[2024/20] p1 & p2 solved

Author: StarlitGhost

.aoc_tiles/tiles/2024/20.png

@@ -0,0 +1,15 @@
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############

2024/20/example

@@ -0,0 +1,98 @@
+from GhostyUtils import aoc
+from GhostyUtils.grid import Grid
+from GhostyUtils.vec2 import Vec2, manhattan_distance as mh_dist
+from GhostyUtils.pathfinding import bfs
+from collections import defaultdict
+from functools import partial
+
+
+aoc.argparser.add_argument("-s", "--save", type=int, default=100, help="picoseconds to save")
+
+
+# non-walls 1 space UDLR from pos
+def neighbours(pos: tuple, grid: Grid) -> list[tuple]:
+    return [n for n in grid.neighbours(pos, diagonal=False) if grid[n] != '#']
+
+
+# non-walls behind walls, 2 spaces UDLR from pos
+def cheat(pos: tuple, grid: Grid) -> list[Vec2]:
+    pos = Vec2(pos)
+
+    def end_pos(pos, n):
+        return pos+(n-pos)*2
+
+    return [end_pos(pos, n) for n in grid.neighbours(pos, diagonal=False)
+            if grid[n] == '#' and grid.in_bounds(end_pos(pos, n)) and grid[end_pos(pos, n)] != '#']
+
+
+def cheat_savings(saves: dict[int], minimum: int):
+    for saving, cheats in sorted(list(saves.items())):
+        if saving < minimum:
+            continue
+        print(f"There are {cheats} cheats that save {saving} picoseconds.")
+
+
+def main():
+    racetrack = Grid(aoc.read_lines())
+    start = racetrack.find('S')
+    end = racetrack.find('E')
+    target = aoc.args.save
+
+    neighbours_func = partial(neighbours, grid=racetrack)
+    clean_path = next(bfs(start, end, neighbours=neighbours_func))
+    clean_time = len(clean_path)
+    # map clean path positions to their index in the list, for faster lookup
+    clean_path_index = {pos: i for i, pos in enumerate(clean_path)}
+
+    # print the clean path
+    if aoc.args.progress or aoc.args.verbose:
+        overlays = [
+            {path: '.' for path in clean_path},
+            {start: 'S', end: 'E'},
+        ]
+        print(racetrack.render_with_overlays(overlays))
+        print(f"clean time: {clean_time} picoseconds")
+
+    saves = defaultdict(int)
+    cheats = {}
+    for i, pos in enumerate(clean_path[:-1]):
+        for c in cheat(pos, racetrack):
+            # skip cheats that take us backwards
+            if clean_path_index[c.as_tuple()] < i:
+                continue
+
+            # calculate the time saved
+            saved = -(i - clean_path_index[c.as_tuple()] + 2)
+            saves[saved] += 1
+            cheats[(pos, c.as_tuple())] = saved
+
+            # draw the cheats on the map
+            if aoc.args.verbose:
+                overlays.append({(pos+(c-pos)/2).as_tuple(): '1',
+                                 c.as_tuple(): '2'})
+                print(racetrack.render_with_overlays(overlays))
+                print(f"^ saves {saved} picoseconds")
+                overlays.pop()
+
+    if aoc.args.progress or aoc.args.verbose:
+        cheat_savings(saves, target)
+    print(f"p1: {sum(times for saved, times in saves.items() if saved >= 100)}")
+
+    saves = defaultdict(int)
+    for i, pos in enumerate(clean_path[:-target]):
+        # loop over path positions that are over 100 positions forward along the path and under
+        # 20 manhattan distance away from the current position
+        for candidate, md in filter(lambda md: md[1] <= 20,
+                                    ((other, mh_dist(pos, other))
+                                     for other in clean_path[i+target:])):
+            # calculate the time saved
+            saved = -(i - clean_path_index[candidate] + md)
+            saves[saved] += 1
+
+    if aoc.args.progress or aoc.args.verbose:
+        cheat_savings(saves, target)
+    print(f"p2: {sum(times for saved, times in saves.items() if saved >= target)}")
+
+
+if __name__ == "__main__":
+    main()

2024/20/script.py

@@ -3,10 +3,10 @@ My solutions to the yearly Advents of Code
 
 <!-- AOC TILES BEGIN -->
 <h1 align="center">
-  Advent of Code - 197/488 ⭐
+  Advent of Code - 199/490 ⭐
 </h1>
 <h1 align="center">
-  2024 - 36 ⭐ - Python
+  2024 - 38 ⭐ - Python
 </h1>
 <a href="2024/1/script.py">
   <img src=".aoc_tiles/tiles/2024/01.png" width="161px">
@@ -65,6 +65,9 @@ My solutions to the yearly Advents of Code
 <a href="2024/19/script.py">
   <img src=".aoc_tiles/tiles/2024/19.png" width="161px">
 </a>
+<a href="2024/20/script.py">
+  <img src=".aoc_tiles/tiles/2024/20.png" width="161px">
+</a>
 <h1 align="center">
   2023 - 47 ⭐ - Python
 </h1>