You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: timestamp_grid.rb
    path of file: ./repos/stratify/lib/punch_card_graph
    
    the code of the file until where you have to start completion: module PunchCardGraph
  class TimestampGrid
    def initialize(timestamps)
      @timestamps = Array(timestamps)
  
    
    def counts_by_day_and_hour
      counts = default_counts_by_day_and_hour
      @timestamps.each do |timestamp|
        counts[timestamp.wday][timestamp.hour] += 1
      end
      counts
    end