Day 20

src/day20.rs

@@ -0,0 +1,78 @@
+use crate::util::maps::*;
+
+const CHEAT_DURATION: u64 = 20;
+
+pub fn day20() {
+    let input = World::from_string(&crate::input(20));
+    let maze = input.map(|&c| c == '#');
+    let (ex, ey, _) = input.enumerate().find(|(_, _, &c)| c == 'E').unwrap();
+
+    let costs = path_costs(&maze, ex, ey);
+
+    let free_spaces: Vec<(usize, usize)> = maze
+        .enumerate()
+        .filter_map(|(x, y, &b)| if !b { Some((x, y)) } else { None })
+        .collect();
+    let all_cheats: Vec<u64> = free_spaces
+        .iter()
+        .flat_map(|&(ax, ay)| {
+            free_spaces
+                .iter()
+                .filter_map(|&(bx, by)| {
+                    if ax == bx && ay == by {
+                        return None;
+                    }
+                    let taxi_cost = taxicab_distance(ax, ay, bx, by) as u64;
+                    if taxi_cost > CHEAT_DURATION {
+                        return None;
+                    }
+                    let a_cost = costs.get(ax, ay).unwrap();
+                    let b_cost = costs.get(bx, by).unwrap();
+                    // a cheat's timesave is the cost of a - the cost of b, minus the time it takes to do the cheat
+                    // and we return None for cheats that have negative timesave (why would you cheat to do worse?)
+                    a_cost.checked_sub(b_cost)?.checked_sub(taxi_cost)
+                })
+                .collect::<Vec<u64>>()
+        })
+        .collect();
+    let good_cheats = all_cheats
+        .iter()
+        .filter(|&&advantage| advantage >= 100)
+        .count();
+
+    println!("{good_cheats}");
+}
+
+fn path_costs(world: &World<bool>, x: usize, y: usize) -> World<u64> {
+    fn calculate_path_costs(
+        world: &World<bool>,
+        x: usize,
+        y: usize,
+        cost_so_far: u64,
+        costs: &mut World<u64>,
+    ) {
+        let adjacents = world
+            .adjacent_locations(x, y)
+            .into_iter()
+            .filter(|&(nx, ny)| !world.get(nx, ny).unwrap());
+        for (nx, ny) in adjacents {
+            let cost = cost_so_far + 1;
+            let old_cost = costs.get(nx, ny).unwrap();
+            if cost < old_cost {
+                costs.set(nx, ny, cost);
+                calculate_path_costs(world, nx, ny, cost, costs)
+            }
+        }
+    }
+
+    let mut costs = World::from_dimensions(world.width(), world.height(), u64::MAX);
+    costs.set(x, y, 0);
+    calculate_path_costs(world, x, y, 0, &mut costs);
+    costs
+}
+
+fn taxicab_distance(ax: usize, ay: usize, bx: usize, by: usize) -> usize {
+    let dx = if ax < bx { bx - ax } else { ax - bx };
+    let dy = if ay < by { by - ay } else { ay - by };
+    dx + dy
+}

src/main.rs

@@ -3,22 +3,6 @@ pub mod util;
 #[allow(dead_code)]
 mod day1;
 #[allow(dead_code)]
-mod day2;
-#[allow(dead_code)]
-mod day3;
-#[allow(dead_code)]
-mod day4;
-#[allow(dead_code)]
-mod day5;
-#[allow(dead_code)]
-mod day6;
-#[allow(dead_code)]
-mod day7;
-#[allow(dead_code)]
-mod day8;
-#[allow(dead_code)]
-mod day9;
-#[allow(dead_code)]
 mod day10;
 #[allow(dead_code)]
 mod day11;
@@ -36,10 +20,28 @@ mod day16;
 mod day17;
 #[allow(dead_code)]
 mod day18;
+#[allow(dead_code)]
 mod day19;
+#[allow(dead_code)]
+mod day2;
+mod day20;
+#[allow(dead_code)]
+mod day3;
+#[allow(dead_code)]
+mod day4;
+#[allow(dead_code)]
+mod day5;
+#[allow(dead_code)]
+mod day6;
+#[allow(dead_code)]
+mod day7;
+#[allow(dead_code)]
+mod day8;
+#[allow(dead_code)]
+mod day9;
 
 fn main() {
-    day19::day19();
+    day20::day20();
 }
 
 pub fn input(day: u8) -> String {
@@ -47,7 +49,8 @@ pub fn input(day: u8) -> String {
     let token = std::env::var("AOC_TOKEN").expect("AOC_TOKEN must be set in .env file");
 
     let client = reqwest::blocking::Client::new();
-    client.get(format!("https://adventofcode.com/2024/day/{}/input", day))
+    client
+        .get(format!("https://adventofcode.com/2024/day/{}/input", day))
         .header("cookie", format!("session={}", token))
         .send()
         .expect("Failed to get input")