provide method to get the best prefix

lib/ruby_units/unit.rb

@@ -71,7 +71,7 @@ class Unit < Numeric
         :substance,
         :luminosity,
         :currency,
-        :memory,
+        :information,
         :angle
     ]
     @@KINDS            = {
@@ -120,7 +120,7 @@ class Unit < Numeric
         63999998     => :illuminance,
         64000000     => :luminous_power,
         1280000000   => :currency,
-        25600000000  => :memory,
+        25600000000  => :information,
         511999999980 => :angular_velocity,
         512000000000 => :angle
     }
@@ -1048,9 +1048,8 @@ def as_json(*args)
 
     # returns the 'unit' part of the Unit object without the scalar
     # @return [String]
-    def units
+    def units(with_prefix = true)
       return "" if @numerator == UNITY_ARRAY && @denominator == UNITY_ARRAY
-      return @unit_name unless @unit_name.nil?
       output_numerator   = []
       output_denominator = []
       num                = @numerator.clone.compact
@@ -1061,7 +1060,9 @@ def units
       else
         while defn = RubyUnits::Unit.definition(num.shift) do
           if defn && defn.prefix?
-            output_numerator << defn.display_name + RubyUnits::Unit.definition(num.shift).display_name
+            if with_prefix
+              output_numerator << (defn.display_name + RubyUnits::Unit.definition(num.shift).display_name)
+            end
           else
             output_numerator << defn.display_name
           end
@@ -1073,7 +1074,9 @@ def units
       else
         while defn = RubyUnits::Unit.definition(den.shift) do
           if defn && defn.prefix?
-            output_denominator << defn.display_name + RubyUnits::Unit.definition(den.shift).display_name
+            if with_prefix
+              output_denominator << (defn.display_name + RubyUnits::Unit.definition(den.shift).display_name)
+            end
           else
             output_denominator << defn.display_name
           end
@@ -1087,7 +1090,6 @@ def units
           map { |x| [x, output_denominator.count(x)] }.
           map { |element, power| ("#{element}".strip + (power > 1 ? "^#{power}" : '')) }
       out = "#{on.join('*')}#{od.empty? ? '' : '/' + od.join('*')}".strip
-      @unit_name = out unless self.kind == :temperature
       return out
     end
 
@@ -1264,6 +1266,16 @@ def coerce(other)
       end
     end
 
+    # returns a new unit that has been
+    def best_prefix
+      _best_prefix =  if (self.kind == :information)
+        @@PREFIX_VALUES.key(2**((Math.log(self.base_scalar,2) / 10.0).floor * 10))
+      else
+        @@PREFIX_VALUES.key(10**((Math.log10(self.base_scalar) / 3.0).floor * 3))
+      end
+      self.to(RubyUnits::Unit.new(@@PREFIX_MAP.key(_best_prefix)+self.units(false)))
+    end
+
     # Protected and Private Functions that should only be called from this class
     protected
 
@@ -1300,6 +1312,7 @@ def unit_signature_vector
       return vector
     end
 
+
     private
 
     # used by #dup to duplicate a Unit
@@ -1516,8 +1529,6 @@ def self.base_units
       return @@base_units ||= @@definitions.dup.delete_if { |_, defn| !defn.base? }.keys.map { |u| RubyUnits::Unit.new(u) }
     end
 
-    private
-
     # parse a string consisting of a number and a unit string
     # @param [String] string
     # @return [Array] consisting of [Numeric, "unit"]

lib/ruby_units/unit_definitions/base.rb

@@ -61,7 +61,7 @@
   unit.scalar    = 1
   unit.numerator = %w{<byte>}
   unit.aliases   = %w{B byte bytes}
-  unit.kind      = :memory
+  unit.kind      = :information
 end
 
 RubyUnits::Unit.define("dollar") do |unit|

spec/ruby-units/unit_spec.rb

@@ -1306,6 +1306,11 @@
     specify { Unit('23 m').div(Unit('2 m')).should == 11 }
   end
 
+  context '#best_prefix' do
+    specify { Unit('1024 KiB').best_prefix.should == Unit('1 MiB')}
+    specify { Unit('1000 m').best_prefix.should == Unit('1 km')}
+  end
+
   context "Time helper functions" do
     before do
       Time.stub!(:now).and_return(Time.utc(2011,10,16))