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What’s the concept behind zip compression? I can understand the concept of removing empty space etc, but presumably something has to be added to say how much/where that free space needs to be added back in during decompression?
What’s the basic process for compressing a stream of bytes?
A good place to start would be to lookup the Huffman compression scheme. The basic idea behind huffman is that in a given file some bytes appear more frequently then others (in a plaintext file many bytes won’t appear at all). Rather then spend 8 bits to encode every byte, why not use a shorter bit sequence to encode the most common characters, and a longer sequences to encode the less common characters (these sequences are determined by creating a huffman tree).
Once you get a handle on using these trees to encode/decode files based on character frequency, imagine that you then start working on word frequency – instead of encoding “they” as a sequence of 4 characters, why not consider it to be a single character due to its frequency, allowing it to be assigned its own leaf in the huffman tree. This is more or less the basis of ZIP and other lossless type compression – they look for common “words” (sequences of bytes) in a file (including sequences of just 1 byte if common enough) and use a tree to encode them. The zip file then only needs to include the tree info (a copy of each sequence and the number of times it appears) to allow the tree to be reconstructed and the rest of the file to be decoded.
Follow up:
To better answer the original question, the idea behind lossless compression is not so much to remove empty space, but to remove redundent information.
If you created a database to store music lyrics, you’d find a lot of space was being used to store the chorus which repeats several times. Instead of using all that space, you could simply place the word CHORUS before the first instance of the chorus lines, and then every time the chorus is to be repeated, just use CHORUS as a place holder (in fact this is pretty much the idea behind LZW compression – in LZW each line of the song would have a number shown before it. If a line repeats later in the song, rather then write out the whole line only the number is shown)