Support added for TAR LZW compression (Unix 'compress' resulting in .…

…Z files)

src/SharpCompress/Common/CompressionType.cs

@@ -15,5 +15,6 @@ public enum CompressionType
     Xz,
     Unknown,
     Deflate64,
-    Shrink
+    Shrink,
+    Lzw
 }

src/SharpCompress/Compressors/Lzw/LzwConstants.cs

@@ -0,0 +1,65 @@
+namespace SharpCompress.Compressors.Lzw
+{
+    /// <summary>
+    /// This class contains constants used for LZW
+    /// </summary>
+    [System.Diagnostics.CodeAnalysis.SuppressMessage(
+        "Naming",
+        "CA1707:Identifiers should not contain underscores",
+        Justification = "kept for backwards compatibility"
+    )]
+    public sealed class LzwConstants
+    {
+        /// <summary>
+        /// Magic number found at start of LZW header: 0x1f 0x9d
+        /// </summary>
+        public const int MAGIC = 0x1f9d;
+
+        /// <summary>
+        /// Maximum number of bits per code
+        /// </summary>
+        public const int MAX_BITS = 16;
+
+        /* 3rd header byte:
+         * bit 0..4 Number of compression bits
+         * bit 5    Extended header
+         * bit 6    Free
+         * bit 7    Block mode
+         */
+
+        /// <summary>
+        /// Mask for 'number of compression bits'
+        /// </summary>
+        public const int BIT_MASK = 0x1f;
+
+        /// <summary>
+        /// Indicates the presence of a fourth header byte
+        /// </summary>
+        public const int EXTENDED_MASK = 0x20;
+
+        //public const int FREE_MASK      = 0x40;
+
+        /// <summary>
+        /// Reserved bits
+        /// </summary>
+        public const int RESERVED_MASK = 0x60;
+
+        /// <summary>
+        /// Block compression: if table is full and compression rate is dropping,
+        /// clear the dictionary.
+        /// </summary>
+        public const int BLOCK_MODE_MASK = 0x80;
+
+        /// <summary>
+        /// LZW file header size (in bytes)
+        /// </summary>
+        public const int HDR_SIZE = 3;
+
+        /// <summary>
+        /// Initial number of bits per code
+        /// </summary>
+        public const int INIT_BITS = 9;
+
+        private LzwConstants() { }
+    }
+}