A Sorting Language

Inspired by Neal Ford's post on patterns and sorting in Ruby:

class Comparator
  def initialize(&default)
    if (defined? default)
      @compare_block=default
    else
      @compare_block=lambda { |a,b| a <=> b }
    end
  end
  
  def compare(a,b)
    @compare_block.call(a,b)
  end
  
  def to_proc
    method(:compare).to_proc
  end
  
  def to_comparator
    self
  end
  
  def then_by(next_aspect)
    next_comparator=next_aspect.to_comparator
    self.class.new do |a,b|
      comparison=compare(a,b)
      if (comparison == 0)
        next_comparator.compare(a,b)
      else
        comparison
      end
    end
  end
  
  def reverse
    self.class.new do |a,b| 
      comparison=compare(a,b)
      if (comparison == 1)
        -1
      elsif (comparison == -1)
        1
      else
        0
      end
    end
  end
end


def by(aspect_to_compare)
  aspect_to_compare.to_comparator
end

class Symbol
  def to_comparator
    Comparator.new { |a,b| a.send(self) <=> b.send(self) }
  end
end

module Enumerable
  def to_comparator
    inject do |thus_far, every|
      thus_far.to_comparator.then_by(every.to_comparator)
    end
  end
end

require 'rubyunit'
class SortTest < Test::Unit::TestCase
  def test_simple
    a=Person.new("blaine", 36)
    b=Person.new("blaine", 12)
    
    result=[a,b].sort(&by(:name).then_by(:age))
    assert_equal([b,a], result)
  end
  
  def test_array
    a=Person.new("blaine", 12)
    b=Person.new("alice", 12)
    
    result=[a,b].sort(&by([:age, :name]))
    assert_equal([b,a], result)
  end
  
  def test_reverse
    a=Person.new("grue", 123)
    b=Person.new("thief", 34)
    
    result=[a,b].sort(&by(:name).reverse)
    assert_equal([b,a], result)
    
    result=[a,b].sort(&by(:age).reverse)
    assert_equal([a,b], result)
  end
end

class Person
  attr_reader :name, :age
  
  def initialize(name, age)
    @name=name
    @age=age
  end
end

I loved Neal's post, but it got me thinking that sometimes I need to sort beyond one field. How could I go about that? I whipped up this example really quick. The function :by is simply syntactic sugar for converting to a comparator. I thought it read better. Neal's solution is the way to go if you need to sort on a single field. But when you need more, the above will work too. This is another one of my little late night coding thoughts. Enjoy.