From 9ca870d1bbf6eadb450b4135a4c56273c7d452cd Mon Sep 17 00:00:00 2001
From: sepia <soerxpso@gmail.com>
Date: Fri, 20 Dec 2024 16:50:19 -0600
Subject: [PATCH] Day 20 (refactor)

---
 .gitignore       |  2 ++
 src/day20.rs     | 82 +++++++++++++-----------------------------------
 src/util/maps.rs | 26 ++++++++++++++-
 3 files changed, 48 insertions(+), 62 deletions(-)
 create mode 100644 .gitignore

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..fedaa2b
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+/target
+.env
diff --git a/src/day20.rs b/src/day20.rs
index c6477db..c05e0a8 100644
--- a/src/day20.rs
+++ b/src/day20.rs
@@ -1,74 +1,34 @@
 use crate::util::maps::*;
+use itertools::Itertools;
 
-const CHEAT_DURATION: u64 = 20;
+const CHEAT_DURATION: usize = 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 input: World<char> = World::from_string(&crate::input(20));
+    let maze: World<bool> = input.map(|&c| c == '#');
 
-    let costs = path_costs(&maze, ex, ey);
+    let (end_x, end_y, _) = input.enumerate().find(|(_, _, &c)| c == 'E').unwrap();
+    let distance_map: World<Option<u64>> = maze.tile_distances(end_x, end_y);
 
-    let free_spaces: Vec<(usize, usize)> = maze
+    let walkable_tiles: Vec<(usize, usize)> = distance_map
         .enumerate()
-        .filter_map(|(x, y, &b)| if !b { Some((x, y)) } else { None })
+        .filter_map(|(x, y, &d)| if d.is_some() { Some((x, y)) } else { None })
         .collect();
-    let all_cheats: Vec<u64> = free_spaces
+    let tile_pairs = walkable_tiles
         .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();
+        .cartesian_product(walkable_tiles.iter())
+        .filter(|(&(ax, ay), &(bx, by))| !(ax == bx && ay == by));
+    let all_cheats = tile_pairs
+        .filter(|(&(ax, ay), &(bx, by))| taxicab_distance(ax, ay, bx, by) <= CHEAT_DURATION)
+        .map(|(&(ax, ay), &(bx, by))| {
+            let taxi_distance = taxicab_distance(ax, ay, bx, by) as i64;
+            let a_distance_to_end = distance_map.get(ax, ay).unwrap().unwrap() as i64;
+            let b_distance_to_end = distance_map.get(bx, by).unwrap().unwrap() as i64;
+            a_distance_to_end - b_distance_to_end - taxi_distance
+        });
+    let good_cheats = all_cheats.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
+    println!("{good_cheats} cheats save 100 or more picoseconds.");
 }
 
 fn taxicab_distance(ax: usize, ay: usize, bx: usize, by: usize) -> usize {
diff --git a/src/util/maps.rs b/src/util/maps.rs
index e7e36a2..a2eeb15 100644
--- a/src/util/maps.rs
+++ b/src/util/maps.rs
@@ -1,4 +1,4 @@
-use std::fmt::Display;
+use std::{fmt::Display, intrinsics::atomic_cxchgweak_acquire_acquire};
 
 use itertools::Itertools;
 
@@ -187,3 +187,27 @@ where
         f.write_str(&final_string)
     }
 }
+
+impl World<bool> {
+    pub fn tile_distances(&self, x: usize, y: usize) -> World<Option<u64>> {
+        let mut costs = World::from_dimensions(self.width(), self.height(), None);
+        let mut tile_stack: Vec<(usize, usize, u64)> = vec![];
+        tile_stack.push((x, y, 0));
+        while let Some((x, y, cost_so_far)) = tile_stack.pop() {
+            let adjacents = self
+                .adjacent_locations(x, y)
+                .into_iter()
+                .filter(|&(nx, ny)| !self.get(nx, ny).unwrap());
+            for (nx, ny) in adjacents {
+                let cost = cost_so_far + 1;
+                let old_cost = costs.get(nx, ny).unwrap().unwrap_or(u64::MAX);
+                if cost < old_cost {
+                    costs.set(nx, ny, Some(cost));
+                    tile_stack.push((nx, ny, cost));
+                }
+            }
+        }
+
+        costs
+    }
+}