AdventOfCode/y2015/src/days/d9.rs

use core::panic;
use std::{
    collections::{HashMap, HashSet},
    error::Error,
};

pub fn process_part1(input: &str) -> u32 {
    let mut distances = HashMap::new();
    let mut locations = HashSet::new();
    input.lines().for_each(|line| {
        let (direction, distance) = line.split_once(" = ").unwrap();
        let (from, to) = direction.split_once(" to ").unwrap();
        let distance = distance.parse::<u32>().unwrap();
        distances.insert((from.to_string(), to.to_string()), distance);
        locations.insert(from.to_string());
        locations.insert(to.to_string());
    });
    let locations = locations
        .iter()
        .map(|place| place.to_string())
        .collect::<Vec<String>>();
    let num_permutations = factorial(locations.len());
    let mut shortest = u32::MAX;
    for idx in 1..=num_permutations {
        let perm = permutation(locations.clone(), idx).unwrap();
        let route_length = get_route_length(perm, &distances);
        if route_length < shortest {
            shortest = route_length;
        }
    }
    shortest
}

fn get_route_length(route: Vec<String>, distances: &HashMap<(String, String), u32>) -> u32 {
    route
        .windows(2)
        .map(|pair| {
            let distance = distances.get(&(pair[0].clone(), pair[1].clone()));
            if let Some(distance) = distance {
                return distance;
            }
            let distance = distances.get(&(pair[1].clone(), pair[0].clone()));
            if let Some(distance) = distance {
                return distance;
            }
            panic!("Should not happen");
        })
        .sum()
}

fn permutation(mut locations: Vec<String>, nth: usize) -> Result<Vec<String>, Box<dyn Error>> {
    locations.sort();
    if nth == 1 {
        return Ok(locations);
    }
    if nth > factorial(locations.len()) || nth == 0 {
        return Err(Box::from("Out of bounds"));
    }
    let mut perm = Vec::new();
    let num_unique_locations = locations.len();
    let mut remainder = nth - 1;
    for idx in 1..=locations.len() {
        let permutations = remainder / factorial(num_unique_locations - idx);
        remainder %= factorial(num_unique_locations - idx);
        perm.push(locations[permutations].clone());
        locations.remove(permutations);
    }
    Ok(perm)
}

fn factorial(num: usize) -> usize {
    let mut fact = 1;
    for n in 1..=num {
        fact *= n;
    }
    fact
}

pub fn process_part2(input: &str) -> u32 {
    let mut distances = HashMap::new();
    let mut locations = HashSet::new();
    input.lines().for_each(|line| {
        let (direction, distance) = line.split_once(" = ").unwrap();
        let (from, to) = direction.split_once(" to ").unwrap();
        let distance = distance.parse::<u32>().unwrap();
        distances.insert((from.to_string(), to.to_string()), distance);
        locations.insert(from.to_string());
        locations.insert(to.to_string());
    });
    let locations = locations
        .iter()
        .map(|place| place.to_string())
        .collect::<Vec<String>>();
    let num_permutations = factorial(locations.len());
    let mut longest = 0;
    for idx in 1..=num_permutations {
        let perm = permutation(locations.clone(), idx).unwrap();
        let route_length = get_route_length(perm, &distances);
        if route_length > longest {
            longest = route_length;
        }
    }
    longest
}

#[cfg(test)]
mod tests {
    use super::*;

    const INPUT: &str = "London to Dublin = 464
London to Belfast = 518
Dublin to Belfast = 141";

    #[test]
    fn part1() {
        let result = process_part1(INPUT);
        assert_eq!(result, 605);
    }

    #[test]
    fn part2() {
        let result = process_part2(INPUT);
        assert_eq!(result, 982);
    }
}