Solve 2025 day 7

2025/d07.py

@@ -0,0 +1,59 @@
+from lib import get_data, Grid2D
+from collections import defaultdict
+
+
+data = """.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+..............."""
+data = get_data(__file__)
+
+g = Grid2D(data)
+beams = [g.find('S')[0][1]]
+
+splits = 0
+for row in range(1, g.n_rows - 1):
+    new_beams = set()
+    for beam in beams:
+        field = g[(row, beam)]
+        if field == "^":
+            splits += 1
+            new_beams.add(beam - 1)
+            new_beams.add(beam + 1)
+        elif field == ".":
+            new_beams.add(beam)
+        else:
+            assert False, "unexpected field"
+    beams = list(new_beams)
+    # print(beams)
+
+print(splits)
+
+beams = {g.find('S')[0][1]: 1}
+for row in range(1, g.n_rows - 1):
+    new_beams = defaultdict(int)
+    for beam, count in beams.items():
+        field = g[(row, beam)]
+        if field == "^":
+            new_beams[beam - 1] += count
+            new_beams[beam + 1] += count
+        elif field == ".":
+            new_beams[beam] += count
+        else:
+            assert False, "unexpected field"
+    beams = new_beams
+
+print(sum(beams.values()))
+

README.md

@@ -20,7 +20,13 @@ stressful and this will actually be more fun. Thank you Eric Wastl and let's go!
   time because I always kind of avoid that. I could probably decrease the
   complexity from O(n^2) to O(n log(n)) but I am happy that I've implemented
   merging at all.
-- Day 6:
+- Day 6: Transposing some rows and cols. Fun and good to keep the brain fit
+  but not really hard. I would have been way too slow for the leaderboard
+  in pre-AI years.
+- Day 7: Grid puzzle that required non-naiv implementation for part 2. Took
+  me a second to realize that I could not save the coordinates by list but
+  had to use a count to be efficient.
+- Day 8:
 
 
 ## AoC 2024