It depends if you have static or adaptive coder.

Static is much cheaper, uses the same probabilities for the entire data block (e.g. 30 kB), probabilities are stored in the header - practically all Huffman and tANS compressors (however, there are considered exceptions: https://en.wikipedia.org/wiki/Adaptive_Huffman_coding ).

Adaptive can start with e.g. uniform probability (no need to store in header) and learns on the way - it is more costly but gives better compression, used with arithmetic coding or rANS. See https://fgiesen.wordpress.com/2015/05/26/models-for-adaptive... https://fgiesen.wordpress.com/2015/12/21/rans-in-practice/