Solve day 16.

README.md

@@ -12,3 +12,4 @@
 - Day 10: 180:00; this one was hard for me.
 - Day 11: 68:00; okay but not elegant and way too slow ofc; x-ray solution would have been neat
 - Day 12: 52:00 and 22:00 for leaderboard; had the right idea and I am good at this type of problem
+- Day 16: 00:27:30 745; best placement so far, of course still horribly slow

d16.py

@@ -0,0 +1,109 @@
+import lib
+
+EXAMPLE = r"""
+.|...\....
+|.-.\.....
+.....|-...
+........|.
+..........
+.........\
+..../.\\..
+.-.-/..|..
+.|....-|.\
+..//.|....
+"""
+
+NEW = {
+    '.': {
+        'w': (0, -1, 'w'),
+        'e': (0,  1, 'e'),
+        's': (1,  0, 's'),
+        'n': (-1, 0, 'n'),
+    },
+    '|': {
+        'w': [(1, 0, 's'), (-1, 0, 'n')],
+        'e': [(1, 0, 's'), (-1, 0, 'n')],
+        's': (1,  0, 's'),
+        'n': (-1, 0, 'n'),
+    },
+    '-': {
+        's': [(0, -1, 'w'), (0, 1, 'e')],
+        'n': [(0, -1, 'w'), (0, 1, 'e')],
+        'e': (0,  1, 'e'),
+        'w': (0, -1, 'w'),
+    },
+    '/': {
+        's': (0, -1, 'w'),
+        'n': (0, 1, 'e'),
+        'e': (-1,  0, 'n'),
+        'w': (1, 0, 's'),
+    },
+    '\\': {
+        's': (0, 1, 'e'),
+        'n': (0, -1, 'w'),
+        'e': (1,  0, 's'),
+        'w': (-1, 0, 'n'),
+    },
+}
+
+def solve(lines: list[str], second=False):
+    res = 0
+
+    g = list(map(list, lines))
+
+    rows = len(g)
+    cols = len(g[0])
+
+    if not second:
+        starts = [(0, 0, 'e')]
+    else:
+        starts = [(r, 0, 'e') for r in range(rows)]
+        starts += [(r, cols - 1, 'w') for r in range(rows)]
+        starts += [(0, c, 's') for c in range(cols)]
+        starts += [(rows - 1, c, 'n') for c in range(cols)]
+
+    for start in starts:
+        visited = set()
+        beams = [start]
+        while beams:
+            b = beams.pop()
+            row, col, d = b
+            visited.add((row, col, d))
+
+            f = g[row][col]
+            new = NEW[f][d]
+            if isinstance(new, tuple):
+                r, c, nd = new
+                nr = row + r
+                nc = col + c
+                if nr >= 0 and nc >= 0 and nr < rows and nc < cols:
+                    if not (nr, nc, nd) in visited:
+                        beams.append((nr, nc, nd))
+            else:
+                for r, c, nd in new:
+                    nr = row + r
+                    nc = col + c
+                    if nr >= 0 and nc >= 0 and nr < rows and nc < cols:
+                        if not (nr, nc, nd) in visited:
+                            beams.append((nr, nc, nd))
+
+        v = set([(r, c) for (r, c, _) in list(visited)])
+        res = max(res, len(v))
+    return res
+
+
+def main():
+    lines = lib.str_to_lines_no_empty(EXAMPLE)
+    print("Example  1:", solve(lines))
+
+    lines = lib.str_to_lines_no_empty(open("i16.txt").read())
+    print("Solution 1:", solve(lines))
+
+    lines = lib.str_to_lines_no_empty(EXAMPLE)
+    print("Example  2:", solve(lines, True))
+
+    lines = lib.str_to_lines_no_empty(open("i16.txt").read())
+    print("Solution 2:", solve(lines, True))
+
+if __name__ == "__main__":
+    main()