Restore SEBPatch

SafeExamBrowser.Configuration/SubStream.cs

@@ -0,0 +1,140 @@
+/*
+ * Copyright (c) 2024 ETH Zürich, IT Services
+ * 
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+using System;
+using System.IO;
+
+namespace SafeExamBrowser.Configuration
+{
+	/// <summary>
+	/// A read-only wrapper for a subsection of another, larger stream.
+	/// </summary>
+	internal class SubStream : Stream
+	{
+		private long length;
+		private long offset;
+		private Stream original;
+
+		public override bool CanRead => original.CanRead;
+		public override bool CanSeek => original.CanSeek;
+		public override bool CanWrite => false;
+		public override long Length => length;
+		public override long Position { get; set; }
+
+		/// <summary>
+		/// Creates a new wrapper for the specified subsection of the given stream.
+		/// </summary>
+		/// <remarks>
+		/// 
+		/// Below an example of a subsection within a stream:
+		/// 
+		/// +==============+==============================================================+==============================+
+		/// |      ...     |####################### subsection ###########################|              ...             |
+		/// +==============+==============================================================+==============================+
+		/// ^              ^                                                              ^                              ^
+		/// |              |                                                              |                              |
+		/// |              + offset                                                       + length                       |
+		/// |                                                                                                            |
+		/// + start of original                                                                          end of original +
+		/// 
+		/// </remarks>
+		/// <exception cref="ArgumentException">In case the original stream does not support <see cref="Stream.CanRead"/>.</exception>
+		/// <exception cref="ArgumentException">In case the original stream does not support <see cref="Stream.CanSeek"/>.</exception>
+		/// <exception cref="ArgumentOutOfRangeException">In case the specified subsection is outside the bounds of the original stream.</exception>
+		public SubStream(Stream original, long offset, long length)
+		{
+			this.original = original;
+			this.offset = offset;
+			this.length = length;
+
+			if (!original.CanRead)
+			{
+				throw new ArgumentException("The original stream must support reading!", nameof(original));
+			}
+
+			if (!original.CanSeek)
+			{
+				throw new ArgumentException("The original stream must support seeking!", nameof(original));
+			}
+
+			if (original.Length < offset + length || offset < 0 || length < 1)
+			{
+				throw new ArgumentOutOfRangeException($"Specified subsection is outside the bounds of the original stream!");
+			}
+		}
+
+		public override void Flush()
+		{
+			throw new NotSupportedException();
+		}
+
+		public override int Read(byte[] buffer, int offset, int count)
+		{
+			var originalPosition = original.Position;
+
+			if (Position < 0 || Position >= Length)
+			{
+				return 0;
+			}
+
+			if (Position + count >= Length)
+			{
+				count = Convert.ToInt32(Length - Position);
+			}
+
+			original.Seek(this.offset + Position, SeekOrigin.Begin);
+
+			var bytesRead = original.Read(buffer, offset, count);
+
+			Position += bytesRead;
+			original.Seek(originalPosition, SeekOrigin.Begin);
+
+			return bytesRead;
+		}
+
+		public override int ReadByte()
+		{
+			if (Position < 0 || Position >= Length)
+			{
+				return -1;
+			}
+
+			return base.ReadByte();
+		}
+
+		public override long Seek(long offset, SeekOrigin origin)
+		{
+			switch (origin)
+			{
+				case SeekOrigin.Begin:
+					Position = offset;
+					break;
+				case SeekOrigin.Current:
+					Position += offset;
+					break;
+				case SeekOrigin.End:
+					Position = length + offset;
+					break;
+				default:
+					throw new NotImplementedException($"Seeking from position '{origin}' is not implemented!");
+			}
+
+			return Position;
+		}
+
+		public override void SetLength(long value)
+		{
+			throw new NotSupportedException();
+		}
+
+		public override void Write(byte[] buffer, int offset, int count)
+		{
+			throw new NotSupportedException();
+		}
+	}
+}