Handy grouping funtion

grp.rb

#!/usr/bin/env ruby
require 'active_support/core_ext'

abort("usage: #{__FILE__} day|week|month") unless %w(day week month).include?(ARGV[0])

$stdin.readlines.map {|l|
  timestamp, amount = l.strip.split(',', 2)
  [Time.at(timestamp.to_i), amount.to_i]
}.group_by {|(time, _)|
  case ARGV[0]
  when 'day'
    time.beginning_of_day
  when 'week'
    time.beginning_of_week
  when 'month'
    time.beginning_of_month
  end
}.inject([]) {|results, (period, entries)|
  results << [period.strftime("%Y-%m-%d"), entries.inject(0){|total, entry| total += entry[1] }]
  results
}.sort_by {|(period, _)|
  period
}.each {|row|
  puts row.join(',')
}