Add day 13 solution.

day13.cr

@@ -0,0 +1,34 @@
+require "advent"
+INPU = input(2020, 13).lines
+INPUT = {INPU[0].to_i32, INPU[1].split(",")}
+
+def part1(input)
+  early, busses = input
+  busses.reject! &.==("x")
+  busses = busses.map &.to_i32
+  bbus = busses.min_by do |b|
+    (early / b).ceil * b
+  end
+  diff = bbus * (((early/bbus).ceil * bbus).to_i32 - early)
+end
+
+def part2(input)
+  _, busses = input
+  busses = busses.map_with_index do |x, i|
+    x.to_i32?.try { |n| { n, i } }
+  end
+  busses = busses.compact
+  n = 0_i64
+  iter = 1_i64
+  busses.each do |m, i|
+    while (n + i) % m != 0
+      n += iter
+    end
+    iter *= m
+  end
+  puts n
+  puts busses
+end
+
+puts part1(INPUT.clone)
+puts part2(INPUT.clone)