wordcount_map_reduce.rb

#!/usr/bin/ruby

# to count the words of "The Complete Works of William Shakespeare", type
# $ curl http://www.gutenberg.org/cache/epub/100/pg100.txt | ./wordcount_map_reduce.rb

def map line
  line.split(' ').each do |word|
    emit(word, 1) 
  end
end

def reduce key, values
  values.inject(0){|sum, value| sum += value}
end

def finalize key_value_rows
  key_value_rows.sort_by{|key, value| value}.reverse
end

def format_output key, value
  "#{value}\t#{key}"
end


KEY_TO_VALUES = {}
def emit key, value
  KEY_TO_VALUES[key] ||= []
  KEY_TO_VALUES[key] << value
end

ARGF.each do |line|
  map(line)
end

key_value_rows = KEY_TO_VALUES.map do |key, values|
  [key, reduce(key, values)]
end

key_value_rows = finalize(key_value_rows)

key_value_rows.each do |key, value|
  puts format_output(key, value)
end