[steerablesafe]

My impression is that arithmetic coding is the inverse. You have N symbols with different known probabilities, and you turn it into a stream of 1/2 probability random bits.

edit: On second thought this is probably a bijection and you can call it "arithmetic coding" in either direction.

[eru]

Agreed.

As far as I can tell, arithmetic coding needs both a compressor and a decompressor. Otherwise it's relatively useless in eg your fancy video file format.