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Re:A Gentle Introduction to Data Compression by userbinator on Monday December 29, 2014 @09:30PM (#8813003)
I think that for beginners it's far better to talk about the LZ-style algorithms first, since they are much easier to understand without requiring any discussion of probabilities and the associated maths; the idea of replacing repeated strings with a reference to them is very intuitive, and so is the implementation of an LZ compressor/decompressor. In fact, an in-memory LZ decompressor core can be written in less than two dozen bytes of x86 instructions, and can easily outperform the fastest Huffman decompressor.

When I was interested enough in compression to read about it, the majority of the information I could find seemed highly theoretical and made me think that Huffman was a good first step so I managed to write a simple static/dynamic Huffman compressor/decompressor, but that didn't work so well. It was "fast enough", but the compression ratio was disappointing. When I discovered LZ the difference was amazing: an extremely simple algorithm, requiring about an order of magnitude less code in implementation, that compressed much better than Huffman for data which would intuitively be compressible.

In retrospect, it's quite clear that Huffman and other "variable length code" techniques only work because of nonuniformity in symbol probabilities, so it is ineffective on data containing repeating patterns but uniform probability. E.g. take a string 256 bytes long containing all the byte values 0, 1, 2, ... 253, 254, 255; and repeat that string multiple times in a file. LZ can easily compress that pattern but Huffman can't. (Does LZ rely on probability nonuniformity? It doesn't look like it.) The fact that many common methods like FLATE use Huffman only to compress the output of LZ on their input probably reflects this, but I don't think I've seen such an intuitive explanation for it before.

Re:A Gentle Introduction to Data Compression by Scaevolus on Tuesday December 30, 2014 @03:33AM (#8813816)
I agree. Transforms (LZ77, BWT, RLE, ...) are generally more interesting and productive than entropy coding. http://mattmahoney.net/dc/dce.html