Solve day 6

2024/06_guard.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+import sys
+
+grid = [line.rstrip() for line in sys.stdin]
+h = len(grid)
+w = len(grid[0])
+guard = next((x, y) for y in range(h) for x in range(w) if grid[y][x] == '^')
+obstacles = {(x, y) for y in range(h) for x in range(w) if grid[y][x] == '#'}
+
+def walk(x, y, dx, dy, obstacles):
+    while 0 <= x < w and 0 <= y < h:
+        if (x, y) in obstacles:
+            x -= dx
+            y -= dy
+            dx, dy = -dy, dx
+        else:
+            yield (x, y), (dx, dy)
+            x += dx
+            y += dy
+
+def loops(path, visited):
+    vis = {(x, y, dx, dy) for (x, y), d in visited.items() for dx, dy in d}
+    for pos in path:
+        if pos in vis:
+            return True
+        vis.add(pos)
+    return False
+
+def add_obstacle(gx, gy, obstacles):
+    positions = set()
+    visited = {}
+    for xy, d in walk(gx, gy, 0, -1, obstacles):
+        if xy not in visited and xy not in positions and \
+                loops(walk(*xy, *d, obstacles | {xy}), visited):
+            positions.add(xy)
+        visited.setdefault(xy, set()).add(d)
+    return positions
+
+print(len(set(xy for xy, _ in walk(*guard, 0, -1, obstacles))))
+print(len(add_obstacle(*guard, obstacles)))

2024/input/6

@@ -0,0 +1,130 @@
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