Oops, forgot to commit this one

Author: grendello

tools/apput/src/Common/SubStream.cs

@@ -0,0 +1,69 @@
+using System;
+using System.IO;
+
+namespace ApplicationUtility;
+
+class SubStream : Stream
+{
+	readonly Stream baseStream;
+	readonly long length;
+	readonly long offsetInParentStream;
+
+	public override bool CanRead => true;
+	public override bool CanSeek => true;
+	public override bool CanWrite => false;
+	public override long Length => length;
+
+	public override long Position {
+		get => throw new NotSupportedException ();
+		set => throw new NotSupportedException ();
+	}
+
+	public SubStream (Stream baseStream, long offsetInParentStream, long length)
+	{
+		if (!baseStream.CanSeek) {
+			throw new InvalidOperationException ($"Base stream must support seeking");
+		}
+
+		if (!baseStream.CanRead) {
+			throw new InvalidOperationException ($"Base stream must support reading");
+		}
+
+		if (offsetInParentStream >= baseStream.Length) {
+			throw new ArgumentOutOfRangeException (nameof (offsetInParentStream), $"{offsetInParentStream} exceeds length of the base stream ({baseStream.Length})");
+		}
+
+		if (offsetInParentStream + length > baseStream.Length) {
+			throw new InvalidOperationException ($"Not enough data in base stream after offset {offsetInParentStream}, length of {length} bytes is too big.");
+		}
+
+		this.baseStream = baseStream;
+		this.length = length;
+		this.offsetInParentStream = offsetInParentStream;
+	}
+
+	public override int Read (byte [] buffer, int offset, int count)
+	{
+		return baseStream.Read (buffer, offset, count);
+	}
+
+	public override long Seek (long offset, SeekOrigin origin)
+	{
+		return baseStream.Seek (offset + offsetInParentStream, origin);
+	}
+
+	public override void Flush ()
+	{
+		throw new NotSupportedException ();
+	}
+
+	public override void SetLength (long value)
+	{
+		throw new NotSupportedException ();
+	}
+
+	public override void Write (byte [] buffer, int offset, int count)
+	{
+		throw new NotSupportedException ();
+	}
+}