day 21 part 2

src/day21.rs

@@ -0,0 +1,299 @@
+use std::{collections::HashMap, fs::read_to_string};
+
+use itertools::Itertools;
+
+pub fn day_main() {
+    let input = read_to_string("input/day21.txt").unwrap();
+    let input = input.trim();
+    println!(" part1: {}", part1(input));
+    println!(" part2: {}", part2(input));
+}
+
+type RiddleResult = i64;
+
+fn part1(input: &str) -> RiddleResult {
+    let mut p = Precomputed::new(2);
+    input
+        .lines()
+        .map(|line| p.shortest(line) * value(line))
+        .sum()
+}
+
+fn value(line: &str) -> i64 {
+    line[0..3].parse::<i64>().unwrap()
+}
+
+fn part2(input: &str) -> RiddleResult {
+    let mut p = Precomputed::new(25);
+    input
+        .lines()
+        .map(|line| p.shortest(line) * value(line))
+        .sum()
+}
+
+struct Precomputed {
+    num_sp: HashMap<(char, char), Vec<&'static str>>,
+    arrow_sp: HashMap<(char, char), Vec<&'static str>>,
+    robot_layers: i64,
+    cache: HashMap<(char, char, i64), i64>,
+}
+
+impl Precomputed {
+    fn new(robot_layers: i64) -> Self {
+        let num_sp = num_sp();
+        let arrow_sp = arrow_sp();
+        Self {
+            num_sp,
+            arrow_sp,
+            robot_layers,
+            cache: HashMap::new(),
+        }
+    }
+
+    fn shortest(&mut self, digit_pad: &str) -> RiddleResult {
+        format!("A{digit_pad}")
+            .chars()
+            .tuple_windows()
+            .map(|(a, b)| {
+                let x = self
+                    .num_sp
+                    .get(&(a, b))
+                    .unwrap()
+                    .clone()
+                    .iter()
+                    .map(|sp| format!("A{sp}A")) // we add the A in the next layer at the end of each sequence
+                    .map(|sp| {
+                        let x = if sp.len() > 1 {
+                            sp.chars()
+                                .tuple_windows()
+                                .map(|(c, d)| self.get(c, d, self.robot_layers))
+                                .sum::<i64>()
+                        } else {
+                            1
+                        };
+                        // println!(" sp {a} to {b} (NUM): {sp} -- {x}");
+                        x
+                    })
+                    .min()
+                    .unwrap();
+                // println!("{a}{b}: {x}");
+                x
+            })
+            // .inspect(|r| println!("shortest part: {r}"))
+            .sum()
+    }
+
+    fn get(&mut self, a: char, b: char, n: i64) -> i64 {
+        if n == 0 {
+            return 1;
+        }
+        if let Some(result) = self.cache.get(&(a, b, n)) {
+            return *result;
+        }
+        let paths = self.arrow_sp.get(&(a, b)).unwrap().clone();
+        let result = paths
+            .iter()
+            .map(|sp| format!("A{sp}A"))
+            .map(|sp| {
+                let x = if sp.len() > 1 {
+                    sp.chars()
+                        .tuple_windows()
+                        .map(|(c, d)| self.get(c, d, n - 1))
+                        .sum::<i64>()
+                } else {
+                    1
+                };
+                // println!(
+                //     "{}sp {a} to {b} ({n}): {sp} -- {x}",
+                //     " ".repeat(self.robot_layers as usize + 2 - n as usize)
+                // );
+                x
+            })
+            .min()
+            .unwrap();
+        self.cache.insert((a, b, n), result);
+        result
+    }
+}
+
+fn arrow_sp() -> HashMap<(char, char), Vec<&'static str>> {
+    let mut starters = HashMap::new();
+
+    starters.insert(('A', '<'), vec!["<v<", "v<<"]);
+    starters.insert(('A', '^'), vec!["<"]);
+    starters.insert(('A', '>'), vec!["v"]);
+    starters.insert(('A', 'v'), vec!["<v", "v<"]);
+
+    starters.insert(('<', '^'), vec![">^"]);
+    starters.insert(('<', '>'), vec![">>"]);
+    starters.insert(('<', 'v'), vec![">"]);
+
+    starters.insert(('^', '>'), vec![">v", "v>"]);
+    starters.insert(('^', 'v'), vec!["v"]);
+
+    starters.insert(('>', 'v'), vec!["<"]);
+
+    let mut result = starters.clone();
+    for ((from, to), paths) in starters {
+        result.insert((to, from), invert(&paths));
+    }
+
+    for c in "A<>v^".chars() {
+        result.insert((c, c), vec![""]);
+    }
+
+    result
+}
+
+fn num_sp() -> HashMap<(char, char), Vec<&'static str>> {
+    let mut starters = HashMap::new();
+
+    starters.insert(('A', '0'), vec!["<"]);
+    starters.insert(('A', '1'), vec!["<^<", "^<<"]);
+    starters.insert(('A', '2'), vec!["<^", "^<"]);
+    starters.insert(('A', '3'), vec!["^"]);
+    starters.insert(('A', '4'), vec!["<^<^", "<^^<", "^<<^", "^<^<", "^^<<"]);
+    starters.insert(('A', '5'), vec!["<^^", "^<^", "^^<"]);
+    starters.insert(('A', '6'), vec!["^^"]);
+    starters.insert(
+        ('A', '7'),
+        vec![
+            "<^<^^", "<^^<^", "<^^^<", "^<<^^", "^<^<^", "^<^^<", "^^^<<",
+        ],
+    );
+    starters.insert(('A', '8'), vec!["<^^^", "^<^^", "^^<^", "^^^<"]);
+    starters.insert(('A', '9'), vec!["^^^"]);
+
+    starters.insert(('0', '1'), vec!["^<"]);
+    starters.insert(('0', '2'), vec!["^"]);
+    starters.insert(('0', '3'), vec!["^>", ">^"]);
+    starters.insert(('0', '4'), vec!["^<^", "^^<"]);
+    starters.insert(('0', '5'), vec!["^^"]);
+    starters.insert(('0', '6'), vec!["^^>", "^>^"]);
+    starters.insert(('0', '7'), vec!["^<^^", "^^<^", "^^^<"]);
+    starters.insert(('0', '8'), vec!["^^^"]);
+    starters.insert(('0', '9'), vec!["^^^>", "^^>^", "^>^^", ">^^^"]);
+
+    starters.insert(('1', '2'), vec![">"]);
+    starters.insert(('1', '3'), vec![">>"]);
+    starters.insert(('1', '4'), vec!["^"]);
+    starters.insert(('1', '5'), vec!["^>", ">^"]);
+    starters.insert(('1', '6'), vec!["^>>", ">^>", ">>^"]);
+    starters.insert(('1', '7'), vec!["^^"]);
+    starters.insert(('1', '8'), vec!["^^>", "^>^", ">^^"]);
+    starters.insert(('1', '9'), vec!["^^>>", "^>^>", "^>>^", ">^>^", ">>^^"]);
+
+    starters.insert(('2', '3'), vec![">"]);
+    starters.insert(('2', '4'), vec!["<^", "^<"]);
+    starters.insert(('2', '5'), vec!["^"]);
+    starters.insert(('2', '6'), vec!["^>", ">^"]);
+    starters.insert(('2', '7'), vec!["<^^", "^<^", "^^<"]);
+    starters.insert(('2', '8'), vec!["^^"]);
+    starters.insert(('2', '9'), vec!["^^>", "^>^", ">^^"]);
+
+    starters.insert(('3', '4'), vec!["<<^", "<^<", "^<<"]);
+    starters.insert(('3', '5'), vec!["<^", "^<"]);
+    starters.insert(('3', '6'), vec!["^"]);
+    starters.insert(
+        ('3', '7'),
+        vec!["<<^^", "<^<^", "<^^<", "^<^<", "^<<^", "^^<<"],
+    );
+    starters.insert(('3', '8'), vec!["<^^", "^<^", "^^<"]);
+    starters.insert(('3', '9'), vec!["^^"]);
+
+    starters.insert(('4', '5'), vec![">"]);
+    starters.insert(('4', '6'), vec![">>"]);
+    starters.insert(('4', '7'), vec!["^"]);
+    starters.insert(('4', '8'), vec!["^>", ">^"]);
+    starters.insert(('4', '9'), vec!["^>>", ">^>", ">>^"]);
+
+    starters.insert(('5', '6'), vec![">"]);
+    starters.insert(('5', '7'), vec!["<^", "^<"]);
+    starters.insert(('5', '8'), vec!["^"]);
+    starters.insert(('5', '9'), vec!["^>", ">^"]);
+
+    starters.insert(('6', '7'), vec!["<<^", "<^<", "^<<"]);
+    starters.insert(('6', '8'), vec!["<^", "^<"]);
+    starters.insert(('6', '9'), vec!["^"]);
+
+    starters.insert(('7', '8'), vec![">"]);
+    starters.insert(('7', '9'), vec![">>"]);
+
+    starters.insert(('8', '9'), vec![">"]);
+
+    let mut result = starters.clone();
+    for ((from, to), paths) in starters {
+        result.insert((to, from), invert(&paths));
+    }
+
+    result
+}
+
+fn invert(paths: &Vec<&'static str>) -> Vec<&'static str> {
+    paths.iter().map(|path| reverse_path(path)).collect()
+}
+
+fn reverse_path(path: &str) -> &'static str {
+    path.chars()
+        .rev()
+        .map(|d| opposite(d))
+        .collect::<String>()
+        .leak()
+}
+
+fn opposite(d: char) -> char {
+    match d {
+        '<' => '>',
+        '^' => 'v',
+        '>' => '<',
+        'v' => '^',
+        _ => panic!("unknown direction {d}"),
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::day21::Precomputed;
+
+    use super::{part1, part2};
+
+    const TEST_INPUT: &str = r"029A
+980A
+179A
+456A
+379A
+";
+
+    #[test]
+    fn example1() {
+        assert_eq!(part1(TEST_INPUT), 126384);
+    }
+
+    #[test]
+    fn example1_mini() {
+        assert_eq!(part1("029A"), 68 * 29);
+    }
+
+    #[test]
+    fn shortest_human() {
+        let precomputed = Precomputed::new(2);
+        assert_eq!(precomputed.get('<', '^', 0), 1);
+    }
+
+    #[test]
+    fn shortest_1st() {
+        let p = Precomputed::new(2);
+        assert_eq!(p.get('A', '^', 1), 2);
+    }
+
+    #[test]
+    fn shortest_2nd() {
+        let p = Precomputed::new(2);
+        assert_eq!(p.get('A', '^', 2), 8);
+    }
+
+    #[test]
+    fn test2() {
+        assert_eq!(part2(TEST_INPUT), 0);
+    }
+}

src/lib.rs

@@ -16,6 +16,7 @@ pub mod day15;
 pub mod day16;
 pub mod day17;
 pub mod day19;
+pub mod day21;
 pub mod day22;
 pub mod day23;
 pub mod day24;

src/main.rs

@@ -23,6 +23,7 @@ fn main() {
         (16, day16::day_main),
         (17, day17::day_main),
         (19, day19::day_main),
+        (21, day21::day_main),
         (22, day22::day_main),
         (23, day23::day_main),
         (23, day23::day_main),