// [dependencies]
// primal = "0.2"

fn compute_transitions(limit: usize) {
    use std::collections::BTreeMap;
    let mut transitions = BTreeMap::new();
    let mut prev = 0;
    for n in primal::Primes::all().take(limit) {
        let digit = n % 10;
        if prev != 0 {
            let key = (prev, digit);
            if let Some(v) = transitions.get_mut(&key) {
                *v += 1;
            } else {
                transitions.insert(key, 1);
            }
        }
        prev = digit;
    }
    println!("First {} prime numbers:", limit);
    for ((from, to), c) in &transitions {
        let freq = 100.0 * (*c as f32) / (limit as f32);
        println!(
            "{} -> {}: count = {:7}, frequency = {:.2} %",
            from, to, c, freq
        );
    }
}

fn main() {
    compute_transitions(1000000);
    println!();
    compute_transitions(100000000);
}