advent-of-code/2022/src/day4.rs

use nom::{self, bytes::complete::tag, character::complete, IResult};
use std::collections::HashSet;

const INPUT: &str = include_str!("../data/day4.txt");

pub fn part1() -> String {
    format!("{}", count_full_containments(&input(INPUT)))
}

pub fn part2() -> String {
    format!("{}", count_overlaps(&input(INPUT)))
}

fn count_full_containments(pairings: &Vec<ElfPairing>) -> u32 {
    pairings
        .into_iter()
        .filter(|pairing| pairing.has_full_containment())
        .count() as u32
}

fn count_overlaps(pairings: &Vec<ElfPairing>) -> u32 {
    pairings
        .into_iter()
        .filter(|pairing| pairing.has_overlap())
        .count() as u32
}

struct ElfPairing {
    elf1: Vec<u32>,
    elf2: Vec<u32>,
}

impl ElfPairing {
    fn has_full_containment(&self) -> bool {
        let elf1 = self.elf1.iter().cloned().collect::<HashSet<u32>>();
        let elf2 = self.elf2.iter().cloned().collect::<HashSet<u32>>();
        elf1.is_subset(&elf2) || elf2.is_subset(&elf1)
    }

    fn has_overlap(&self) -> bool {
        let element_count = self.elf1.len() + self.elf2.len();
        let combined_set = self
            .elf1
            .iter()
            .chain(self.elf2.iter())
            .cloned()
            .collect::<HashSet<u32>>();
        combined_set.len() < element_count
    }
}

fn input(data: &str) -> Vec<ElfPairing> {
    data.lines()
        .map(|line| {
            let (_, pairing) = elf_pair(line).unwrap();
            pairing
        })
        .collect::<Vec<ElfPairing>>()
}

fn elf_pair(input: &str) -> IResult<&str, ElfPairing> {
    let (input, elf1) = elf_range(input)?;
    let (input, _) = tag(",")(input)?;
    let (input, elf2) = elf_range(input)?;

    Ok((input, ElfPairing { elf1, elf2 }))
}

fn elf_range(input: &str) -> IResult<&str, Vec<u32>> {
    let (input, start) = complete::u32(input)?;
    let (input, _) = tag("-")(input)?;
    let (input, end) = complete::u32(input)?;

    Ok((input, (start..end + 1).collect::<Vec<u32>>()))
}

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

    const TEST_DATA: &str = "2-4,6-8
2-3,4-5
5-7,7-9
2-8,3-7
6-6,4-6
2-6,4-8";

    #[test]
    fn it_gets_a_hashset_from_a_range() {
        assert_eq!(vec![2, 3, 4], (2..5).collect::<Vec<u32>>());
    }

    fn with_input<F>(test: F)
    where
        F: Fn(Vec<ElfPairing>) -> () + std::panic::UnwindSafe,
    {
        test(input(TEST_DATA));
    }

    #[test]
    fn it_parses_input_correctly() {
        with_input(|pairings| {
            assert_eq!(vec![2, 3, 4], pairings[0].elf1);
            assert_eq!(vec![6, 7, 8], pairings[0].elf2);

            assert_eq!(vec![6], pairings[4].elf1);
            assert_eq!(vec![4, 5, 6], pairings[4].elf2);
        });
    }

    #[test]
    fn it_checks_full_containment() {
        with_input(|pairings| {
            assert!(!pairings[0].has_full_containment());
            assert!(!pairings[1].has_full_containment());
            assert!(!pairings[2].has_full_containment());
            assert!(pairings[3].has_full_containment());
            assert!(pairings[4].has_full_containment());
            assert!(!pairings[5].has_full_containment());
        });
    }

    #[test]
    fn it_counts_full_containments() {
        with_input(|pairings| {
            assert_eq!(2, count_full_containments(&pairings));
        });
    }

    #[test]
    fn it_checks_overlap() {
        with_input(|pairings| {
            assert!(!pairings[0].has_overlap());
            assert!(!pairings[1].has_overlap());
            assert!(pairings[2].has_overlap());
            assert!(pairings[3].has_overlap());
            assert!(pairings[4].has_overlap());
            assert!(pairings[5].has_overlap());
        });
    }

    #[test]
    fn it_checks_part1() {
        let pairings = input(INPUT);
        assert_eq!(466, count_full_containments(&pairings));
        assert_eq!(865, count_overlaps(&pairings));
    }
}