From 23abca01329e44f53884efcc9f87fcc9212ce839 Mon Sep 17 00:00:00 2001
From: "Peter J. Holzer" <hjp@hjp.at>
Date: Sat, 26 Dec 2020 23:06:13 +0100
Subject: [PATCH] Complete day 16, part 2

---
 16/part2 | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 92 insertions(+)
 create mode 100755 16/part2

diff --git a/16/part2 b/16/part2
new file mode 100755
index 0000000..4a705d0
--- /dev/null
+++ b/16/part2
@@ -0,0 +1,92 @@
+#!/usr/bin/python3
+
+import re
+from pprint import pprint
+
+with open("input") as fh:
+    state = "rules"
+    rules = {}
+    nearby = []
+    for ln in fh:
+        ln = ln.strip()
+        if state == "rules":
+            if m := re.match(r"([a-z ]+): ([0-9]+)-([0-9]+) or ([0-9]+)-([0-9]+)", ln):
+                rules[m.group(1)] = ((int(m.group(2)), int(m.group(3))),
+                                     (int(m.group(4)), int(m.group(5))))
+            elif ln == "your ticket:":
+                state = "your"
+            elif ln == "":
+                pass
+            else:
+                raise RuntimeError(ln)
+        elif state == "your":
+            if re.match(r"[0-9,]+", ln):
+                my_ticket = [int(x) for x in ln.split(",")]
+            elif ln == "nearby tickets:":
+                state = "nearby"
+            elif ln == "":
+                pass
+            else:
+                raise RuntimeError(ln)
+        elif state == "nearby":
+            if re.match(r"[0-9,]+", ln):
+                ticket = [int(x) for x in ln.split(",")]
+                nearby.append(ticket)
+            elif ln == "":
+                pass
+            else:
+                raise RuntimeError(ln)
+
+print(len(nearby), "tickets")
+valid_tickets = []
+for ticket in nearby:
+    ticket_ok = True
+    for num in ticket:
+        ok = False
+        for rule in rules.values():
+            if rule[0][0] <= num <= rule[0][1] or rule[1][0] <= num <= rule[1][1]:
+                ok = True
+                break
+        if not ok:
+            ticket_ok = False
+            break
+    if ticket_ok:
+        valid_tickets.append(ticket)
+
+print(len(valid_tickets), "tickets valid")
+
+candidates = {x: set(range(len(valid_tickets[0]))) for x in rules.keys()}
+pprint(candidates)
+
+for name, rule in rules.items():
+    for ticket in valid_tickets:
+        for i in list(candidates[name]):
+            num = ticket[i]
+            if rule[0][0] <= num <= rule[0][1] or rule[1][0] <= num <= rule[1][1]:
+                pass
+            else:
+                print(num, "doesn't match", rule, "eliminating", i, "from", name)
+                candidates[name] -= {i}
+
+pprint(candidates)
+
+column = {}
+while candidates:
+    for name, columns in candidates.items():
+        if len(columns) == 1:
+            column[name] = c = columns.pop()
+            for columns in candidates.values():
+                columns -= {c}
+            break
+        elif len(columns) == 0:
+            # eliminated earlier
+            del candidates[name]
+            break
+pprint(column)
+
+res = 1
+for name, c in column.items():
+    if name.startswith("departure"):
+        res *= my_ticket[c]
+
+print(res)