From 9d50931fe92b7c484904c5b93b45a8dc60abc4e7 Mon Sep 17 00:00:00 2001
From: Kees van Kempen <git@keesvankempen.nl>
Date: Thu, 19 Dec 2024 00:40:02 +0100
Subject: [PATCH] 2024(4): fixed the first puzzle!

---
 2024/4/puzzle1.py | 46 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 40 insertions(+), 6 deletions(-)

diff --git a/2024/4/puzzle1.py b/2024/4/puzzle1.py
index 58689bc..95e3c49 100644
--- a/2024/4/puzzle1.py
+++ b/2024/4/puzzle1.py
@@ -31,7 +31,7 @@ def rotate(list_2d):
 			substring += list_2d[x][y]
 			x += 1
 			y += 1
-		diagonal_list.append(substring)	
+		diagonal_list.append(substring)
 
 	# Reverse
 	diagonal_list = diagonal_list[::-1]
@@ -45,7 +45,39 @@ def rotate(list_2d):
 			substring += list_2d[x][y]
 			x += 1
 			y += 1
-		diagonal_list.append(substring)	
+		diagonal_list.append(substring)
+
+	return diagonal_list
+
+def rotate_neg(list_2d):
+	x_max, y_max = len(list_2d), len(list_2d[0])
+	diagonal_list = []
+
+	# Diagonals starting from (0,0), for the first row.
+	for i in range(x_max):
+		substring = ""
+		x = x_max - 1 - i
+		y = 0
+		while x >= 0 and y < y_max:
+			#print(x, x_max, y, y_max)
+			substring += list_2d[x][y]
+			x -= 1
+			y += 1
+		diagonal_list.append(substring)
+
+	# Reverse
+	diagonal_list = diagonal_list[::-1]
+
+	# Diagonals starting from (0,1), for the first column.
+	for j in range(1, y_max):
+		substring = ""
+		x = x_max - 1
+		y = j
+		while x >= 0 and y < len(list_2d[x]):
+			substring += list_2d[x][y]
+			x -= 1
+			y += 1
+		diagonal_list.append(substring)
 
 	return diagonal_list
 
@@ -53,24 +85,26 @@ if __name__ == "__main__":
 	result = 0
 	
 	with open("input", "r") as fp:
-		# Skip last line as it is empty
+		# Skip last line as it is empty.
 		full_string = fp.read().split("\n")[:-1]
 		for line in full_string:
 			result += scan_line(line)
 			result += scan_line(line[::-1])
+		# Transpose for vertical hits.
 		for line in transpose(full_string):
 			result += scan_line(line)
 			result += scan_line(line[::-1])
 
+		# Rotate 45 degrees to one side for diagonals.
 		diagonal_list = rotate(full_string)
 		for line in diagonal_list:
 			result += scan_line(line)
 			result += scan_line(line[::-1])
-		# TODO: Transposion after rotating is ill-defined.
-		for line in rotate(transpose(full_string)):
+		diagonal_list = rotate_neg(full_string)
+		# Rotate the other way round.
+		for line in diagonal_list:
 			result += scan_line(line)
 			result += scan_line(line[::-1])
 
-		#transposed_file = 
 	print(f"The sum of all horizontal XMAS's equals {result}.")