I am looking to implement the Rosetta Code LZSW Decompression method in C# and I need some help. The original code is available here: http://rosettacode.org/wiki/LZW_compression#C.23
I am only focusing on the Decompress method as I "simply" (if only) want to decompress .Z-files in my C# program in .NET 6.
I want my version take a byte[] as input and return a byte[] (as I am reading .ReadAllBytes() from file and want to create a new file with the decompressed result).
My problem comes from the fact that in C#, chars are 16bit (2 bytes) and not 8bit (1byte). This really messes with my head as that consequently (in my mind) means that each char should be represented by two bytes. In the code at Rosetta Code, the intial dictionary created only contains integer keys of 0 -> 255 meaning up to 1 byte, not two. I am thinking if this is an error in their implementation? What do you think? And how would you go about converting this algorithm to a method with the signature: byte[] Decompress(byte[]) ?
Thanks
CodePudding user response：
No, there is no error. No, you don't need to convert the algorithm to work on 16-bit values. The usual lossless compression libraries operate on sequences of bytes. Your string of characters would first need to be converted to a sequence of bytes, e.g. to UTF-8, e.g. byte[] bs = Encoding.UTF8.GetBytes(str);. UTF-8 would be the right choice, since that encoding gives the compressor the best shot at compressing. In fact, just encoding UTF-16 to UTF-8 will almost always compress the strings, which makes it a good starting point. (In fact, using UTF-16 as the standard for character strings in .NET is a terrible choice for exactly this reason, but I digress.)
Any data you compress would first be serialized to a sequence of bytes for these compressors, if it isn't bytes already, in a way that permits reversing the transformation after decompression on the other end.
Since you are decompressing, someone encoded the characters into a sequence of bytes, so you need to first find out what they did. It may just be a sequence of ASCII characters, which are already one byte per character. Then you would use System.Text.Encoding.ASCII.GetString(bs); to make a character string out of it.
CodePudding user response：
When compressing data we usually talk about sequences of symbols. A symbol in this context might be a byte, a character, or something completely different.
Your example obviously uses characters as it's symbols, but there should not be any real problem just replacing this with bytes instead. The more difficult part will be its use of strings to represent sequences of characters. You will need an equivalent representation of byte sequences that provides functionality like:
- Concatenation/appending
- Equality
- GetHashCode (for performance)
- Immutability (i.e. appending a byte should produce a new sequence, not modify the existing sequence)
Note that there LZW implementations have to agree on some specific things to be compatible, so implementing the posted example may or may not not allow you to decode .Z files encoded with another implementation. If your goal is to decode actual files you may have better luck asking on software recommendations for a preexisting decompression library.