Friday, February 14, 2014

FSE decoding : wrap up

 With all the previous articles written on FSE, it seems we now have all elements to build a complete FSE decoder. Let's wrap up all this learning into a single piece.

To start with, we consider that all symbols are attributed a weight, of minimum 1, normalized over a sum which is a power of 2 (8, 16, 32, etc.).
The size of the decoding table is this sum.

Each symbol will appear in the table exactly as many times as it weights.

We have seen that, to maximize table's capability to capture fractional bits, we have to spread each symbol over the table, at roughly regular distances (the more regular, the better the compression performance).
For example, using the previous example of a symbol A with weight 5/16 :

We have seen that destination sub-ranges must sum up to cover the complete table, with each sub-range having a size in power of 2, defining an exact number of bits to use. We have seen that the smaller ranges must be placed at the beginning.

We now must associate each symbol in the table with a destination range.
To do that, we will number the ranges, beginning with the larger ones.

So the final assembling becomes :

If you build a decoding table using the above construction method, you have a simple decoding algorithm as a result. The hot loop is a mere 4-lines, as follows :

// Persistent variables across the loops : state and bitStream
       BYTE symbol = decodeTable[state].symbol;
       int nbBits = decodeTable[state].nbBits;
       int rest = getBits (bitStream, nbBits);
       state = decodeTable[state].newStateBaseline + rest;

This is all it takes to build your own FSE decoder.
You can always use the code at Github for further guidance.

Next part will be a study of the compression algorithm part of FSE, which is a bit more complex...

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