--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/day13b.rb	Wed Dec 13 20:32:13 2023 +0000
@@ -0,0 +1,35 @@
+#! /usr/bin/env ruby
+
+if ARGV.length != 1
+        abort("Incorrect arguments - needs input file")
+elsif not File.exist? (ARGV[0])
+	abort("File #{ARGV[0]} did not exist")
+end
+
+file = ARGV[0]
+
+maps = File.open(file, "r").each_line().map(&:chomp).chunk_while {|before, after| before.length == after.length}.filter {|chunk| chunk != [""]}.map {|chunk| chunk.map{|row| row.each_char.to_a}}
+
+def count_num_changes(row_a, row_b)
+	row_a.zip(row_b).map {|a,b| (a != b) ? 1 : 0}.sum
+end
+
+def find_imperfect_reflection(data)
+	candidate_start = data.each_cons(2).with_index.flat_map {|vals,idx| count_num_changes(*vals) <= 1 ? [idx] : []}
+	valid_starts = candidate_start.flat_map do |start|
+		pos = start
+		reflected = start + 1
+		num_changes = count_num_changes(data[pos], data[reflected])
+		while pos > 0 and reflected < data.length - 1
+			pos -= 1
+			reflected += 1
+			num_changes_in_row = count_num_changes(data[pos], data[reflected])
+			num_changes += num_changes_in_row
+			break unless num_changes <= 1
+		end
+		num_changes == 1 ? [start + 1] : []
+	end
+	valid_starts[0]
+end
+
+puts maps.flat_map {|data| [find_imperfect_reflection(data).to_i * 100, find_imperfect_reflection(data.transpose()).to_i]}.sum
\ No newline at end of file