Solve 2024:6 p1-2 "Guard Gallivant"

Pt 1 was easy, Pt 2 was pure horror.

Earlier drafts of the code tried to be way too
smart. At one point, I just came to the conclusion
to place a new obstacle (#) on the grid and just
rerun the thing to look for loops.

2 things:

- the visited positions from pt 1 can be used as a
subset for positions to consider for the extra
"#".
- The track of loops can be optimized to look at
bounces on "#"s instead of each individual y,x pos,
given that the direction is remembered.

pt 2 is familiar, the last time a puzzle required
look detection the puzzle used lazer beams and
reflectors. Not sure what Event or day it was.

2024-python/output/day_06.py

@@ -0,0 +1,56 @@
+from output import cw, matrix
+
+
+def solve(data):
+    M, H, W = matrix(data)
+
+    y, x = [(r, c) for r in range(H) for c in range(W) if M[r][c] == "^"][0]
+    delta_y, delta_x = (-1, 0)
+    seen, _ = travel(M, H, W, y, x, delta_y, delta_x)
+
+    p1 = len(seen)
+
+    p2 = 0
+    for row, col in seen:
+        grid = [list(row) for row in M]
+        grid[row][col] = "#"
+        _, loop_found = travel(grid, H, W, y, x, delta_y, delta_x)
+
+        if loop_found:
+            p2 += 1
+
+    return p1, p2
+
+
+def travel(M, H, W, y, x, delta_y, delta_x):
+    seen = set()
+    hit = set()
+    loop_found = False
+
+    while True:
+        seen.add((y, x))
+
+        if y == 0 or y == H - 1 or x == 0 or x == W - 1:
+            break
+
+        if M[y + delta_y][x + delta_x] == "#":
+            if (y + delta_y, x + delta_x, delta_y, delta_x) in hit:
+                loop_found = True
+                break
+
+            hit.add((y + delta_y, x + delta_x, delta_y, delta_x))
+            delta_y, delta_x = cw(delta_y, delta_x)
+        else:
+            y, x = y + delta_y, x + delta_x
+
+    return seen, loop_found
+
+
+if __name__ == "__main__":
+    with open("./input/06.txt", "r") as f:
+        inp = f.read().strip()
+
+    p1, p2 = solve(inp)
+
+    print(p1)
+    print(p2)