Solve 2024:20 p1-2 "Race Condition"

BFS to get the path from start to end.

pt 1 took some time to wrap around, and initial
versions of the code used defaultdict() to keep
counts of all cheats to check if the example
puzzle input gace the right counts for the cheats.

But since only 100 or more was requested, a counter
do the job in the final version of the code.

For pt 2, a permutation is used.

2024-python/output/__init__.py

@@ -118,6 +118,22 @@ def bfs(S, E=None):
     # return insights
 
 
+def mhd_search(r, c, R=20):
+    """returns all coords that are within R manhattan distance from (r,c)"""
+    p = set()
+    for d in range(1, R + 1):
+        p.add((r, c + d))
+        p.add((r, c - d))
+        p.add((r + d, c))
+        p.add((r - d, c))
+        for dd in range(d):
+            p.add((r - dd, c - d + dd))
+            p.add((r + dd, c - d + dd))
+            p.add((r - dd, c - dd + d))
+            p.add((r + dd, c - dd + d))
+    return p
+
+
 def dijkstras(grid, start, target):
     """
     1. Create an array that holds the distance of each vertex from the starting

2024-python/output/day_20.py

@@ -0,0 +1,58 @@
+from collections import deque
+
+from output import matrix, D, mhd
+
+
+def solve(data):
+    grid, H, W = matrix(data)
+    S = next((r, c) for r in range(H) for c in range(W) if grid[r][c] == "S")
+    E = next((r, c) for r in range(H) for c in range(W) if grid[r][c] == "E")
+    seen = set()
+    Q = deque([(S, 0, [])])
+    while Q:
+        pos, w, path = Q.pop()
+        r, c = pos
+        if grid[r][c] == "#" or pos in seen:
+            continue
+        seen.add(pos)
+        if pos == E:
+            og = path + [(r, c)]
+            break
+        for dr, dc in D:
+            Q.append(((r + dr, c + dc), w + 1, path + [(r, c)]))
+
+    track = {rc: i for i, rc in enumerate(og)}
+    T = 100
+    p1 = 0
+    p2 = 0
+
+    cheats = set()
+    for r, c in track.keys():
+        for dr, dc in D:
+            rr, cc = r + dr * 2, c + dc * 2
+            if (
+                (rr, cc) not in cheats
+                and (rr, cc) in track
+                and T + 2 <= abs(track[(rr, cc)] - track[(r, c)])
+            ):
+                p1 += 1
+        cheats.add((r, c))
+
+    tk = [k for k in track.keys()]
+    for b in range(T, len(tk)):
+        for a in range(b - T):
+            distance = mhd(tk[a], tk[b])
+            if distance <= 20 and b - a - distance >= T:
+                p2 += 1
+
+    return p1, p2
+
+
+if __name__ == "__main__":
+    with open("./input/20.txt", "r") as f:
+        inp = f.read().strip()
+
+    p1, p2 = solve(inp)
+
+    print(p1)
+    print(p2)