Add support for custom version schemes

A version scheme is a class that inherits from Version and reimplements
Version#<=>. This will allow formulae to specify a custom comparison
method that will be used instead of the default, for cases where the
default is insufficient.

Library/Homebrew/formula_support.rb

@@ -52,10 +52,13 @@ class SoftwareSpec
   end
 
   def version val=nil
-    if val.nil?
-      @version ||= Version.parse(@url)
-    else
-      @version = Version.new(val)
+    @version ||= case val
+      when nil then Version.parse(@url)
+      when Hash
+        key, value = val.shift
+        scheme = VersionSchemeDetector.new(value).detect
+        scheme.new(key)
+      else Version.new(val)
       end
   end
 

Library/Homebrew/test/test_formula.rb

@@ -278,4 +278,11 @@ class FormulaTests < Test::Unit::TestCase
       when :mountainlion then '8badf00d8badf00d8badf00d8badf00d8badf00d'
       end, f.bottle.checksum.hexdigest
   end
+
+  def test_custom_version_scheme
+    f = CustomVersionSchemeTestBall.new
+
+    assert_version_equal '1.0', f.version
+    assert_instance_of CustomVersionScheme, f.version
+  end
 end

Library/Homebrew/test/testball.rb

@@ -271,3 +271,17 @@ class RevisedBottleSpecTestBall < Formula
     super "revisedbottlespectestball"
   end
 end
+
+class CustomVersionScheme < Version
+end
+
+class CustomVersionSchemeTestBall < Formula
+  homepage 'http://example.com'
+  url 'file:///foo.com/testball-0.1.tbz'
+  sha1 '482e737739d946b7c8cbaf127d9ee9c148b999f5'
+  version '1.0' => CustomVersionScheme
+
+  def initialize name=nil
+    super "customversionschemetestball"
+  end
+end

Library/Homebrew/version.rb

@@ -126,3 +126,24 @@ class Version
     return m.captures.first unless m.nil?
   end
 end
+
+class VersionSchemeDetector
+  def initialize scheme
+    @scheme = scheme
+  end
+
+  def detect
+    if @scheme.is_a? Class and @scheme.ancestors.include? Version
+      @scheme
+    elsif @scheme.is_a? Symbol then detect_from_symbol
+    else
+      raise "Unknown version scheme #{@scheme} was requested."
+    end
+  end
+
+  private
+
+  def detect_from_symbol
+    raise "Unknown version scheme #{@scheme} was requested."
+  end
+end