The classpoly package provides a low-level C implementation of the algorithms to compute class polynomials that are described in the papers:
Andrew V. Sutherland, Computing Hilbert class polynomials with the Chinese Remainder Theorem, Mathematics of Computation 80 (2011), 501-538,You are welcome to use and/or modify this software in your research provided you cite the papers above if/when you publish your work.
Portions of this software have been adapted and incorporated into Pari/GP by Hamish Ivey-Law where they can be accessed via the polclass function.
Download link: classpoly_v1.0.2.tar (1.2MB)