Day 21 done and dusted, not general fits the input only

2023/day-21/src/part1.rs

@@ -0,0 +1,101 @@
+#![warn(clippy::all, clippy::pedantic)]
+
+use std::collections::HashSet;
+
+use glam::IVec2;
+use itertools::Itertools;
+use nom::{
+    branch::alt,
+    bytes::complete::tag,
+    character::complete,
+    combinator::eof,
+    multi::{fold_many1, many1},
+    sequence::terminated,
+    IResult, Parser,
+};
+use nom_locate::LocatedSpan;
+
+type Span<'a> = LocatedSpan<&'a str>;
+type SpanIVec2<'a> = LocatedSpan<&'a str, IVec2>;
+
+fn next_step(loc: IVec2, boulders: &HashSet<IVec2>) -> Vec<IVec2> {
+    [IVec2::X, IVec2::NEG_X, IVec2::Y, IVec2::NEG_Y]
+        .iter()
+        .map(|dir| loc + *dir)
+        .filter(|loc| boulders.get(loc).is_none())
+        .collect()
+}
+
+/// day 21 part 1 of aoc 2023
+///
+/// # Arguments
+/// - input the input for today's puzzle
+///
+/// # Panics
+/// panics whne it cannot parse the input OR when ever the number of game numbers is greater than
+/// usize
+#[must_use]
+pub fn part1(input: &str, steps: u32) -> String {
+    let (_, (start, boulders)) = parse_input(Span::from(input)).expect("AOC input should be valid");
+    let mut current = [start].into_iter().collect::<HashSet<_>>();
+    for _i in 0..steps {
+        current = current
+            .iter()
+            .flat_map(|loc| next_step(*loc, &boulders))
+            .unique()
+            .collect::<HashSet<_>>();
+    }
+    current.len().to_string()
+}
+
+fn with_xy(span: Span) -> SpanIVec2 {
+    let x = i32::try_from(span.get_column()).expect("overflow") - 1;
+    let y = i32::try_from(span.location_line()).expect("wrap around") - 1;
+    span.map_extra(|()| IVec2::new(x, y))
+}
+
+fn parse_input(input: Span) -> IResult<Span, (IVec2, HashSet<IVec2>)> {
+    fold_many1(
+        terminated(
+            many1(alt((tag("S"), tag("."), tag("#"))).map(with_xy)),
+            alt((complete::line_ending, eof)),
+        ),
+        || (IVec2::splat(0), HashSet::new()),
+        |(mut start, mut set), row| {
+            for spot in row {
+                if spot.fragment() == &"S" {
+                    start = spot.extra;
+                }
+                if spot.fragment() == &"#" {
+                    set.insert(spot.extra);
+                }
+            }
+            (start, set)
+        },
+    )(input)
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    use rstest::rstest;
+
+    const INPUT: &str = "...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........";
+    #[rstest]
+    #[case(6, "16")]
+    fn part1_works(#[case] steps: u32, #[case] expected: &str) {
+        let result = part1(INPUT, steps);
+        assert_eq!(result, expected.to_string());
+    }
+}