BTreeMap first free key

first_free_key.rs

use std::collections::BTreeMap;

fn first_free_key<V>(map: &BTreeMap<usize, V>) -> usize {
    map.keys()
        .zip(map.keys().skip(1))
        .find(|(a, b)| (*b - *a) > 1)
        .map(|(a, _)| *a + 1)
        .unwrap_or_else(|| map.keys().last().map(|x| x + 1).unwrap_or(0))
}

fn main() {
    let mut map = BTreeMap::new();
    map.insert(0usize, "Hi");
    map.insert(2usize, "Hi");
    map.insert(3usize, "Hi");

    let idx = first_free_key(&map);
    println!("IDX: {}", idx);
}