Modular composition via factorization

Modular composition is the problem to compute the composition of two univariate polynomials modulo a third one. For polynomials with coefficients in a finite field, Kedlaya and Umans proved in 2008 that the theoretical bit complexity for performing this task could be made arbitrarily close to linear. Unfortunately, beyond its major theoretical impact, this result has not led to practically faster implementations yet. In this article, we explore how polynomial factorization may help modular composition. Most of our algorithms assume that the modulus is fixed, which allows us to precompute factorizations of the modulus over suitable extension fields. This significant restriction with respect to Kedlaya and Umans' algorithm is acceptable whenever we need to perform many compositions modulo the same modulus. For polynomials over a finite field, we show that compositions modulo a fixed modulus of composite degree are cheaper than general modular compositions. In some very favorable cases of fixed moduli of very smooth degree, we show that the complexity even becomes quasi-linear.