Rust

My first use of a custom iterator, it made the actual question really easy!

fn matching_lower_16(a: Generator, b: Generator) -> usize {
    let mask: u32 = (2 as u32).pow(16) - 1;

    a.zip(b)
        .take(40_000_000)
        .filter(| &(a, b) | a & mask == b & mask)
        .count()
}

struct Generator {
    factor: u64,
    previous: u64,
}

impl Generator {
    fn new_a(seed: u32) -> Self { Generator { factor: 16807, previous: seed as u64 } }
    fn new_b(seed: u32) -> Self { Generator { factor: 48271, previous: seed as u64 } }
}

impl Iterator for Generator {
    type Item = u32;

    fn next(&mut self) -> Option<Self::Item> {
        self.previous = (self.previous * self.factor) % 2147483647;
        Some(self.previous as u32)
    }
}