ModernUO/Projects/Server/Buffers/BufferReader.cs

// Copyright (c) Harry Pierson. All rights reserved.
// Licensed under the MIT license.
// See LICENSE file in the project root for full license information.

using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
using System.Threading;

namespace System.Buffers
{
  public ref struct BufferReader<T>
  {
    private readonly bool usingSequence;
    private readonly ReadOnlySequence<T> sequence;
    private SequencePosition currentPosition;
    private SequencePosition nextPosition;
    private bool moreData;
    private readonly long length;

    public BufferReader(ReadOnlySpan<T> span)
    {
      usingSequence = false;
      CurrentSpanIndex = 0;
      Consumed = 0;
      sequence = default;
      currentPosition = default;
      length = span.Length;

      CurrentSpan = span;
      nextPosition = default;
      moreData = span.Length > 0;
    }

    public BufferReader(in ReadOnlySequence<T> sequence)
    {
      usingSequence = true;
      CurrentSpanIndex = 0;
      Consumed = 0;
      this.sequence = sequence;
      currentPosition = sequence.Start;
      length = -1;

      var first = sequence.First.Span;
      CurrentSpan = first;
      nextPosition = sequence.GetPosition(first.Length);
      moreData = first.Length > 0;

      if (!moreData && !sequence.IsSingleSegment)
      {
        moreData = true;
        GetNextSpan();
      }
    }

    public readonly bool End => !moreData;

    public ReadOnlySpan<T> CurrentSpan { get; private set; }

    public int CurrentSpanIndex { get; private set; }

    public readonly ReadOnlySpan<T> UnreadSpan
    {
      [MethodImpl(MethodImplOptions.AggressiveInlining)]
      get => CurrentSpan.Slice(CurrentSpanIndex);
    }

    public long Consumed { get; private set; }

    public readonly long Remaining => Length - Consumed;

    public readonly long Length
    {
      get
      {
        if (length < 0)
        {
          // Cast-away readonly to initialize lazy field
          Volatile.Write(ref Unsafe.AsRef(length), sequence.Length);
        }

        return length;
      }
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public readonly bool TryPeek([MaybeNullWhen(false)] out T value)
    {
      if (moreData)
      {
        value = CurrentSpan[CurrentSpanIndex];
        return true;
      }

      value = default!;
      return false;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public bool TryRead([MaybeNullWhen(false)] out T value)
    {
      if (End)
      {
        value = default!;
        return false;
      }

      value = CurrentSpan[CurrentSpanIndex];
      CurrentSpanIndex++;
      Consumed++;

      if (CurrentSpanIndex >= CurrentSpan.Length)
      {
        if (usingSequence)
          GetNextSpan();
        else
          moreData = false;
      }

      return true;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Rewind(long count)
    {
      if ((ulong)count > (ulong)Consumed) throw new ArgumentOutOfRangeException(nameof(count));

      Consumed -= count;

      if (CurrentSpanIndex >= count)
      {
        CurrentSpanIndex -= (int)count;
        moreData = true;
      }
      else if (usingSequence)
      {
        // Current segment doesn't have enough data, scan backward through segments
        RetreatToPreviousSpan(Consumed);
      }
      else
      {
        throw new ArgumentOutOfRangeException(nameof(count), $"Rewind went past the start of the memory by {count}.");
      }
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private void RetreatToPreviousSpan(long consumed)
    {
      ResetReader();
      Advance(consumed);
    }

    private void ResetReader()
    {
      CurrentSpanIndex = 0;
      Consumed = 0;
      currentPosition = sequence.Start;
      nextPosition = currentPosition;

      if (sequence.TryGet(ref nextPosition, out var memory))
      {
        moreData = true;

        if (memory.Length == 0)
        {
          CurrentSpan = default;
          // No data in the first span, move to one with data
          GetNextSpan();
        }
        else
        {
          CurrentSpan = memory.Span;
        }
      }
      else
      {
        // No data in any spans and at end of sequence
        moreData = false;
        CurrentSpan = default;
      }
    }

    private void GetNextSpan()
    {
      if (!sequence.IsSingleSegment)
      {
        var previousNextPosition = nextPosition;
        while (sequence.TryGet(ref nextPosition, out var memory))
        {
          currentPosition = previousNextPosition;
          if (memory.Length > 0)
          {
            CurrentSpan = memory.Span;
            CurrentSpanIndex = 0;
            return;
          }

          CurrentSpan = default;
          CurrentSpanIndex = 0;
          previousNextPosition = nextPosition;
        }
      }

      moreData = false;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Advance(long count)
    {
      const long TooBigOrNegative = unchecked((long)0xFFFFFFFF80000000);
      if ((count & TooBigOrNegative) == 0 && CurrentSpan.Length - CurrentSpanIndex > (int)count)
      {
        CurrentSpanIndex += (int)count;
        Consumed += count;
      }
      else if (usingSequence)
      {
        // Can't satisfy from the current span
        AdvanceToNextSpan(count);
      }
      else if (CurrentSpan.Length - CurrentSpanIndex == (int)count)
      {
        CurrentSpanIndex += (int)count;
        Consumed += count;
        moreData = false;
      }
      else
      {
        throw new ArgumentOutOfRangeException(nameof(count));
      }
    }

    private void AdvanceToNextSpan(long count)
    {
      if (count < 0) throw new ArgumentOutOfRangeException(nameof(count));

      Consumed += count;
      while (moreData)
      {
        var remaining = CurrentSpan.Length - CurrentSpanIndex;

        if (remaining > count)
        {
          CurrentSpanIndex += (int)count;
          count = 0;
          break;
        }

        // As there may not be any further segments we need to
        // push the current index to the end of the span.
        CurrentSpanIndex += remaining;
        count -= remaining;

        GetNextSpan();

        if (count == 0) break;
      }

      if (count != 0)
      {
        // Not enough data left- adjust for where we actually ended and throw
        Consumed -= count;
        throw new ArgumentOutOfRangeException(nameof(count));
      }
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public readonly bool TryCopyTo(Span<T> destination)
    {
      // This API doesn't advance to facilitate conditional advancement based on the data returned.
      // We don't provide an advance option to allow easier utilizing of stack allocated destination spans.
      // (Because we can make this method readonly we can guarantee that we won't capture the span.)

      var firstSpan = UnreadSpan;
      if (firstSpan.Length >= destination.Length)
      {
        firstSpan.Slice(0, destination.Length).CopyTo(destination);
        return true;
      }

      // Not enough in the current span to satisfy the request, fall through to the slow path
      return TryCopyMultisegment(destination);
    }

    internal readonly bool TryCopyMultisegment(Span<T> destination)
    {
      // If we don't have enough to fill the requested buffer, return false
      if (Remaining < destination.Length)
        return false;

      var firstSpan = UnreadSpan;
      firstSpan.CopyTo(destination);
      var copied = firstSpan.Length;

      var next = nextPosition;
      while (sequence.TryGet(ref next, out var nextSegment))
        if (nextSegment.Length > 0)
        {
          var nextSpan = nextSegment.Span;
          var toCopy = Math.Min(nextSpan.Length, destination.Length - copied);
          nextSpan.Slice(0, toCopy).CopyTo(destination.Slice(copied));
          copied += toCopy;
          if (copied >= destination.Length) break;
        }

      return true;
    }
  }
}