Sunday, September 16, 2007

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.

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