Add day 4 solution.

day4.cr

@@ -0,0 +1,52 @@
+require "./common.cr"
+
+lines = File.read("day4_input").split("\n")
+lines.pop
+lines.sort!
+
+REGEX = /\[(\d+)-(\d+)-(\d+) (\d+):(\d+)\].+/
+GUARD_REGEX = /.+#(\d+).+/
+
+active = 0
+last_time = {0, 1, 0, 0, 0}
+minutes = {} of Int32 => Array(Int32)
+
+lines.each do |line|
+  match = line.match(REGEX).not_nil!
+  year = match[1].to_i32
+  month = match[2].to_i32
+  day = match[3].to_i32
+  hour = match[4].to_i32
+  minute = match[5].to_i32
+  new_time = {year, month, day, hour, minute}
+
+  if line.includes? "Guard"
+    active = line.match(GUARD_REGEX).not_nil![1].to_i32
+  elsif line.includes? "wakes"
+    old_year, old_month, old_day, old_hour, old_minute = last_time
+    difference = (day - old_day) * 24 * 60 +
+      (hour - old_hour) * 60 +
+      (minute - old_minute)
+
+    array = minutes[active]? || Array.new(60, 0)
+    60.times do |m|
+      array[m] += difference / 60
+    end
+    (difference % 60).times do |m|
+      actual_minute = (old_minute + m) % 60
+      array[actual_minute] += 1
+    end
+    minutes[active] = array
+  end
+  last_time = new_time
+end
+
+guard_num, guard_minutes = minutes.max_by &.[1].sum
+puts "Guard num: #{guard_num}"
+puts "Guard minute: #{(guard_minutes.index guard_minutes.max).not_nil!}"
+puts "Answer num: #{guard_num * (guard_minutes.index guard_minutes.max).not_nil!}"
+
+guard_num, guard_minutes = minutes.max_by &.[1].max
+puts "Guard num: #{guard_num}"
+puts "Guard minute: #{(guard_minutes.index guard_minutes.max).not_nil!}"
+puts "Answer num: #{guard_num * (guard_minutes.index guard_minutes.max).not_nil!}"