defines Module#qualified_const_(defined?|get|set) and String#deconstantize

This commit also implements a faster version of #demodulize I was unable
to isolate with git add --patch.

Not a big fan of the name #deconstantize. It complements #demodulize
getting rid of the rightmost constant, hence the name, but it is
unrelated to the well-known #constantize. So unsure. Could not come
with anything better, please feel free to rename.

1 files changed, 64 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/core_ext/module/qualified_const.rb b/activesupport/lib/active_support/core_ext/module/qualified_const.rb
new file mode 100644
index 0000000000..d1a0ee2f83
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/module/qualified_const.rb
@@ -0,0 +1,64 @@
+require 'active_support/core_ext/string/inflections'
+
+#--
+# Allows code reuse in the methods below without polluting Module.
+#++
+module QualifiedConstUtils
+  def self.raise_if_absolute(path)
+    raise NameError, "wrong constant name #$&" if path =~ /\A::[^:]+/
+  end
+
+  def self.names(path)
+    path.split('::')
+  end
+end
+
+##
+# Extends the API for constants to be able to deal with qualified names. Arguments
+# are assumed to be relative to the receiver.
+#
+#--
+# Qualified names are required to be relative because we are extending existing
+# methods that expect constant names, ie, relative paths of length 1. For example,
+# Object.const_get("::String") raises NameError and so does qualified_const_get.
+#++
+class Module
+  if method(:const_defined?).arity == 1
+    def qualified_const_defined?(path)
+      QualifiedConstUtils.raise_if_absolute(path)
+
+      QualifiedConstUtils.names(path).inject(self) do |mod, name|
+        return unless mod.const_defined?(name)
+        mod.const_get(name)
+      end
+      return true
+    end
+  else
+    def qualified_const_defined?(path, search_parents=true)
+      QualifiedConstUtils.raise_if_absolute(path)
+
+      QualifiedConstUtils.names(path).inject(self) do |mod, name|
+        return unless mod.const_defined?(name, search_parents)
+        mod.const_get(name)
+      end
+      return true
+    end
+  end
+
+  def qualified_const_get(path)
+    QualifiedConstUtils.raise_if_absolute(path)
+
+    QualifiedConstUtils.names(path).inject(self) do |mod, name|
+      mod.const_get(name)
+    end
+  end
+
+  def qualified_const_set(path, value)
+    QualifiedConstUtils.raise_if_absolute(path)
+
+    const_name = path.demodulize
+    mod_name = path.deconstantize
+    mod = mod_name.empty? ? self : qualified_const_get(mod_name)
+    mod.const_set(const_name, value)
+  end
+end