Allow negative indices in nth list function

Allow negative indices between -1 and (-1 * list length) in the `nth`
function.

doc-src/SASS_CHANGELOG.md

@@ -59,6 +59,8 @@ Thanks to Alexander Pavlov for implementing this.
 * Add a function \{Sass::Script::Function#unique\_id `unique-id()`} that will
   return a CSS identifier that is unique within the scope of a single CSS file.
 
+* Allow negative indices into lists when using `nth()`.
+
 ### Backwards Incompatibilities -- Must Read!
 
 * Sass will now throw an error when `@extend` is used to extend a selector

lib/sass/script/functions.rb

@@ -1486,23 +1486,23 @@ def length(list)
     # @example
     #   nth(10px 20px 30px, 1) => 10px
     #   nth((Helvetica, Arial, sans-serif), 3) => sans-serif
+    #   nth((red, green, blue), -2) => green
     # @param list [Value] The list
     # @param n [Sass::Script::Value::Number] The index into the list
     # @return [Sass::Script::Value::Base] The nth item in the list
-    # @raise [ArgumentError] If `n` isn't an integer between 1 and the list's length.
+    # @raise [ArgumentError] If `n` isn't an integer whose absolute value is between 1 and the list's length.
     def nth(list, n)
       assert_type n, :Number
-      if !n.int?
-        raise ArgumentError.new("List index #{n} must be an integer")
-      elsif n.to_i < 1
-        raise ArgumentError.new("List index #{n} must be greater than or equal to 1")
+      if !n.int? || n.to_i == 0
+        raise ArgumentError.new("List index #{n} must be a non-zero integer")
       elsif list.to_a.size == 0
         raise ArgumentError.new("List index is #{n} but list has no items")
-      elsif n.to_i > (size = list.to_a.size)
+      elsif n.to_i.abs > (size = list.to_a.size)
         raise ArgumentError.new("List index is #{n} but list is only #{size} item#{'s' if size != 1} long")
       end
 
-      list.to_a[n.to_i - 1]
+      index = n.to_i > 0 ? n.to_i - 1 : n.to_i
+      list.to_a[index]
     end
     declare :nth, [:list, :n]
 

test/sass/functions_test.rb

@@ -1017,12 +1017,14 @@ def test_length
   def test_nth
     assert_equal("1", evaluate("nth(1 2 3, 1)"))
     assert_equal("2", evaluate("nth(1 2 3, 2)"))
+    assert_equal("3", evaluate("nth(1 2 3, -1)"))
+    assert_equal("1", evaluate("nth(1 2 3, -3)"))
     assert_equal("3", evaluate("nth((1, 2, 3), 3)"))
     assert_equal("foo", evaluate("nth(foo, 1)"))
     assert_equal("bar baz", evaluate("nth(foo (bar baz) bang, 2)"))
-    assert_error_message("List index 0 must be greater than or equal to 1 for `nth'", "nth(foo, 0)")
-    assert_error_message("List index -10 must be greater than or equal to 1 for `nth'", "nth(foo, -10)")
-    assert_error_message("List index 1.5 must be an integer for `nth'", "nth(foo, 1.5)")
+    assert_error_message("List index 0 must be a non-zero integer for `nth'", "nth(foo, 0)")
+    assert_error_message("List index is -10 but list is only 1 item long for `nth'", "nth(foo, -10)")
+    assert_error_message("List index 1.5 must be a non-zero integer for `nth'", "nth(foo, 1.5)")
     assert_error_message("List index is 5 but list is only 4 items long for `nth'", "nth(1 2 3 4, 5)")
     assert_error_message("List index is 2 but list is only 1 item long for `nth'", "nth(foo, 2)")
     assert_error_message("List index is 1 but list has no items for `nth'", "nth((), 1)")