From 0c1f1d0ac6bd3049f8a8abd5b9990aa7db152982 Mon Sep 17 00:00:00 2001
From: Kees van Kempen <git@keesvankempen.nl>
Date: Thu, 19 Dec 2024 12:04:41 +0100
Subject: [PATCH] 2024(5): not done, but this is a start

---
 2024/5/puzzle1.py | 67 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 2024/5/puzzle1.py

diff --git a/2024/5/puzzle1.py b/2024/5/puzzle1.py
new file mode 100644
index 0000000..76ca2bd
--- /dev/null
+++ b/2024/5/puzzle1.py
@@ -0,0 +1,67 @@
+verbose = False
+
+def indices(lst, element):
+	"""
+	See <https://stackoverflow.com/questions/6294179/how-to-find-all-occurrences-of-an-element-in-a-list>.
+	"""
+
+	result = []
+	offset = -1
+	while True:
+		try:
+			offset = lst.index(element, offset+1)
+		except ValueError:
+			return result
+		result.append(offset)
+
+def load_input(filename="input"):
+	with open(filename, "r") as fp:
+		lines = fp.read().splitlines()
+
+	# Find the empty line/separator.
+	sep = lines.index("")
+
+	# Handle rules
+	rule_lines = lines[:sep]
+	rules = [tuple(map(int, l.split("|"))) for l in rule_lines]
+
+	# Handle lines
+	update_lines = lines[sep + 1:]
+	updates = [list(map(int, l.split(","))) for l in update_lines]
+
+	return rules, updates
+
+def is_update_legal(update, rules) -> bool:
+	for rule in rules:
+		# Find indices of left hand side and right hand side
+		lhs, rhs = rule
+
+		if not lhs in update or not rhs in update:
+			continue
+		# This reminds me of limes inferior and limes superior.
+		# Thanks, Mueger.
+		if max(indices(update, lhs)) > min(indices(update, rhs)):
+			return False
+	return True
+
+def middle_page_num(update) -> int:
+	return len(update) // 2
+
+if __name__ == "__main__":
+	rules, updates = load_input()
+
+	indices_legal = []
+	middle_sum = 0
+	for idx, update in enumerate(updates):
+		if is_update_legal(update, rules):
+			if verbose:
+				print(update, " is legal")
+			indices_legal.append(idx)
+			middle_sum += middle_page_num(update)
+		else:
+			if verbose:		
+				print(update, " is illegal")
+
+	if verbose:
+		print(indices_legal)
+	print(f"The sum of middle page numbers of legal updates is {middle_sum}.")