Implements Padding parsing

This is a straight copy/paste from margin.

Implemented per http://www.w3.org/TR/CSS21/box.html#propdef-padding and
http://www.w3.org/TR/CSS/

Author: zmoazeni

lib/csscss.rb

@@ -18,3 +18,4 @@
 require "csscss/parser/background"
 require "csscss/parser/list_style"
 require "csscss/parser/margin"
+require "csscss/parser/padding"

lib/csscss/parser/padding.rb

@@ -0,0 +1,59 @@
+module Csscss
+  module Parser
+    module Padding
+      extend Parser::Base
+
+      class Parser < Parslet::Parser
+        include Common
+
+        rule(:padding_side) {
+          length | percent | symbol("inherit")
+        }
+
+        rule(:padding) {
+          (
+           symbol("inherit") >> eof | (
+             padding_side.maybe.as(:top)    >>
+             padding_side.maybe.as(:right)  >>
+             padding_side.maybe.as(:bottom) >>
+             padding_side.maybe.as(:left)
+           )
+          ).as(:padding)
+        }
+        root(:padding)
+      end
+
+      class Transformer < Parslet::Transform
+        rule(padding: simple(:inherit)) {[]}
+
+        SIDE = proc {|side, value| Declaration.from_parser("padding-#{side}", value) }
+
+        rule(padding: {
+          top:simple(:top),
+          right:simple(:right),
+          bottom:simple(:bottom),
+          left:simple(:left)
+        }) {
+          values = [top, right, bottom, left].compact
+          case values.size
+          when 4
+            %w(top right bottom left).zip(values).map {|side, value| SIDE[side, value] }
+          when 3
+            %w(top right bottom).zip(values).map {|side, value| SIDE[side, value] }.tap do |declarations|
+              declarations << SIDE["left", values[1]]
+            end
+          when 2
+            %w(top right).zip(values).map {|side, value| SIDE[side, value] }.tap do |declarations|
+              declarations << SIDE["bottom", values[0]]
+              declarations << SIDE["left", values[1]]
+            end
+          when 1
+            %w(top right bottom left).map do |side|
+              SIDE[side, values[0]]
+            end
+          end
+        }
+      end
+    end
+  end
+end

lib/csscss/redundancy_analyzer.rb

@@ -88,7 +88,8 @@ def shorthand_parser(property)
       case property
       when "background" then Parser::Background
       when "list-style" then Parser::ListStyle
-      when "margin" then Parser::Margin
+      when "margin"     then Parser::Margin
+      when "padding"    then Parser::Padding
       end
     end
   end

test/csscss/parser/padding_test.rb

@@ -0,0 +1,50 @@
+require "test_helper"
+
+module Csscss::Parser
+  module Padding
+    describe Padding do
+      include CommonParserTests
+
+      before do
+        @parser = Parser.new
+        @trans = Transformer.new
+      end
+
+      it "converts shorthand rules to longhand" do
+        trans("1px 10% inherit 4em").must_equal([
+          dec("padding-top", "1px"),
+          dec("padding-right", "10%"),
+          dec("padding-bottom", "inherit"),
+          dec("padding-left", "4em")
+        ])
+
+        trans("1px 10% inherit").must_equal([
+          dec("padding-top", "1px"),
+          dec("padding-right", "10%"),
+          dec("padding-bottom", "inherit"),
+          dec("padding-left", "10%")
+        ])
+
+        trans("1px 10%").must_equal([
+          dec("padding-top", "1px"),
+          dec("padding-right", "10%"),
+          dec("padding-bottom", "1px"),
+          dec("padding-left", "10%")
+        ])
+
+        trans("1px").must_equal([
+          dec("padding-top", "1px"),
+          dec("padding-right", "1px"),
+          dec("padding-bottom", "1px"),
+          dec("padding-left", "1px")
+        ])
+      end
+
+      it "tries the parse and returns false if it doesn't work" do
+        @parser.try_parse("foo").must_equal(false)
+        parsed = @parser.try_parse("1px")
+        parsed[:padding][:top].must_equal("1px")
+      end
+    end
+  end
+end