--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/day13.rb	Wed Dec 13 20:32:13 2023 +0000
@@ -0,0 +1,30 @@
+#! /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 find_reflection(data)
+	candidate_start = data.each_cons(2).with_index.flat_map {|vals,idx| a,b = vals; a == b ? [idx] : []}
+	valid_starts = candidate_start.flat_map do |start|
+		pos = start
+		reflected = start + 1
+		match = true
+		while pos > 0 and reflected < data.length - 1
+			pos -= 1
+			reflected += 1
+			match = data[pos] == data[reflected]
+			break unless match
+		end
+		match ? [start + 1] : []
+	end
+	valid_starts[0]
+end
+
+puts maps.flat_map {|data| [find_reflection(data).to_i * 100, find_reflection(data.transpose()).to_i]}.sum
\ No newline at end of file