Add support for complex nth-child selectors in omega()

app/assets/stylesheets/functions/_private.scss

@@ -116,3 +116,39 @@
 
   @return $opposite-direction;
 }
+
+
+@function to-number($string) {
+  $string: str-replace($string, " ", "");
+  $strings: "0" "1" "2" "3" "4" "5" "6" "7" "8" "9";
+  $numbers:  0 1 2 3 4 5 6 7 8 9;
+  $result: 0;
+
+  @for $i from 1 through str-length($string) {
+    $character: str-slice($string, $i, $i);
+    $index: index($strings, $character);
+
+    @if not $index {
+      @warn "Unknown character `#{$character}`.";
+      @return false;
+    }
+
+    $number: nth($numbers, $index);
+    $result: $result * 10 + $number;
+  }
+
+  @return $result;
+}
+
+@function str-replace($string, $search, $replace: "") {
+  $index: str-index($string, $search);
+
+  @if $index {
+    $first: str-slice($string, 1, $index - 1);
+    $last-slice: str-slice($string, $index + str-length($search));
+    $last: str-replace($last-slice, $search, $replace);
+    @return $first + $replace + $last;
+  }
+
+  @return $string;
+}

app/assets/stylesheets/grid/_omega.scss

@@ -79,7 +79,32 @@
     margin-#{$direction}: 0;
   }
 
-  @if type-of($query) == number and unit($query) == "n" {
+  @if type-of($query) == string {
+    $query: str-replace($query, " ", "");
+    $operator: false;
+
+    @if str_index($query, "+") {
+      $operator: "+";
+    } @else if str_index($query, "-") {
+      $operator: "-";
+    }
+
+    @if $operator {
+      $operator-index: str_index($query, $operator);
+      $first: str-slice($query, 0, ($operator-index - 1));
+      $last: to-number(str-slice($query, ($operator-index + 1), -1));
+      @if $operator == "+" {
+        $last: $last + 1;
+      } @else if $operator == "-" {
+        $last: $last - 1;
+      }
+      $nth: "#{$first}#{$operator}#{$last}";
+
+      &:nth-child(#{$nth}) {
+        clear: $opposite-direction;
+      }
+    }
+  } @else if type-of($query) == number && unit($query) == "n" {
     &:nth-child(#{$query}+1) {
       clear: $opposite-direction;
     }

spec/neat/omega_spec.rb

@@ -7,36 +7,59 @@
 
   context "with no argument" do
     it "removes right margin" do
-      expect(".omega-default").to have_rule("margin-right: 0")
+      selector = ".omega-default"
+      expect(selector).to have_rule("margin-right: 0")
     end
   end
 
   context "with argument (4n)" do
     it "removes right margin of nth-child(4n)" do
-      expect(".omega-nth-default:nth-child(4n)").to have_rule("margin-right: 0")
+      selector = ".omega-nth-default:nth-child(4n)"
+      expect(selector).to have_rule("margin-right: 0")
     end
 
     it "adds clear to nth-child(4n+1)" do
-      expect(".omega-nth-default:nth-child(4n+1)").to have_rule("clear: left")
+      selector = ".omega-nth-default:nth-child(4n+1)"
+      expect(selector).to have_rule("clear: left")
     end
   end
 
   context "with argument ('4n+1')" do
     it "removes right margin of nth-child(4n+1)" do
-      expect(".omega-complex-nth:nth-child(4n+1)").to have_rule("margin-right: 0")
+      selector = ".omega-complex-nth:nth-child(4n+1)"
+      expect(selector).to have_rule("margin-right: 0")
+    end
+
+    it "adds clear to nth-child('4n+2')" do
+      selector = ".omega-complex-nth:nth-child(4n+2)"
+      expect(selector).to have_rule("clear: left")
+    end
+  end
+
+  context "with argument ('3n-1')" do
+    it "removes right margin of nth-child(3n-1)" do
+      selector = ".omega-complex-nth-negative:nth-child(3n-1)"
+      expect(selector).to have_rule("margin-right: 0")
+    end
+
+    it "adds clear to nth-child('3n-0')" do
+      selector = ".omega-complex-nth-negative:nth-child(3n-0)"
+      expect(selector).to have_rule("clear: left")
     end
   end
 
   context "when called inside an RTL row" do
     context "with no argument" do
       it "removes left margin" do
-        expect("section .omega-default-left").to have_rule("margin-left: 0")
+        selector = "section .omega-default-left"
+        expect(selector).to have_rule("margin-left: 0")
       end
     end
 
     context "with argument (4n block)" do
       it "removes left margin of nth-child(4n)" do
-        expect("section .omega-nth-default-left:nth-child(4n)").to have_rule("margin-left: 0")
+        selector = "section .omega-nth-default-left:nth-child(4n)"
+        expect(selector).to have_rule("margin-left: 0")
       end
     end
   end

test/omega.scss

@@ -12,6 +12,10 @@
   @include omega("4n+1");
 }
 
+.omega-complex-nth-negative {
+  @include omega("3n-1");
+}
+
 section {
   @include row($direction: RTL);