using System;
using System.IO;
using System.Runtime.CompilerServices;
using Unity.Collections.LowLevel.Unsafe;
using UnityEngine;

namespace Mirror
{
    /// <summary>Network Reader for most simple types like floats, ints, buffers, structs, etc. Use NetworkReaderPool.GetReader() to avoid allocations.</summary>
    // Note: This class is intended to be extremely pedantic,
    // and throw exceptions whenever stuff is going slightly wrong.
    // The exceptions will be handled in NetworkServer/NetworkClient.
    public class NetworkReader
    {
        // internal buffer
        // byte[] pointer would work, but we use ArraySegment to also support
        // the ArraySegment constructor
        ArraySegment<byte> buffer;

        /// <summary>Next position to read from the buffer</summary>
        // 'int' is the best type for .Position. 'short' is too small if we send >32kb which would result in negative .Position
        // -> converting long to int is fine until 2GB of data (MAX_INT), so we don't have to worry about overflows here
        public int Position;

        /// <summary>Total number of bytes to read from buffer</summary>
        public int Length
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get => buffer.Count;
        }

        /// <summary>Remaining bytes that can be read, for convenience.</summary>
        public int Remaining
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get => Length - Position;
        }

        public NetworkReader(byte[] bytes)
        {
            buffer = new ArraySegment<byte>(bytes);
        }

        public NetworkReader(ArraySegment<byte> segment)
        {
            buffer = segment;
        }

        // sometimes it's useful to point a reader on another buffer instead of
        // allocating a new reader (e.g. NetworkReaderPool)
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void SetBuffer(byte[] bytes)
        {
            buffer = new ArraySegment<byte>(bytes);
            Position = 0;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void SetBuffer(ArraySegment<byte> segment)
        {
            buffer = segment;
            Position = 0;
        }

        // ReadBlittable<T> from DOTSNET
        // this is extremely fast, but only works for blittable types.
        // => private to make sure nobody accidentally uses it for non-blittable
        //
        // Benchmark: see NetworkWriter.WriteBlittable!
        //
        // Note:
        //   ReadBlittable assumes same endianness for server & client.
        //   All Unity 2018+ platforms are little endian.
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal unsafe T ReadBlittable<T>()
            where T : unmanaged
        {
            // check if blittable for safety
#if UNITY_EDITOR
            if (!UnsafeUtility.IsBlittable(typeof(T)))
            {
                throw new ArgumentException($"{typeof(T)} is not blittable!");
            }
#endif

            // calculate size
            //   sizeof(T) gets the managed size at compile time.
            //   Marshal.SizeOf<T> gets the unmanaged size at runtime (slow).
            // => our 1mio writes benchmark is 6x slower with Marshal.SizeOf<T>
            // => for blittable types, sizeof(T) is even recommended:
            // https://docs.microsoft.com/en-us/dotnet/standard/native-interop/best-practices
            int size = sizeof(T);

            // enough data to read?
            if (Position + size > buffer.Count)
            {
                throw new EndOfStreamException($"ReadBlittable<{typeof(T)}> out of range: {ToString()}");
            }

            // read blittable
            T value;
            fixed (byte* ptr = &buffer.Array[buffer.Offset + Position])
            {
#if UNITY_ANDROID
                // on some android systems, reading *(T*)ptr throws a NRE if
                // the ptr isn't aligned (i.e. if Position is 1,2,3,5, etc.).
                // here we have to use memcpy.
                //
                // => we can't get a pointer of a struct in C# without
                //    marshalling allocations
                // => instead, we stack allocate an array of type T and use that
                // => stackalloc avoids GC and is very fast. it only works for
                //    value types, but all blittable types are anyway.
                //
                // this way, we can still support blittable reads on android.
                // see also: https://github.com/vis2k/Mirror/issues/3044
                // (solution discovered by AIIO, FakeByte, mischa)
                T* valueBuffer = stackalloc T[1];
                UnsafeUtility.MemCpy(valueBuffer, ptr, size);
                value = valueBuffer[0];
#else
                // cast buffer to a T* pointer and then read from it.
                value = *(T*)ptr;
#endif
            }
            Position += size;
            return value;
        }

        // blittable'?' template for code reuse
        // note: bool isn't blittable. need to read as byte.
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal T? ReadBlittableNullable<T>()
            where T : unmanaged =>
                ReadByte() != 0 ? ReadBlittable<T>() : default(T?);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public byte ReadByte() => ReadBlittable<byte>();

        /// <summary>Read 'count' bytes into the bytes array</summary>
        // NOTE: returns byte[] because all reader functions return something.
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public byte[] ReadBytes(byte[] bytes, int count)
        {
            // check if passed byte array is big enough
            if (count > bytes.Length)
            {
                throw new EndOfStreamException($"ReadBytes can't read {count} + bytes because the passed byte[] only has length {bytes.Length}");
            }
            // check if within buffer limits
            if (Position + count > buffer.Count)
            {
                throw new EndOfStreamException($"ReadBytesSegment can't read {count} bytes because it would read past the end of the stream. {ToString()}");
            }

            Array.Copy(buffer.Array, buffer.Offset + Position, bytes, 0, count);
            Position += count;
            return bytes;
        }

        /// <summary>Read 'count' bytes allocation-free as ArraySegment that points to the internal array.</summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ArraySegment<byte> ReadBytesSegment(int count)
        {
            // check if within buffer limits
            if (Position + count > buffer.Count)
            {
                throw new EndOfStreamException($"ReadBytesSegment can't read {count} bytes because it would read past the end of the stream. {ToString()}");
            }

            // return the segment
            ArraySegment<byte> result = new ArraySegment<byte>(buffer.Array, buffer.Offset + Position, count);
            Position += count;
            return result;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override string ToString() =>
            $"NetworkReader pos={Position} len={Length} buffer={BitConverter.ToString(buffer.Array, buffer.Offset, buffer.Count)}";

        /// <summary>Reads any data type that mirror supports. Uses weaver populated Reader(T).read</summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T Read<T>()
        {
            Func<NetworkReader, T> readerDelegate = Reader<T>.read;
            if (readerDelegate == null)
            {
                Debug.LogError($"No reader found for {typeof(T)}. Use a type supported by Mirror or define a custom reader");
                return default;
            }
            return readerDelegate(this);
        }
    }

    /// <summary>Helper class that weaver populates with all reader types.</summary>
    // Note that c# creates a different static variable for each type
    // -> Weaver.ReaderWriterProcessor.InitializeReaderAndWriters() populates it
    public static class Reader<T>
    {
        public static Func<NetworkReader, T> read;
    }
}