collatz_20.rs

use std::collections::BinaryHeap;
use std::collections::HashSet;
use std::cmp::Reverse;

fn main() {
    println!("strict digraph {{");

    let mut v : BinaryHeap<(Reverse<u128>, u8)> = Default::default();
    let mut visited : HashSet<u128> = Default::default();
    
    v.push((Reverse(1), 0));
    while let Some((Reverse(c), n)) = v.pop() {
        if visited.contains(&c) { continue }
        visited.insert(c);
    
        let a = [
            Some(2*c),
            if c>1 && (c-1)%3 == 0 && ((c-1)/3)%2==1 { Some((c-1)/3) }
            else { None}
        ];
        for &i in a.iter().flatten() {
            if n < 20 { v.push((Reverse(i), n+1)); }
            println!("{} -> {}", i, c);
            println!("{} [shape=circle, style=filled, fillcolor=\"/spectral11/{}\"]", c, 1 + (((c/9) as f64).ln() as u32));
        }
    }

    println!("}}");
    
}