Implemented `/usr/bin/cal` (sans opts) in Ruby as a fun challenge my friend mentioned doing in an interview.

cal.rb

#!/usr/bin/env ruby
require 'date'

today         = Date.today
first_day     = Date.new(today.year, today.month, 1)
last_day      = Date.new(today.year, today.month, -1) # A trick from JavaScript, heh...

first_week_blank_count = first_day.wday
last_week_blank_count  = last_day.wday

day_slots = Array.new(first_week_blank_count) + (first_day..last_day).to_a + Array.new(last_week_blank_count)

def highlight(str)
  "\e[7m#{str}\e[27m"
end

week_str = day_slots.slice_after { |d| d && d.saturday? }.map { |week|
  week.map { |date|
    if date
      date_str = date.day.to_s.rjust(2)
      date == today ? highlight(date_str) : date_str
    else
      '  '
    end
  }.join(' ')
}.join(?\n)

month_header  = today.strftime('%B %Y').center(20)
day_header    = 'Su Mo Tu We Th Fr Sa' # Should really be a CONSTANT, but also, should be localized...

puts  month_header,
      day_header,
      week_str