PeekStream.cs

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namespace Engine.Media {
    public class PeekStream : Stream {
        Stream m_stream;
 
        byte[] m_buffer;
 
        long m_position;
 
        int m_end;
 
        public override bool CanRead => true;
 
        public override bool CanWrite => false;
 
        public override bool CanSeek => m_stream.CanSeek;
 
        public override long Length => m_stream.Length;
 
        public override long Position {
            get => CanSeek ? m_position : throw new NotSupportedException();
            set {
                if (CanSeek) {
                    m_position = value;
                    m_stream.Position = Math.Max(m_position, m_end);
                    return;
                }
                throw new NotSupportedException();
            }
        }
 
        public PeekStream(Stream stream, int peekSize) {
            ArgumentNullException.ThrowIfNull(stream);
            if (!stream.CanRead) {
                throw new ArgumentException("Stream is not readable.");
            }
            ArgumentOutOfRangeException.ThrowIfNegative(peekSize);
            m_stream = stream;
            m_buffer = new byte[peekSize];
            m_end = stream.Read(m_buffer, 0, peekSize);
        }
 
        public MemoryStream GetInitialBytesStream() => new(m_buffer, 0, m_end, false);
 
        public override long Seek(long offset, SeekOrigin origin) {
            switch (origin) {
                case SeekOrigin.Begin: Position = offset; break;
                case SeekOrigin.End: Position = Length + offset; break;
                case SeekOrigin.Current: Position += offset; break;
                default: throw new ArgumentException("Invalid origin.", nameof(origin));
            }
            return Position;
        }
 
        public override void SetLength(long value) {
            m_stream.SetLength(value);
        }
 
        public override int Read(byte[] buffer, int offset, int count) {
            ArgumentNullException.ThrowIfNull(buffer);
            if (offset < 0) {
                throw new ArgumentOutOfRangeException(nameof(offset));
            }
            if (offset + count > buffer.Length) {
                throw new ArgumentOutOfRangeException(nameof(count));
            }
            int num = 0;
            if (m_position < m_end) {
                int num2 = MathUtils.Min(m_end - (int)m_position, count);
                Array.Copy(m_buffer, (int)m_position, buffer, offset, num2);
                m_position += num2;
                offset += num2;
                count -= num2;
                num += num2;
            }
            if (count > 0) {
                int num3 = m_stream.Read(buffer, offset, count);
                num += num3;
                m_position += num3;
            }
            return num;
        }
 
        public override void Write(byte[] buffer, int offset, int count) {
            throw new InvalidOperationException();
        }
 
        public override int ReadByte() {
            if (m_position < m_end) {
                return m_buffer[m_position++];
            }
            int num = m_stream.ReadByte();
            if (num >= 0) {
                m_position++;
            }
            return num;
        }
 
        public override void Flush() {
            m_stream.Flush();
        }
 
        protected override void Dispose(bool disposing) {
            if (disposing) {
                m_stream.Dispose();
            }
            base.Dispose(disposing);
        }
    }
}