Solve day 24 and update readme.

README.md

@@ -26,3 +26,11 @@
 - Day 18: 12:00 and 32:00; really straightforward and of course way too slow.
 - Day 19: Slow.
 - Day 20: Struggled way too much.
+- Day 21: Straightforward and relatively fast.
+- Day 22: Very hard and wasn't able to do hands free. Even the best guys took
+  over an hour.
+- Day 23: Super straightforward, but took me way longer than it should have
+  because I loose focus and make silly errors. Like back in Middleschool.
+- Day 24: Easy. Should have been faster, but no crazy mistakes. Still way too
+  slow for something easy like this.
+- Day 25:

d24.py

@@ -0,0 +1,93 @@
+from lib import *
+
+EXAMPLE = """#.#####
+#.....#
+#>....#
+#.....#
+#...v.#
+#.....#
+#####.#
+"""
+
+EXAMPLE = """#.######
+#>>.<^<#
+#.<..<<#
+#>v.><>#
+#<^v^^>#
+######.#
+"""
+
+B2DIR = {
+    ">": (0, 1),
+    "v": (1, 0),
+    "<": (0, -1),
+    "^": (-1, 0),
+}
+
+ADJ5 = [
+    (-1, 0),
+    (0, 1),
+    (1, 0),
+    (0, -1),
+    (0, 0),
+]
+
+def solve(input: Input, second=False):
+    g = input.grid2()
+    start = (0, g.rows()[0].index('.'))
+    poss = set([(start, 0)])
+    end = (g.n_rows - 1, g.rows()[-1].index('.'))
+    bzs = []
+    for d in B2DIR.keys():
+        for b in g.find(d):
+            bzs.append((b, B2DIR[d]))
+
+    for time in range(10**9):
+        nbzs = []
+        bzsset = set()
+        for pos, dir in bzs:
+            row, col = add2(pos, dir)
+            if row == 0:
+                row = g.n_rows - 2
+            elif row == g.n_rows - 1:
+                row = 1
+            if col == 0:
+                col = g.n_cols - 2
+            elif col == g.n_cols - 1:
+                col = 1
+            nbzs.append(((row, col), dir))
+            bzsset.add((row, col))
+        bzs = nbzs
+
+        nposs = set()
+        while poss:
+            pos, round = poss.pop()
+            for adj in ADJ5:
+                npos = add2(pos, adj)
+                if npos[0] < 0 or npos[0] >= g.n_rows or npos[1] < 0 or npos[1] >= g.n_cols:
+                    continue
+                if not second:
+                    if npos not in bzsset and g[npos] != '#':
+                        nposs.add((npos, round))
+                    if npos == end:
+                        return time + 1
+                else:
+                    if npos == end and round == 0:
+                        nposs.add((npos, round + 1))
+                    elif npos == end and round == 2:
+                        return time + 1
+                    elif npos == start and round == 1:
+                        nposs.add((npos, round + 1))
+                    elif npos not in bzsset and g[npos] != '#':
+                        nposs.add((npos, round))
+        poss = nposs
+
+def main():
+    DAY_INPUT = "i24.txt"
+    print("Example  1:", solve(Input(EXAMPLE)))
+    print("Solution 1:", solve(Input(DAY_INPUT)))
+    print("Example  2:", solve(Input(EXAMPLE), True))
+    print("Solution 2:", solve(Input(DAY_INPUT), True))
+
+if __name__ == "__main__":
+    main()