From bce004b604b72be9f88ab07d181fe51ad7feb1e0 Mon Sep 17 00:00:00 2001
From: the <stefanforstenlechner@gmail.com>
Date: Mon, 21 Dec 2020 15:34:21 +0000
Subject: [PATCH] day19

---
 day19.py | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)
 create mode 100644 day19.py

diff --git a/day19.py b/day19.py
new file mode 100644
index 0000000..15e835b
--- /dev/null
+++ b/day19.py
@@ -0,0 +1,58 @@
+def read_input():
+    with open('day19/input') as reader:
+        text = reader.read()
+        rulesAsText, messagesAsText = text.split('\n\n')
+        messages = filter(lambda m: m != '', messagesAsText.split('\n'))
+        rulesToParse = rulesAsText.split('\n')
+
+    return rulesToParse, messages
+
+
+def parse_rules(rulesToParse):
+    rules = {}
+    for line in rulesToParse:
+        name, production_of_rule = line.split(': ')
+        if production_of_rule in ('"a"', '"b"'):
+            rules[int(name)] = production_of_rule[1]
+            continue
+        split_productions = production_of_rule.split(' | ')
+        productions = list(map(lambda p: [int(x) for x in p.split(' ')], split_productions))
+        rules[int(name)] = productions
+    return rules
+
+
+rulesToParse, messages = read_input()
+rules = parse_rules(rulesToParse)
+
+
+def verify_rule(message):
+    global rules
+
+    state = [([0], 0)]
+    while state:
+        current_rules, index = state.pop()
+        current_rule = current_rules.pop()
+        productions = rules[current_rule]
+        if productions in ('a', 'b'):
+            if message[index] == productions:
+                index += 1
+                if index >= len(message):
+                    if len(current_rules) == 0:
+                        # solution found
+                        return True
+                    else:
+                        continue
+                elif len(current_rules) == 0:
+                    # no more rules, but message continues. current state will not find anything anymore
+                    continue
+                state.append((list(current_rules), index))
+            continue
+
+        for p in productions:
+            state.append((list(current_rules) + list(reversed(p)), index))
+
+    return False
+
+
+res = sum(map(lambda m: verify_rule(m), messages))
+print(res)