Day 16

2020/day16.py

@@ -0,0 +1,73 @@
+import sys
+from collections import defaultdict
+from functools import reduce
+from itertools import permutations
+
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+
+    step = 0
+    rules = {}
+    nearby = []
+    for line in lines:
+        if not line:
+            step += 1
+            continue
+        if step == 0:
+            name, ranges = line.split(": ")
+            rules[name] = tuple(map(
+                lambda rs: tuple(map(int, rs.split("-"))),
+                [rs for rs in ranges.split(" or ")]
+            ))
+        elif step == 1 and not line.startswith("your"):
+            your_ticket = list(map(int, line.split(",")))
+            ticket_len = len(your_ticket)
+        elif step == 2 and not line.startswith("nearby"):
+            nearby.append(list(map(int, line.split(","))))
+
+    # Part 1
+    acc = 0
+    invalids = []
+    for nt in nearby:
+        for n in nt:
+            for rule in rules.values():
+                if n_sat_rule(n, rule):
+                    break
+            else:  # satisfies no rules
+                invalids.append(nt)
+                acc += n
+    print("Part 1:", acc)
+
+    # Part 2
+    tickets = [your_ticket, *filter(lambda t: t not in invalids, nearby)]
+    isat = {}
+    for i in range(ticket_len):
+        possible_rules = set(rules.keys())
+        for ticket in tickets:
+            n = ticket[i]
+            for rname, r in rules.items():
+                if not n_sat_rule(n, r):
+                    possible_rules.remove(rname)
+        isat[i] = possible_rules
+    # for some reason, len(s) for s in isat.values() is unique per i, which
+    # means we could easily solve the whole field attribution incrementally!
+    solved_fields = set()
+    field_map = {}
+    for i, s in sorted(isat.items(), key=lambda item: len(item[1])):
+        s -= solved_fields
+        assert len(s) == 1
+        f = s.pop()
+        field_map[i] = f
+        solved_fields.add(f)
+    dep_fields = [i for i, f in field_map.items() if f.startswith("departure")]
+    your_ticket_values = [your_ticket[i] for i in dep_fields]
+    print("Part 2:", reduce(lambda a, b: a * b, your_ticket_values, 1))
+
+
+def n_sat_rule(n, r):
+    return r[0][0] <= n <= r[0][1] or r[1][0] <= n <= r[1][1]
+
+
+if __name__ == "__main__":
+    main()