External command "brew which"

A work-in-progress command, "brew which" will show which versions of formulae
you have installed based on what symlinks exist from HOMEBREW_PREFIX back into
the Cellar.

Thus, if you happen to have symlinks into two different versions of a brew in
the Cellar (typically an error), both of those versions will be listed.

This is a diagnostic command, but some of this code will be the basis for
possible future better handling of multiple-versions-at-once.

Library/Contributions/examples/brew-which.rb

@@ -0,0 +1,53 @@
+require 'extend/pathname'
+
+REAL_CELLAR = HOMEBREW_CELLAR.realpath
+
+class String
+  def starts_with?(prefix)
+    prefix = prefix.to_s
+    self[0, prefix.length] == prefix
+  end
+end
+
+class Pathname
+  def starts_with?(prefix)
+    prefix = prefix.to_s
+    self.to_s[0, prefix.length] == prefix
+  end
+end
+
+
+def audit
+  brew_links = Array.new
+  version_map = Hash.new
+
+  # paths=%w[bin sbin etc lib include share].collect {|d| HOMEBREW_PREFIX+d}
+  paths=%w[bin].collect {|d| HOMEBREW_PREFIX+d}
+
+  paths.each do |path|
+    path.find do |path|
+      next unless path.symlink? && path.resolved_path_exists?
+      brew_links << Pathname.new(path.realpath)
+    end
+  end
+
+  brew_links = brew_links.collect{|p|p.relative_path_from(REAL_CELLAR).to_s}.reject{|p|p.starts_with?("../")}
+  brew_links.each do |p|
+    parts = p.split("/")
+    next if parts.count < 2 # Shouldn't happen
+    brew = parts.shift
+    version = parts.shift
+
+    versions = version_map[brew] || []
+    versions << version unless versions.include? version
+    version_map[brew] = versions
+  end
+
+  return version_map
+end
+
+brews = audit
+brews.keys.sort.each do |b|
+  puts "#{b}: #{brews[b].sort*' '}"
+end
+puts