AoC2019/src/day3.rs

use std::io::BufRead;
use std::str::FromStr;
use std::num::ParseIntError;
use std::fmt::{self, Display};
use std::collections::{HashSet, HashMap};

use Segment::*;

#[derive(Debug, PartialEq)]
enum Segment {
    Up(i32),
    Down(i32),
    Right(i32),
    Left(i32),
}

enum SegmentError {
    UnknownDirection(char),
    ParseIntError(ParseIntError)
}

impl Display for SegmentError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            SegmentError::UnknownDirection(direction) => write!(f, "Unknown direction `{}`", direction),
            SegmentError::ParseIntError(ref e) => e.fmt(f),
        }
    }
}

impl FromStr for Segment {
    type Err = SegmentError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let direction = s.chars().next().unwrap();
        let length = s[1..].parse().map_err(|e| SegmentError::ParseIntError(e))?;

        let segment = match direction {
            'U' => Up(length),
            'D' => Down(length),
            'R' => Right(length),
            'L' => Left(length),
            direction => {
                return Err(SegmentError::UnknownDirection(direction));
            }
        };
        Ok(segment)
    }
}

impl Segment {
    #[inline]
    fn direction(&self) -> (i32, i32) {
        match self {
            Up(_) => (0, 1),
            Down(_) => (0, -1),
            Right(_) => (1, 0),
            Left(_) => (-1, 0),
        }
    }

    #[inline]
    fn length(&self) -> i32 {
        match self {
            Up(l) | Down(l) | Right(l) | Left(l) => *l
        }
    }
}

fn parse_input<F: BufRead> (input: &mut F) -> Vec<Segment> {
    let mut buf = String::new();
    let _num_bytes = input.read_line(&mut buf).expect("Unable to read a line of input file");
    buf.trim().split(",").filter_map(|segment| segment.parse().ok()).collect()
}

pub fn part1<F: BufRead> (mut input: F) {
    let path1 = parse_input(&mut input);
    let path2 = parse_input(&mut input);
    let res = compute_closest_intersection(path1, path2);
    println!("{}", res);
}

fn compute_closest_intersection(path1: Vec<Segment>, path2: Vec<Segment>) -> i32 {
    let steps1 = build_full_path(path1, 0, (0, 0), HashMap::new(), 1);
    let steps2 = build_full_path(path2, 0, (0, 0), HashMap::new(), 1);

    let seen1: HashSet<_> = steps1.keys().collect();
    let seen2: HashSet<_> = steps2.keys().collect();

    seen1.intersection(&seen2).map(|(x, y)| x.abs() + y.abs()).min().unwrap()
}

pub fn part2<F: BufRead> (mut input: F) {
    let path1 = parse_input(&mut input);
    let path2 = parse_input(&mut input);
    let res = compute_first_intersection(path1, path2);
    println!("{}", res);
}

fn compute_first_intersection(path1: Vec<Segment>, path2: Vec<Segment>) -> i32 {
    let steps1 = build_full_path(path1, 0, (0, 0), HashMap::new(), 1);
    let steps2 = build_full_path(path2, 0, (0, 0), HashMap::new(), 1);

    let seen1: HashSet<_> = steps1.keys().collect();
    let seen2: HashSet<_> = steps2.keys().collect();

    seen1.intersection(&seen2).map(|coord| steps1[coord] + steps2[coord]).min().unwrap()
}

fn build_full_path(
    path: Vec<Segment>,
    index: usize,
    (x0, y0): (i32, i32),
    mut steps: HashMap<(i32, i32), i32>,
    current_step: i32
) -> HashMap<(i32, i32), i32> {
    if index == path.len() {
        steps
    } else {
        let segment = &path[index];
        let (dx, dy) = segment.direction();
        let length = segment.length();
        let mut current_step = current_step;

        for i in 1..(length + 1) {
            steps.entry((x0 + i * dx, y0 + i * dy)).or_insert(current_step);
            current_step += 1;
        }

        build_full_path(path, index + 1, (x0 + length * dx, y0 + length * dy), steps, current_step)
    }
}

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

    #[test]
    fn test_input_parsing() {
        let mut input = "U7,R6,D4,L4\n".as_bytes();
        let expected = vec![Up(7), Right(6), Down(4), Left(4)];

        let res = parse_input(&mut input);
        assert_eq!(res, expected);
    }

    #[test]
    fn test_compute_closest_intersection() {
        let mut input = r#"R8,U5,L5,D3
U7,R6,D4,L4
R75,D30,R83,U83,L12,D49,R71,U7,L72
U62,R66,U55,R34,D71,R55,D58,R83
R98,U47,R26,D63,R33,U87,L62,D20,R33,U53,R51
U98,R91,D20,R16,D67,R40,U7,R15,U6,R7
"#.as_bytes();
        let test_results = [6, 159, 135];
        for expected in &test_results {
            let path1 = parse_input(&mut input);
            let path2 = parse_input(&mut input);
            let res = compute_closest_intersection(path1, path2);
            assert_eq!(res, *expected)
        }
    }

    #[test]
    fn test_compute_first_intersection() {
        let mut input = r#"R8,U5,L5,D3
U7,R6,D4,L4
R75,D30,R83,U83,L12,D49,R71,U7,L72
U62,R66,U55,R34,D71,R55,D58,R83
R98,U47,R26,D63,R33,U87,L62,D20,R33,U53,R51
U98,R91,D20,R16,D67,R40,U7,R15,U6,R7
"#.as_bytes();
        let test_results = [30, 610, 410];
        for expected in &test_results {
            let path1 = parse_input(&mut input);
            let path2 = parse_input(&mut input);
            let res = compute_first_intersection(path1, path2);
            assert_eq!(res, *expected)
        }
    }
}