2025 day 7

adventofcode/2025/day7/day7.py

@@ -0,0 +1,67 @@
+#!/usr/bin/env python3
+import fileinput
+
+
+def main(inp):
+    start_pos = None
+    splitters = set()
+    x, y = 0, 0
+    for y, row in enumerate(inp):
+        for x, ch in enumerate(row):
+            if ch == "S":
+                start_pos = x, y
+            elif ch == "^":
+                splitters.add((x, y))
+
+    part1(start_pos, splitters, max_y=y)
+    part2(start_pos, splitters, x, max_y=y)
+
+
+def part1(pos, splitters, max_y):
+    "BFS the grid and count the splitters"
+    queue = [pos]
+    visited = set()
+    splitter_cnt = 0
+    while True:
+        current_pos = queue.pop(0)
+        visited.add(current_pos)
+        new_pos = current_pos[0], current_pos[1] + 1
+        if new_pos in visited:
+            continue
+        if new_pos[1] > max_y:
+            break
+        if new_pos in splitters:
+            visited.add(new_pos)
+            splitter_cnt += 1
+            split_left_pos = new_pos[0] - 1, new_pos[1]
+            split_right_pos = new_pos[0] + 1, new_pos[1]
+            if split_left_pos not in visited:
+                queue.append(split_left_pos)
+            if split_right_pos not in visited:
+                queue.append(split_right_pos)
+        else:
+            queue.append(new_pos)
+    print("Part 1: ", splitter_cnt)
+
+
+def part2(start_pos, splitters, max_x, max_y):
+    # store the number of timelines at each position
+    current_row = [0] * (max_x + 1)
+    current_row[start_pos[0]] = 1  # set start pos in the first row
+    for y in range(max_y):
+        new_row = [0] * (max_x + 1)
+        for i, cnt in enumerate(current_row):
+            if cnt > 0:
+                new_pos = i, y + 1
+                if new_pos in splitters:
+                    new_row[i - 1] += cnt
+                    new_row[i + 1] += cnt
+                else:
+                    new_row[i] += cnt
+        current_row = new_row
+    print("Part 2: ", sum(current_row))
+
+
+if __name__ == "__main__":
+    lines = [x.rstrip() for x in fileinput.input()]
+    main(lines)