day20 part 1

src/day16.rs

@@ -1,6 +1,6 @@
+use crate::split_once;
 use aoc_runner_derive::aoc;
 use std::ops::RangeInclusive;
-use crate::split_once;
 
 struct Rule<'a> {
     name: &'a str,

src/day20.rs

@@ -0,0 +1,220 @@
+use crate::split_once;
+use aoc_runner_derive::aoc;
+use std::collections::HashMap;
+
+#[derive(Debug)]
+struct Tile {
+    id: usize,
+    data: [bool; 100],
+}
+impl Tile {
+    fn parse(input: &str) -> Option<Tile> {
+        let (id_line, input_data) = split_once(input, ":\n")?;
+        let (_, id) = split_once(id_line, " ")?;
+        let mut data = [false; 100];
+        for idx in input_data
+            .as_bytes()
+            .iter()
+            .cloned()
+            .filter(|&b| b != b'\n')
+            .enumerate()
+            .filter_map(|(idx, b)| if b == b'#' { Some(idx) } else { None })
+        {
+            data[idx] = true;
+        }
+        Some(Tile {
+            id: id.parse().ok()?,
+            data,
+        })
+    }
+
+    fn sides(&self) -> Vec<u16> {
+        let top = self.data[0..10]
+            .iter()
+            .cloned()
+            .enumerate()
+            .filter(|&(_, b)| b)
+            .fold(0u16, |accum, (idx, _)| accum | (1 << idx));
+        let bottom = self.data[90..100]
+            .iter()
+            .cloned()
+            .enumerate()
+            .filter(|&(_, b)| b)
+            .fold(0u16, |accum, (idx, _)| accum | (1 << idx));
+
+        let left = self
+            .data
+            .iter()
+            .step_by(10)
+            .cloned()
+            .enumerate()
+            .filter(|&(_, b)| b)
+            .fold(0u16, |accum, (idx, _)| accum | (1 << idx));
+
+        let right = self
+            .data
+            .iter()
+            .skip(9)
+            .step_by(10)
+            .cloned()
+            .enumerate()
+            .filter(|&(_, b)| b)
+            .fold(0u16, |accum, (idx, _)| accum | (1 << idx));
+
+        // Put the id's in a consistent order.
+        fn consistent_id(id: u16) -> u16 {
+            let reversed = id.reverse_bits() >> 6;
+            if id < reversed {
+                reversed
+            } else {
+                id
+            }
+        }
+        vec![
+            consistent_id(top),
+            consistent_id(right),
+            consistent_id(bottom),
+            consistent_id(left),
+        ]
+    }
+}
+
+const EXAMPLE: &str = "Tile 2311:
+..##.#..#.
+##..#.....
+#...##..#.
+####.#...#
+##.##.###.
+##...#.###
+.#.#.#..##
+..#....#..
+###...#.#.
+..###..###
+
+Tile 1951:
+#.##...##.
+#.####...#
+.....#..##
+#...######
+.##.#....#
+.###.#####
+###.##.##.
+.###....#.
+..#.#..#.#
+#...##.#..
+
+Tile 1171:
+####...##.
+#..##.#..#
+##.#..#.#.
+.###.####.
+..###.####
+.##....##.
+.#...####.
+#.##.####.
+####..#...
+.....##...
+
+Tile 1427:
+###.##.#..
+.#..#.##..
+.#.##.#..#
+#.#.#.##.#
+....#...##
+...##..##.
+...#.#####
+.#.####.#.
+..#..###.#
+..##.#..#.
+
+Tile 1489:
+##.#.#....
+..##...#..
+.##..##...
+..#...#...
+#####...#.
+#..#.#.#.#
+...#.#.#..
+##.#...##.
+..##.##.##
+###.##.#..
+
+Tile 2473:
+#....####.
+#..#.##...
+#.##..#...
+######.#.#
+.#...#.#.#
+.#########
+.###.#..#.
+########.#
+##...##.#.
+..###.#.#.
+
+Tile 2971:
+..#.#....#
+#...###...
+#.#.###...
+##.##..#..
+.#####..##
+.#..####.#
+#..#.#..#.
+..####.###
+..#.#.###.
+...#.#.#.#
+
+Tile 2729:
+...#.#.#.#
+####.#....
+..#.#.....
+....#..#.#
+.##..##.#.
+.#.####...
+####.#.#..
+##.####...
+##..#.##..
+#.##...##.
+
+Tile 3079:
+#.#.#####.
+.#..######
+..#.......
+######....
+####.#..#.
+.#...#.##.
+#.#####.##
+..#.###...
+..#.......
+..#.###...
+";
+
+#[aoc(day20, part1)]
+fn solve_d20_p1(input: &str) -> usize {
+    let tiles: Vec<_> = input
+        .split("\n\n")
+        .map(|i| Tile::parse(i).unwrap())
+        .collect();
+    let mut side_to_tile: HashMap<_, Vec<_>> = HashMap::new();
+    for tile in &tiles {
+        for side in tile.sides() {
+            side_to_tile.entry(side).or_default().push(tile.id);
+        }
+    }
+    tiles
+        .iter()
+        .filter_map(|tile| {
+            let num_unique_sides = tile.sides().iter().fold(0, |accum, side| {
+                if side_to_tile.get(&side).unwrap().len() == 1 {
+                    accum + 1
+                } else {
+                    accum
+                }
+            });
+            if num_unique_sides == 2 {
+                Some(tile.id)
+            } else {
+                None
+            }
+        })
+        .product()
+}

src/lib.rs

@@ -10,6 +10,7 @@ pub mod day17;
 pub mod day18;
 pub mod day19;
 pub mod day2;
+pub mod day20;
 pub mod day3;
 pub mod day4;
 pub mod day5;
@@ -23,4 +24,4 @@ aoc_runner_derive::aoc_lib! { year = 2020 }
 fn split_once<'a>(input: &'a str, delimeter: &str) -> Option<(&'a str, &'a str)> {
     let idx = input.find(delimeter)?;
     Some((&input[..idx], &input[idx + delimeter.len()..]))
-}
+}