Dyadic source matched to a complete prefix code ($\overline{\ell }=H(p)⟺p_i=2^{-{\ell }_i}$)

For a complete binary prefix code with lengths ${\ell }_i$, the Kraft inequality is an equality: ${\sum }_i{2}^{-{\ell }_i}=1$. Put $q_i=2^{-{\ell }_i}$. The difference between expected codeword length and Shannon entropy isThe Kullback-Leibler divergence is nonnegative and vanishes exactly when $p=q$. Hence a complete code is entropy-tight exactly for its dyadic matching source. For example, lengths $(1,2,2)$ match probabilities $(1/2,1/4,1/4)$. Being an optimal prefix code among symbol-by-symbol codes is a weaker condition and does not imply this equality.