// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Buffers.Binary;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
 
namespace System.Buffers
{
    public static partial class SequenceReaderExtensions
    {
        /// <summary>
        /// Try to read the given type out of the buffer if possible. Warning: this is dangerous to use with arbitrary
        /// structs- see remarks for full details.
        /// </summary>
        /// <remarks>
        /// IMPORTANT: The read is a straight copy of bits. If a struct depends on specific state of it's members to
        /// behave correctly this can lead to exceptions, etc. If reading endian specific integers, use the explicit
        /// overloads such as <see cref="TryReadLittleEndian(ref SequenceReader{byte}, out short)"/>
        /// </remarks>
        /// <returns>
        /// True if successful. <paramref name="value"/> will be default if failed (due to lack of space).
        /// </returns>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static unsafe bool TryRead<T>(ref this SequenceReader<byte> reader, out T value) where T : unmanaged
        {
            ReadOnlySpan<byte> span = reader.UnreadSpan;
            if (span.Length < sizeof(T))
                return TryReadMultisegment(ref reader, out value);
 
            value = Unsafe.ReadUnaligned<T>(ref MemoryMarshal.GetReference(span));
            reader.Advance(sizeof(T));
            return true;
        }
 
        private static unsafe bool TryReadMultisegment<T>(ref SequenceReader<byte> reader, out T value) where T : unmanaged
        {
            Debug.Assert(reader.UnreadSpan.Length < sizeof(T));
 
            // Not enough data in the current segment, try to peek for the data we need.
            T buffer = default;
            Span<byte> tempSpan = new Span<byte>(&buffer, sizeof(T));
 
            if (!reader.TryCopyTo(tempSpan))
            {
                value = default;
                return false;
            }
 
            value = Unsafe.ReadUnaligned<T>(ref MemoryMarshal.GetReference(tempSpan));
            reader.Advance(sizeof(T));
            return true;
        }
 
        /// <summary>
        /// Reads an <see cref="short"/> as little endian.
        /// </summary>
        /// <returns>False if there wasn't enough data for an <see cref="short"/>.</returns>
        public static bool TryReadLittleEndian(ref this SequenceReader<byte> reader, out short value)
        {
            if (BitConverter.IsLittleEndian)
            {
                return reader.TryRead(out value);
            }
 
            return TryReadReverseEndianness(ref reader, out value);
        }
 
        /// <summary>
        /// Reads an <see cref="short"/> as big endian.
        /// </summary>
        /// <returns>False if there wasn't enough data for an <see cref="short"/>.</returns>
        public static bool TryReadBigEndian(ref this SequenceReader<byte> reader, out short value)
        {
            if (!BitConverter.IsLittleEndian)
            {
                return reader.TryRead(out value);
            }
 
            return TryReadReverseEndianness(ref reader, out value);
        }
 
        private static bool TryReadReverseEndianness(ref SequenceReader<byte> reader, out short value)
        {
            if (reader.TryRead(out value))
            {
                value = BinaryPrimitives.ReverseEndianness(value);
                return true;
            }
 
            return false;
        }
 
        /// <summary>
        /// Reads an <see cref="int"/> as little endian.
        /// </summary>
        /// <returns>False if there wasn't enough data for an <see cref="int"/>.</returns>
        public static bool TryReadLittleEndian(ref this SequenceReader<byte> reader, out int value)
        {
            if (BitConverter.IsLittleEndian)
            {
                return reader.TryRead(out value);
            }
 
            return TryReadReverseEndianness(ref reader, out value);
        }
 
        /// <summary>
        /// Reads an <see cref="int"/> as big endian.
        /// </summary>
        /// <returns>False if there wasn't enough data for an <see cref="int"/>.</returns>
        public static bool TryReadBigEndian(ref this SequenceReader<byte> reader, out int value)
        {
            if (!BitConverter.IsLittleEndian)
            {
                return reader.TryRead(out value);
            }
 
            return TryReadReverseEndianness(ref reader, out value);
        }
 
        private static bool TryReadReverseEndianness(ref SequenceReader<byte> reader, out int value)
        {
            if (reader.TryRead(out value))
            {
                value = BinaryPrimitives.ReverseEndianness(value);
                return true;
            }
 
            return false;
        }
 
        /// <summary>
        /// Reads an <see cref="long"/> as little endian.
        /// </summary>
        /// <returns>False if there wasn't enough data for an <see cref="long"/>.</returns>
        public static bool TryReadLittleEndian(ref this SequenceReader<byte> reader, out long value)
        {
            if (BitConverter.IsLittleEndian)
            {
                return reader.TryRead(out value);
            }
 
            return TryReadReverseEndianness(ref reader, out value);
        }
 
        /// <summary>
        /// Reads an <see cref="long"/> as big endian.
        /// </summary>
        /// <returns>False if there wasn't enough data for an <see cref="long"/>.</returns>
        public static bool TryReadBigEndian(ref this SequenceReader<byte> reader, out long value)
        {
            if (!BitConverter.IsLittleEndian)
            {
                return reader.TryRead(out value);
            }
 
            return TryReadReverseEndianness(ref reader, out value);
        }
 
        private static bool TryReadReverseEndianness(ref SequenceReader<byte> reader, out long value)
        {
            if (reader.TryRead(out value))
            {
                value = BinaryPrimitives.ReverseEndianness(value);
                return true;
            }
 
            return false;
        }
    }
}