2024 day 8

Author: ictrobot

crates/year2024/examples/day08_example0.txt

@@ -0,0 +1,12 @@
+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............

crates/year2024/examples/day08_example1.txt

@@ -0,0 +1,10 @@
+..........
+..........
+..........
+....a.....
+..........
+.....a....
+..........
+..........
+..........
+..........

crates/year2024/examples/day08_example2.txt

@@ -0,0 +1,10 @@
+..........
+..........
+..........
+....a.....
+........a.
+.....a....
+..........
+..........
+..........
+..........

crates/year2024/examples/day08_example3.txt

@@ -0,0 +1,10 @@
+..........
+..........
+..........
+....a.....
+........a.
+.....a....
+..........
+......A...
+..........
+..........

crates/year2024/examples/day08_example4.txt

@@ -0,0 +1,10 @@
+T.........
+...T......
+.T........
+..........
+..........
+..........
+..........
+..........
+..........
+..........

crates/year2024/src/day08.rs

@@ -0,0 +1,127 @@
+use utils::array::ArrayVec;
+use utils::prelude::*;
+
+/// Plotting lines between nodes of the same frequency.
+#[derive(Clone, Debug)]
+pub struct Day08 {
+    cols: usize,
+    len: usize,
+    antennas: [ArrayVec<usize, MAX_ANTENNA>; FREQUENCY_COUNT],
+}
+
+const MAX_ANTENNA: usize = 4;
+const FREQUENCY_COUNT: usize = 62;
+
+impl Day08 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let mut lines = input.lines().peekable();
+        let Some(cols) = lines.peek().map(|s| s.len()) else {
+            return Err(InputError::new(input, 0, "expected grid"));
+        };
+        if cols == 0 {
+            return Err(InputError::new(input, 0, "expected at least one column"));
+        }
+
+        let mut antennas = std::array::from_fn(|_| ArrayVec::default());
+        let mut index = 0;
+        for line in lines {
+            if line.len() != cols {
+                return Err(InputError::new(
+                    input,
+                    &line[cols.min(line.len())..],
+                    format!("expected {cols} columns"),
+                ));
+            }
+            for b in line.bytes() {
+                let freq = if b.is_ascii_lowercase() {
+                    b - b'a'
+                } else if b.is_ascii_uppercase() {
+                    b - b'A' + 26
+                } else if b.is_ascii_digit() {
+                    b - b'0' + 52
+                } else if b == b'.' {
+                    index += 1;
+                    continue;
+                } else {
+                    return Err(InputError::new(input, b as char, "expected . or frequency"));
+                };
+
+                if antennas[freq as usize].push(index).is_err() {
+                    return Err(InputError::new(
+                        input,
+                        line,
+                        format!("expected at most {MAX_ANTENNA} '{}' antennas", b as char),
+                    ));
+                }
+                index += 1;
+            }
+        }
+
+        Ok(Self {
+            cols,
+            len: index,
+            antennas,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> usize {
+        self.count_antinode_locations(false)
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> usize {
+        self.count_antinode_locations(true)
+    }
+
+    #[inline]
+    fn count_antinode_locations(&self, part2: bool) -> usize {
+        let mut antinodes = vec![false; self.len];
+        for indexes in &self.antennas {
+            for (i, &index1) in indexes.iter().enumerate() {
+                for &index2 in &indexes[i + 1..] {
+                    let offset = index2 - index1;
+                    // Whether adding offset should move to the right/increase the column number.
+                    // Used to prevent wrapping around onto the next/previous row when
+                    // adding/subtracting the offset from the previous index.
+                    let right = index1 % self.cols < index2 % self.cols;
+
+                    let mut prev = index1;
+                    while let Some(index) = prev.checked_sub(offset) {
+                        if (index % self.cols < prev % self.cols) == right {
+                            antinodes[index] = true;
+                            prev = index;
+                            if part2 {
+                                continue;
+                            }
+                        }
+                        break;
+                    }
+
+                    let mut prev = index2;
+                    while let Some(index) = prev.checked_add(offset) {
+                        if index < self.len && ((prev % self.cols < index % self.cols) == right) {
+                            antinodes[index] = true;
+                            prev = index;
+                            if part2 {
+                                continue;
+                            }
+                        }
+                        break;
+                    }
+                }
+
+                antinodes[index1] |= part2;
+            }
+        }
+        antinodes.iter().filter(|&&x| x).count()
+    }
+}
+
+examples!(Day08 -> (usize, usize) [
+    {file: "day08_example0.txt", part1: 14, part2: 34},
+    {file: "day08_example1.txt", part1: 2},
+    {file: "day08_example2.txt", part1: 4},
+    {file: "day08_example3.txt", part1: 4},
+    {file: "day08_example4.txt", part2: 9},
+]);

crates/year2024/src/lib.rs

@@ -9,4 +9,5 @@ utils::year!(2024 => year2024, ${
     5 => day05::Day05,
     6 => day06::Day06,
     7 => day07::Day07,
+    8 => day08::Day08,
 });