add Geometry#resize_to to calculate dimensions of new source

lib/paperclip/geometry.rb

@@ -101,6 +101,33 @@ module Paperclip
       [ scale_geometry, crop_geometry ]
     end
 
+    # resize to a new geometry
+    # @param geometry [String] the Paperclip geometry definition to resize to
+    # @example
+    #   Paperclip::Geometry.new(150, 150).resize_to('50x50!')
+    #   #=> Paperclip::Geometry(50, 50)
+    def resize_to(geometry)
+      new_geometry = Paperclip::Geometry.parse geometry
+      case new_geometry.modifier
+      when '!', '#'
+        new_geometry
+      when '>'
+        if new_geometry.width >= self.width && new_geometry.height >= self.height
+          self
+        else
+          scale_to new_geometry
+        end
+      when '<'
+        if new_geometry.width <= self.width || new_geometry.height <= self.height
+          self
+        else
+          scale_to new_geometry
+        end
+      else
+        scale_to new_geometry
+      end
+    end
+
     private
 
     def scaling dst, ratio
@@ -118,5 +145,11 @@ module Paperclip
         "%dx%d+%d+%d" % [ dst.width, dst.height, (self.width * scale - dst.width) / 2, 0 ]
       end
     end
+
+    # scale to the requested geometry and preserve the aspect ratio
+    def scale_to(new_geometry)
+      scale = [new_geometry.width.to_f / self.width.to_f , new_geometry.height.to_f / self.height.to_f].min
+      Paperclip::Geometry.new (self.width * scale).round, (self.height * scale).round
+    end
   end
 end

test/geometry_test.rb

+require 'rubygems'
 require './test/helper'
 
 class GeometryTest < Test::Unit::TestCase
@@ -202,5 +203,24 @@ class GeometryTest < Test::Unit::TestCase
         end
       end
     end
+
+    [['256x256', '150x150!' => [150, 150], '150x150#' => [150, 150], '150x150>' => [150, 150], '150x150<' => [256, 256], '150x150' => [150, 150]],
+     ['256x256', '512x512!' => [512, 512], '512x512#' => [512, 512], '512x512>' => [256, 256], '512x512<' => [512, 512], '512x512' => [512, 512]],
+     ['600x400', '512x512!' => [512, 512], '512x512#' => [512, 512], '512x512>' => [512, 341], '512x512<' => [600, 400], '512x512' => [512, 341]]].each do |original_size, options|
+      options.each_pair do |size, dimensions|
+        context "#{original_size} resize_to #{size}" do
+          setup do
+            @source = Paperclip::Geometry.parse original_size
+            @new_geometry = @source.resize_to size
+          end
+          should "have #{dimensions.first} width" do
+            assert_equal dimensions.first, @new_geometry.width
+          end
+          should "have #{dimensions.last} height" do
+            assert_equal dimensions.last, @new_geometry.height
+          end
+        end
+      end
+    end
   end
 end