add Collator#collation_key method

similar to direct comparison, but more efficient if you need to compare
the same string multiple times

lib/ffi-icu/collation.rb

@@ -80,10 +80,18 @@ module ICU
       def rules
         @rules ||= begin
           length = FFI::MemoryPointer.new(:int)
-          ptr = ICU::Lib.ucol_getRules(@c, length)
+          ptr = Lib.ucol_getRules(@c, length)
           ptr.read_array_of_uint16(length.read_int).pack("U*")
         end
       end
+
+      def collation_key(string)
+        ptr = UCharPointer.from_string(string)
+        size = Lib.ucol_getSortKey(@c, ptr, string.jlength, nil, 0)
+        buffer = FFI::MemoryPointer.new(:char, size)
+        Lib.ucol_getSortKey(@c, ptr, string.jlength, buffer, size)
+        buffer.read_bytes(size - 1)
+      end
     end # Collator
 
   end # Collate

lib/ffi-icu/lib.rb

@@ -290,6 +290,7 @@ module ICU
     attach_function :ucol_greaterOrEqual,   "ucol_greaterOrEqual#{suffix}",   [:pointer,   :pointer,  :int32_t,  :pointer, :int32_t], :bool
     attach_function :ucol_equal,            "ucol_equal#{suffix}",            [:pointer,   :pointer,  :int32_t,  :pointer, :int32_t], :bool
     attach_function :ucol_getRules,         "ucol_getRules#{suffix}",         [:pointer,   :pointer], :pointer
+    attach_function :ucol_getSortKey,       "ucol_getSortKey#{suffix}",       [:pointer, :pointer, :int, :pointer, :int], :int
 
     # Transliteration
     #

spec/collation_spec.rb

@@ -61,6 +61,9 @@ module ICU
         collator.rules.include?('ö<<<Ö').should be_true
       end
 
+      it "returns usable collation keys" do
+        collator.collation_key("abc").should be < collator.collation_key("xyz")
+      end
     end
   end # Collate
 end # ICU