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Discussion on: Advent of Code 2020 Solution Megathread - Day 7: Handy Haversacks

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Neil Gall

Oh dear, I didn't look at the input and assumed there were just the nine colours in the example. Modelled them in an enum and wrote parsers for them all, which failed on the first line of input text. Lesson: look at the real data!

The problem is a directed acyclic graph one. I modelled the graph as a Vec of edges with the start and end nodes, which meant quite expensive searching. It only took 10 or 20 seconds to run but I knew it was a mistake. Reworked to a HashMap where the key is the start of each edge and the value is the Vec of possible end nodes.

The actual graph traversals are the bread and butter of my day job. Those were easy. I wasted a lot of time today on parsing and bad modelling.

use std::collections::HashMap;
use std::fs::File;
use std::io::prelude::*;

mod parser;
use parser::*;

// --- file read

fn read_file(filename: &str) -> std::io::Result<String> {
    let mut file = File::open(filename)?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    Ok(contents)
}

// --- model

#[derive(Debug, Clone, Eq, Hash, PartialEq)]
struct BagColor(String, String);

impl BagColor {
    fn of(adj: &str, col: &str) -> Self {
        BagColor(String::from(adj), String::from(col))
    }
}

#[derive(Debug, Eq, PartialEq)]
struct Content {
    color: BagColor,
    count: usize
}

#[derive(Debug, Eq, PartialEq)]
struct ContainsRule {
    container: BagColor,
    contents: Vec<Content>
}

#[derive(Debug)]
struct RuleSet {
    rules: HashMap<BagColor, Vec<Content>>
}

fn parse_rule<'a>() -> impl Parser<'a, ContainsRule> {
    fn bag_color<'b>() -> impl Parser<'b, BagColor> {
        let adjective = one_or_more(letter).map(|ls| ls.into_iter().collect());
        let color = one_or_more(letter).map(|ls| ls.into_iter().collect());

        pair(first(adjective, whitespace), color).map(|(a, c)| BagColor(a, c))
    }

    fn container<'b>() -> impl Parser<'b, BagColor> {
        first(bag_color(), string(" bags contain "))
    }

    let bag_or_bags = string(" bags, ").or(string(" bag, ")).or(string(" bags.")).or(string(" bag."));
    let contained = pair(first(integer, whitespace), first(bag_color(), bag_or_bags));

    let contents_rule = pair(container(), one_or_more(contained)).map(|(color, contents)| 
        ContainsRule {
            container: color.clone(),
            contents: contents.iter().map(|(n, c)| Content {
                color: c.clone(),
                count: *n as usize
            }).collect()
        }
    );

    let no_contents_rule = first(container(), string("no other bags.")).map(|color| 
        ContainsRule {
            container: color,
            contents: vec![]
        }
    );

    contents_rule.or(no_contents_rule)
}

fn parse_input(input: &str) -> ParseResult<RuleSet> {
    let rule_set = one_or_more(first(parse_rule(), whitespace));

    rule_set.parse(input).map(|(rest, rules)| {
        let rule_set = RuleSet { 
            rules: rules.into_iter().map(|r| (r.container, r.contents)).collect()
        };
        (rest, rule_set)
    })
}

impl RuleSet {
    fn can_contain(&self, from: &BagColor, to: &BagColor) -> bool {
        self.rules.get(from)
            .map(|contents| 
                contents.iter().any(|c| &c.color == to))
            .unwrap_or(false)
    }

    fn can_contain_indirectly(&self, from: &BagColor, to: &BagColor) -> bool {
        self.can_contain(from, to) 
            || self.rules.get(from).map(|contents|
                contents.iter().any(|c| self.can_contain_indirectly(&c.color, to)))
                .unwrap_or(false)
    }

    fn number_of_contained_bags(&self, from: &BagColor) -> usize {
        self.rules.get(from)
            .map(|contents| contents.iter()
                .map(|c| c.count * (1 + self.number_of_contained_bags(&c.color)))
                .sum())
            .unwrap_or(0)
    }

    // --- problems 

    fn part1(&self) -> usize {
        self.rules.keys()
            .filter(|color| self.can_contain_indirectly(&color, &BagColor::of("shiny", "gold")))
            .count()
    }

    fn part2(&self) -> usize {
        self.number_of_contained_bags(&BagColor::of("shiny", "gold"))
    }
}

fn main() {
    let input = read_file("./input.txt").unwrap();
    let rules: RuleSet = parse_input(&input).unwrap().1;

    println!("part1 {}", rules.part1());
    println!("part2 {}", rules.part2());
}


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

    #[test]
    fn test_parse_with_single_clause() {
        assert_eq!(
            parse_rule().parse("light red bags contain 1 bright white bag."),
            Ok(("", ContainsRule {
                container: BagColor::of("light", "red"),
                contents: vec![
                    Content { color: BagColor::of("bright", "white"), count: 1 }
                ]
            }))
        );
    }

    #[test]
    fn test_parse_with_two_clauses() {
        assert_eq!(
            parse_rule().parse("light red bags contain 1 bright white bag, 2 muted yellow bags."),
            Ok(("", ContainsRule { 
                container: BagColor::of("light", "red"),
                contents: vec![
                    Content { color: BagColor::of("bright", "white"), count: 1 },
                    Content { color: BagColor::of("muted", "yellow"), count: 2 }
                ]
            }))
        );
    }

    #[test]
    fn test_parse_with_many_clauses() {
        assert_eq!(
            parse_rule().parse("dotted silver bags contain 2 dotted orange bags, 3 bright fuchsia bags, 5 bright tomato bags, 3 faded turquoise bags."),
            Ok(("", ContainsRule {
                container: BagColor::of("dotted", "silver"),
                contents: vec![
                    Content { color: BagColor::of("dotted", "orange"), count: 2 },
                    Content { color: BagColor::of("bright", "fuchsia"), count: 3 },
                    Content { color: BagColor::of("bright", "tomato"), count: 5 },
                    Content { color: BagColor::of("faded", "turquoise"), count: 3 }
                ]
            }))
        );
    }

    #[test]
    fn test_parse_with_no_contents() {
        assert_eq!(
            parse_rule().parse("faded blue bags contain no other bags."),
            Ok(("", ContainsRule {
                container: BagColor::of("faded", "blue"),
                contents: vec![]
            }))
        );
    }

    #[test]
    fn test_parse_records_separated_by_lines() {
        let p = one_or_more(first(letter, whitespace));
        assert_eq!(p.parse("a\nb\nc\n"), Ok(("", vec!['a', 'b', 'c'])));
    }
}
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