt64(IFormatProvider provider)
        {
            throw new InvalidCastException();
        }
    }
}