Day 5: Cafeteria

Rust

I didn't look at the input at first so tried a naive version with sets, but it was too slow even for part one. I got smarter and wrote a version with less code that runs in under half a millisecond.

type Id = usize;

struct Ingredients {
    fresh: Vec<RangeInclusive<Id>>,
    available: HashSet<Id>,
}

impl Ingredients {
    fn is_fresh(&self, id: Id) -> bool {
        self.fresh.iter().any(|range| range.contains(&id))
    }

    fn num_fresh_available(&self) -> usize {
        self.available
            .iter()
            .filter(|&&id| self.is_fresh(id))
            .count()
    }

    fn num_fresh_all(&self) -> usize {
        self.fresh
            .iter()
            .map(|range| 1 + range.end() - range.start())
            .sum()
    }
}

fn merge_ranges(mut ranges: Vec<RangeInclusive<Id>>) -> Vec<RangeInclusive<Id>> {
    if ranges.is_empty() {
        return ranges;
    }
    ranges.sort_by_key(|range| *range.start());

    let mut merged = Vec::new();
    let mut start = ranges[0].start();
    let mut end = ranges[0].end();
    for range in ranges.iter().skip(1) {
        if range.start() > end {
            merged.push(RangeInclusive::new(*start, *end));
            start = range.start();
            end = range.end();
        }
        if range.end() > end {
            end = range.end();
        }
    }
    merged.push(RangeInclusive::new(*start, *end));

    merged
}

impl FromStr for Ingredients {
    type Err = Report;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let Some((fresh_str, avail_str)) = s.split_once("\n\n") else {
            bail!("missing divider");
        };

        let fresh = fresh_str
            .lines()
            .map(|l| -> Result<RangeInclusive<_>, Self::Err> {
                let (start, end) = l.split_once('-').ok_or_eyre("bad range")?;
                let start: Id = start.parse()?;
                let end: Id = end.parse()?;
                Ok(start..=end)
            })
            .collect::<Result<_, _>>()?;

        let available = avail_str
            .lines()
            .map(|l| l.parse())
            .collect::<Result<_, _>>()?;

        Ok(Ingredients {
            fresh: merge_ranges(fresh),
            available,
        })
    }
}

::: spoiler Full code

use std::{collections::HashSet, fs, ops::RangeInclusive, str::FromStr};

use color_eyre::eyre::{OptionExt, Report, Result, bail};

#[derive(Debug, PartialEq, Eq)]
struct Ingredients {
    fresh: Vec<RangeInclusive<Id>>,
    available: HashSet<Id>,
}

type Id = usize;

impl Ingredients {
    fn is_fresh(&self, id: Id) -> bool {
        self.fresh.iter().any(|range| range.contains(&id))
    }

    fn num_fresh_available(&self) -> usize {
        self.available
            .iter()
            .filter(|&&id| self.is_fresh(id))
            .count()
    }

    fn num_fresh_all(&self) -> usize {
        self.fresh
            .iter()
            .map(|range| 1 + range.end() - range.start())
            .sum()
    }
}

fn merge_ranges(mut ranges: Vec<RangeInclusive<Id>>) -> Vec<RangeInclusive<Id>> {
    if ranges.is_empty() {
        return ranges;
    }
    ranges.sort_by_key(|range| *range.start());

    let mut merged = Vec::new();
    let mut start = ranges[0].start();
    let mut end = ranges[0].end();
    for range in ranges.iter().skip(1) {
        if range.start() > end {
            merged.push(RangeInclusive::new(*start, *end));
            start = range.start();
            end = range.end();
        }
        if range.end() > end {
            end = range.end();
        }
    }
    merged.push(RangeInclusive::new(*start, *end));

    merged
}

impl FromStr for Ingredients {
    type Err = Report;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let Some((fresh_str, avail_str)) = s.split_once("\n\n") else {
            bail!("missing divider");
        };

        let fresh = fresh_str
            .lines()
            .map(|l| -> Result<RangeInclusive<_>, Self::Err> {
                let (start, end) = l.split_once('-').ok_or_eyre("bad range")?;
                let start: Id = start.parse()?;
                let end: Id = end.parse()?;
                Ok(start..=end)
            })
            .collect::<Result<_, _>>()?;

        let available = avail_str
            .lines()
            .map(|l| l.parse())
            .collect::<Result<_, _>>()?;

        Ok(Ingredients {
            fresh: merge_ranges(fresh),
            available,
        })
    }
}

fn part1(filepath: &str) -> Result<usize> {
    let input = fs::read_to_string(filepath)?;
    let ingrd = Ingredients::from_str(&input).unwrap();
    Ok(ingrd.num_fresh_available())
}

fn part2(filepath: &str) -> Result<usize> {
    let input = fs::read_to_string(filepath)?;
    let ingrd = Ingredients::from_str(&input).unwrap();
    Ok(ingrd.num_fresh_all())
}

fn main() -> Result<()> {
    color_eyre::install()?;

    println!("Part 1: {}", part1("d05/input.txt")?);
    println!("Part 2: {}", part2("d05/input.txt")?);
    Ok(())
}