Avoid 2/3 hashmap lookups

2021/benches/days.rs

@@ -7,7 +7,7 @@ use criterion::criterion_main;
 use criterion::BenchmarkId;
 use criterion::Criterion;
 
-const DAYS_IMPLEMENTED: usize = 18;
+const DAYS_IMPLEMENTED: usize = 20;
 
 fn read_input(day: usize) -> Vec<u8> {
     let input_path = format!("inputs/{:02}.txt", day);

2021/inputs/20.txt

@@ -0,0 +1,102 @@
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2021/src/day20.rs

@@ -1,9 +1,113 @@
+use std::collections::HashSet;
 use std::io::Read;
+use std::mem::swap;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+type Translation = [bool; 512];
+type Point = (i32, i32);
+type Field = HashSet<Point>;
+
+fn read_input(input: &mut dyn Read) -> (Translation, Field) {
+    let mut buffer = Vec::new();
+    input.read_to_end(&mut buffer).unwrap();
+
+    let mut translation = [false; 512];
+
+    let mut it = buffer.split(|&b| b == b'\n');
+
+    translation
+        .iter_mut()
+        .zip(it.next().unwrap())
+        .for_each(|(t, &c)| *t = c == b'#');
+
+    let mut field = Field::default();
+
+    for (y, line) in it.skip(1).enumerate() {
+        for (x, _) in line.iter().enumerate().filter(|(_, &c)| c == b'#') {
+            field.insert((x as i32, y as i32));
+        }
+    }
+
+    (translation, field)
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+fn find_dimensions(field: &Field) -> ((i32, i32), (i32, i32)) {
+    field
+        .iter()
+        .fold(((0, 0), (0, 0)), |((xmin, xmax), (ymin, ymax)), &(x, y)| {
+            ((xmin.min(x), xmax.max(x)), (ymin.min(y), ymax.max(y)))
+        })
+}
+
+fn advance(translation: &Translation, field: &Field, new_field: &mut Field, infinity: &mut bool) {
+    const INDEX_MASK: usize = (1 << 9) - 1;
+    new_field.clear();
+
+    let ((xmin, xmax), (ymin, ymax)) = find_dimensions(field);
+
+    for x in (xmin - 1)..=(xmax + 1) {
+        let mut index = if *infinity { INDEX_MASK } else { 0 };
+
+        for y in (ymin - 1)..=(ymax + 1) {
+            for dx in -1..=1 {
+                index <<= 1;
+
+                let nx = x + dx;
+                let ny = y + 1;
+
+                if nx < xmin || nx > xmax || ny < ymin || ny > ymax {
+                    index |= *infinity as usize;
+                } else if field.contains(&(nx, ny)) {
+                    index |= 1;
+                }
+            }
+
+            index &= INDEX_MASK;
+
+            if translation[index] {
+                new_field.insert((x, y));
+            }
+        }
+    }
+
+    *infinity = translation[if *infinity { 511 } else { 0 }]
+}
+
+fn parts_common(input: &mut dyn Read, count: usize) -> String {
+    let (translation, mut field) = read_input(input);
+    let mut new_field = Field::new();
+    let mut infinity = false;
+
+    for _ in 0..count {
+        advance(&translation, &field, &mut new_field, &mut infinity);
+        swap(&mut field, &mut new_field);
+    }
+
+    field.len().to_string()
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    parts_common(input, 2)
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    parts_common(input, 50)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/20.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 35);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation(part2, SAMPLE, 3351);
+    }
 }

2021/src/samples/20.txt

@@ -0,0 +1,7 @@
+..#.#..#####.#.#.#.###.##.....###.##.#..###.####..#####..#....#..#..##..###..######.###...####..#..#####..##..#.#####...##.#.#..#.##..#.#......#.###.######.###.####...#.##.##..#..#..#####.....#.#....###..#.##......#.....#..#..#..##..#...##.######.####.####.#.#...#.......#..#.#.#...####.##.#......#..#...##.#.##..#...##.#.##..###.#......#.#.......#.#.#.####.###.##...#.....####.#..#..#.##.#....##..#.####....##...##..#...#......#.#.......#.......##..####..#...#.#.#...##..#.#..###..#####........#..####......#..#
+
+#..#.
+#....
+##..#
+..#..
+..###