Function ecm_factor 

pub fn ecm_factor(
    n: &BigInt,
    b1: u64,
    curves: usize,
    seed: u64,
) -> Option<BigInt>

Lenstra’s ECM, stage 1 only. Try curves Suyama curves at smoothness bound b1; a nontrivial factor of n, or None. Cost tracks the smallest factor, not n. None for n ≤ 3.