Very dirty solution day 13

2021/inputs/13.txt

@@ -0,0 +1,894 @@
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+
+fold along x=655
+fold along y=447
+fold along x=327
+fold along y=223
+fold along x=163
+fold along y=111
+fold along x=81
+fold along y=55
+fold along x=40
+fold along y=27
+fold along y=13
+fold along y=6

2021/src/day13.rs

@@ -1,9 +1,151 @@
+use std::collections::HashSet;
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+use nom::branch::alt;
+use nom::bytes::complete::tag;
+use nom::sequence::tuple;
+use nom::IResult;
+
+use crate::common::LineIter;
+
+type Coords = (u16, u16);
+
+enum Fold {
+    X(u16),
+    Y(u16),
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+fn parse_coordinates(input: &str) -> IResult<&str, Coords> {
+    use nom::character::complete::char;
+    use nom::character::complete::u16;
+
+    let (input, (x, _, y)) = tuple((u16, char(','), u16))(input)?;
+
+    Ok((input, (x, y)))
+}
+
+fn parse_fold(input: &str) -> IResult<&str, Fold> {
+    use nom::character::complete::char;
+    use nom::character::complete::u16;
+
+    let (input, (_, axis, _, coord)) = tuple((
+        tag("fold along "),
+        alt((char('x'), char('y'))),
+        char('='),
+        u16,
+    ))(input)?;
+
+    let fold = match axis {
+        'x' => Fold::X(coord),
+        'y' => Fold::Y(coord),
+        _ => unreachable!("Should be filtered by nom"),
+    };
+
+    Ok((input, fold))
+}
+
+fn apply_fold(dots: &mut HashSet<Coords>, fold: Fold) {
+    let mut to_fold = Vec::new();
+
+    match fold {
+        Fold::X(coord) => dots.retain(|&(x, y)| {
+            if x < coord {
+                true
+            } else {
+                to_fold.push((2 * coord - x, y));
+                false
+            }
+        }),
+        Fold::Y(coord) => dots.retain(|&(x, y)| {
+            if y < coord {
+                true
+            } else {
+                to_fold.push((x, 2 * coord - y));
+                false
+            }
+        }),
+    }
+
+    dots.extend(to_fold.drain(..));
+}
+
+fn print_dots(dots: &HashSet<Coords>) -> String {
+    let (x, y) = dots
+        .iter()
+        .fold((0, 0), |(xc, yc), &(xn, yn)| (xc.max(xn), yc.max(yn)));
+
+    let mut buffer = String::with_capacity((x as usize + 1) * y as usize);
+
+    for y in 0..=y {
+        for x in 0..=x {
+            if dots.contains(&(x, y)) {
+                buffer.push('#');
+            } else {
+                buffer.push(' ');
+            }
+        }
+        buffer.push('\n');
+    }
+
+    buffer.pop();
+
+    buffer
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    let mut reader = LineIter::new(input);
+
+    let mut dots = HashSet::new();
+
+    while let Some(line) = reader.next() {
+        if line == "" {
+            break;
+        }
+
+        let (_, coords) = parse_coordinates(line).unwrap();
+
+        dots.insert(coords);
+    }
+
+    let fold = parse_fold(reader.next().unwrap()).unwrap().1;
+    apply_fold(&mut dots, fold);
+
+    dots.len().to_string()
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    let mut reader = LineIter::new(input);
+
+    let mut dots = HashSet::new();
+
+    while let Some(line) = reader.next() {
+        if line == "" {
+            break;
+        }
+
+        let (_, coords) = parse_coordinates(line).unwrap();
+
+        dots.insert(coords);
+    }
+
+    while let Some(line) = reader.next() {
+        let fold = parse_fold(line).unwrap().1;
+        apply_fold(&mut dots, fold);
+    }
+
+    print_dots(&dots)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::test_implementation;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/13.txt");
+
+    #[test]
+    fn sample_part() {
+        test_implementation(part1, SAMPLE, 17);
+    }
 }

2021/src/samples/13.txt

@@ -0,0 +1,21 @@
+6,10
+0,14
+9,10
+0,3
+10,4
+4,11
+6,0
+6,12
+4,1
+0,13
+10,12
+3,4
+3,0
+8,4
+1,10
+2,14
+8,10
+9,0
+
+fold along y=7
+fold along x=5