which: allow <formulae> args

Also clean-up this external command in preparation
for becoming a built-in command.

Make which_versions available in Homebrew module so
that it can be used by other commands in the future.

Library/Contributions/examples/brew-which.rb

@@ -1,19 +1,13 @@
 require 'extend/pathname'
 
-REAL_CELLAR = HOMEBREW_CELLAR.realpath
 
-class String
+module Homebrew extend self
-  def starts_with? prefix
+  def which_versions which_brews=nil
-    prefix = prefix.to_s
-    self[0, prefix.length] == prefix
-  end
-end
-
-
-def audit
     brew_links = Array.new
     version_map = Hash.new
 
+    real_cellar = HOMEBREW_CELLAR.realpath
+
     # paths=%w[bin sbin etc lib include share].collect {|d| HOMEBREW_PREFIX+d}
     paths=%w[bin].collect {|d| HOMEBREW_PREFIX+d}
 
@@ -24,23 +18,33 @@ def audit
       end
     end
 
-  brew_links = brew_links.collect{|p|p.relative_path_from(REAL_CELLAR).to_s}.reject{|p|p.starts_with?("../")}
+    brew_links = brew_links.collect{|p|p.relative_path_from(real_cellar).to_s}.reject{|p|p.start_with?("../")}
+
     brew_links.each do |p|
       parts = p.split("/")
-    next if parts.count < 2 # Shouldn't happen
+      next if parts.count < 2 # Shouldn't happen for normally installed brews
       brew = parts.shift
       version = parts.shift
 
+      next unless which_brews.include? brew if which_brews
+
       versions = version_map[brew] || []
       versions << version unless versions.include? version
       version_map[brew] = versions
     end
 
     return version_map
+  end
+
+  def which
+    which_brews = ARGV.named.empty? ? nil : ARGV.named
+
+    brews = which_versions which_brews
+    brews.keys.sort.each do |b|
+      puts "#{b}: #{brews[b].sort*' '}"
+    end
+    puts
+  end
 end
 
-brews = audit
+Homebrew.which
-brews.keys.sort.each do |b|
-  puts "#{b}: #{brews[b].sort*' '}"
-end
-puts