Initial read-only support for LZip

LZip has no notion of flienames, so an LzipReader wouldn't make very much sense;
I've just implemented the stream, and hooked it into tar support.

src/SharpCompress/Common/CompressionType.cs

@@ -11,6 +11,7 @@
         LZMA,
         BCJ,
         BCJ2,
+        LZip,
         Unknown
     }
 }

src/SharpCompress/Compressors/LZMA/LZipStream.cs

@@ -0,0 +1,153 @@
+using System;
+using System.IO;
+
+namespace SharpCompress.Compressors.LZMA
+{
+    // TODO:
+    // - Write as well as read
+    // - Multi-volume support
+    // - Use of the data size / member size values at the end of the stream
+
+    /// <summary>
+    /// Stream supporting the LZIP format, as documented at http://www.nongnu.org/lzip/manual/lzip_manual.html
+    /// </summary>
+    public class LZipStream : Stream
+    {
+        private readonly Stream stream;
+        private bool disposed;
+        private readonly bool leaveOpen;
+
+        public LZipStream(Stream stream, CompressionMode mode)
+            : this(stream, mode, false)
+        {
+        }
+
+        public LZipStream(Stream stream, CompressionMode mode, bool leaveOpen)
+        {
+            if (mode != CompressionMode.Decompress)
+            {
+                throw new NotImplementedException("Only LZip decompression is currently supported");
+            }
+            Mode = mode;
+            this.leaveOpen = leaveOpen;
+            int dictionarySize = ValidateAndReadSize(stream);
+            if (dictionarySize == 0)
+            {
+                throw new IOException("Not an LZip stream");
+            }
+            byte[] properties = GetProperties(dictionarySize);
+            this.stream = new LzmaStream(properties, stream);
+        }
+
+        #region Stream methods
+
+        protected override void Dispose(bool disposing)
+        {
+            if (disposed)
+            {
+                return;
+            }
+            disposed = true;
+            if (disposing && !leaveOpen)
+            {
+                stream.Dispose();
+            }
+        }
+
+        public CompressionMode Mode { get; }
+
+        public override bool CanRead => stream.CanRead;
+
+        public override bool CanSeek => false;
+
+        public override bool CanWrite => false;
+
+        public override void Flush()
+        {
+            stream.Flush();
+        }
+    
+        // TODO: Both Length and Position are sometimes feasible, but would require
+        // reading the output length when we initialize.
+        public override long Length { get { throw new NotImplementedException(); } }
+
+        public override long Position { get { throw new NotImplementedException(); } set { throw new NotImplementedException(); } }
+
+        public override int Read(byte[] buffer, int offset, int count) => stream.Read(buffer, offset, count);
+
+        public override long Seek(long offset, SeekOrigin origin)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override void SetLength(long value)
+        {
+            throw new NotImplementedException();
+        }
+
+        public override void Write(byte[] buffer, int offset, int count)
+        {
+            throw new NotImplementedException();
+        }
+        #endregion
+
+        /// <summary>
+        /// Determines if the given stream is positioned at the start of a v1 LZip
+        /// file, as indicated by the ASCII characters "LZIP" and a version byte
+        /// of 1, followed by at least one byte.
+        /// </summary>
+        /// <param name="stream">The stream to read from. Must not be null.</param>
+        /// <returns><c>true</c> if the given stream is an LZip file, <c>false</c> otherwise.</returns>
+        public static bool IsLZipFile(Stream stream) => ValidateAndReadSize(stream) != 0;
+
+        /// <summary>
+        /// Reads the 6-byte header of the stream, and returns 0 if either the header
+        /// couldn't be read or it isn't a validate LZIP header, or the dictionary
+        /// size if it *is* a valid LZIP file.
+        /// </summary>
+        private static int ValidateAndReadSize(Stream stream)
+        {
+            if (stream == null)
+            {
+                throw new ArgumentNullException(nameof(stream));
+            }
+            // Read the header
+            byte[] header = new byte[6];
+            int n = stream.Read(header, 0, header.Length);
+
+            // TODO: Handle reading only part of the header?
+
+            if (n != 6)
+            {
+                return 0;
+            }
+
+            if (header[0] != 'L' || header[1] != 'Z' || header[2] != 'I' || header[3] != 'P' || header[4] != 1 /* version 1 */)
+            {
+                return 0;
+            }
+            int basePower = header[5] & 0x1F;
+            int subtractionNumerator = (header[5] & 0xE0) >> 5;
+            return (1 << basePower) - subtractionNumerator * (1 << (basePower - 4));
+        }
+
+        /// <summary>
+        /// Creates a byte array to communicate the parameters and dictionary size to LzmaStream.
+        /// </summary>
+        private static byte[] GetProperties(int dictionarySize) =>
+            new byte[]
+            {
+                // Parameters as per http://www.nongnu.org/lzip/manual/lzip_manual.html#Stream-format
+                // but encoded as a single byte in the format LzmaStream expects.
+                // literal_context_bits = 3
+                // literal_pos_state_bits = 0
+                // pos_state_bits = 2
+                93,
+                // Dictionary size as 4-byte little-endian value
+                (byte)(dictionarySize & 0xff),
+                (byte)((dictionarySize >> 8) & 0xff),
+                (byte)((dictionarySize >> 16) & 0xff),
+                (byte)((dictionarySize >> 24) & 0xff)
+            };
+    }
+}

src/SharpCompress/Readers/ReaderFactory.cs

@@ -13,6 +13,7 @@ using SharpCompress.Readers.GZip;
 using SharpCompress.Readers.Rar;
 using SharpCompress.Readers.Tar;
 using SharpCompress.Readers.Zip;
+using SharpCompress.Compressors.LZMA;
 
 namespace SharpCompress.Readers
 {
@@ -64,6 +65,18 @@ namespace SharpCompress.Readers
                 }
             }
 
+            rewindableStream.Rewind(false);
+            if (LZipStream.IsLZipFile(rewindableStream))
+            {
+                rewindableStream.Rewind(false);
+                LZipStream testStream = new LZipStream(rewindableStream, CompressionMode.Decompress, true);
+                if (TarArchive.IsTarFile(testStream))
+                {
+                    rewindableStream.Rewind(true);
+                    return new TarReader(rewindableStream, options, CompressionType.LZip);
+                }
+            }
+
             rewindableStream.Rewind(false);
             if (RarArchive.IsRarFile(rewindableStream, options))
             {

src/SharpCompress/Readers/Tar/TarReader.cs

@@ -9,6 +9,7 @@ using SharpCompress.Compressors;
 using SharpCompress.Compressors.BZip2;
 using SharpCompress.Compressors.Deflate;
 using SharpCompress.IO;
+using SharpCompress.Compressors.LZMA;
 
 namespace SharpCompress.Readers.Tar
 {
@@ -38,6 +39,10 @@ namespace SharpCompress.Readers.Tar
                 {
                     return new GZipStream(stream, CompressionMode.Decompress);
                 }
+                case CompressionType.LZip:
+                {
+                    return new LZipStream(stream, CompressionMode.Decompress);
+                }
                 case CompressionType.None:
                 {
                     return stream;
@@ -87,6 +92,19 @@ namespace SharpCompress.Readers.Tar
                 }
                 throw new InvalidFormatException("Not a tar file.");
             }
+
+            rewindableStream.Rewind(false);
+            if (LZipStream.IsLZipFile(rewindableStream))
+            {
+                rewindableStream.Rewind(false);
+                LZipStream testStream = new LZipStream(rewindableStream, CompressionMode.Decompress, false);
+                if (TarArchive.IsTarFile(testStream))
+                {
+                    rewindableStream.Rewind(true);
+                    return new TarReader(rewindableStream, options, CompressionType.LZip);
+                }
+                throw new InvalidFormatException("Not a tar file.");
+            }
             rewindableStream.Rewind(true);
             return new TarReader(rewindableStream, options, CompressionType.None);
         }

test/SharpCompress.Test/Tar/TarReaderTests.cs

@@ -33,6 +33,12 @@ namespace SharpCompress.Test
             Read("Tar.tar.gz", CompressionType.GZip);
         }
 
+        [Fact]
+        public void Tar_LZip_Reader()
+        {
+            Read("Tar.tar.lz", CompressionType.LZip);
+        }
+
         [Fact]
         public void Tar_BZip2_Entry_Stream()
         {