The code function produces code words. The expected length of the code word is limited by the entropy from the source probability $p$.

The Shannon information content, aka self-information, is described by

$$- \log_2 p(x=a),$$

for the case that $x=a$.

The Shannon entropy is the expected information content for the whole sequence with probability distribution $p(x)$,

$$\mathcal H = - \sum_x p(x\in X) \log_2 p(x).$$

The Shannon source coding theorem says that for $N$ samples from the source, we can roughly compress it into $N\mathcal H$.

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Lei Ma (2021). 'Coding Theory Concepts', Datumorphism, 02 April. Available at: https://datumorphism.leima.is/cards/information/coding-theory-concepts/.

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• cards/information/coding-theory-concepts.md