Added lighten-by() and darken-by() BIFs

docs/bifs.md

@@ -208,6 +208,38 @@ Darken the given `color` by `amount`.
     darken(white, 50%)
     // => #808080
 
+### lighten-by(color | hsl, amount)
+
+ Lighten by `amount` of the current lightness value.
+
+    lighten-by(black, 50%)
+    // => #808080
+
+    lighten-by(lighten-by(black, 50%), 50%)
+    // => #bfbfbf
+
+    lighten-by(lighten-by(lighten-by(black, 50%), 50%), 50%)
+    // => #fff
+
+essentially the same as:
+
+    black + hsl(0,0,50%)
+    black + hsl(0,0,75%)
+    black + hsl(0,0,100%)
+
+### darken-by(color | hsl, amount)
+
+ Darken by `amount` of the current lightness value.
+
+    darken-by(white, 50%)
+    // => #808080
+
+    darken-by(darken-by(white, 50%), 50%)
+    // => #404040
+
+    darken-by(darken-by(darken-by(white, 50%), 50%), 50%)
+    // => #202020
+
 ### desaturate(color | hsl, amount)
 
 Desaturate the given `color` by `amount`.

lib/functions/index.js

@@ -491,6 +491,26 @@ exports.desaturate = function(color, amount){
     , hsl.a);
 };
 
+/**
+ * Lighten `color` by `amount` of the current value.
+ *
+ * @param {Color|HSLA} color
+ * @param {Unit} amount
+ * @return {HSLA}
+ * @api public
+ */
+
+exports['lighten-by'] = function(color, amount){
+  utils.assertColor(color, 'color');
+  utils.assertType(amount, nodes.Unit, 'amount');
+  var hsl = color.hsl;
+  return new nodes.HSLA(
+      hsl.h
+    , hsl.s
+    , hsl.l + (hsl.l || 100) * amount.val / 100
+    , hsl.a);
+};
+
 /**
  * Lighten `color` by `amount`.
  *
@@ -531,6 +551,26 @@ exports.darken = function(color, amount){
     , hsl.a);
 };
 
+/**
+ * Darken `color` by `amount` of the current value.
+ *
+ * @param {Color|HSLA} color
+ * @param {Unit} amount
+ * @return {HSLA}
+ * @api public
+ */
+
+exports['darken-by'] = function(color, amount){
+  utils.assertColor(color, 'color');
+  utils.assertType(amount, nodes.Unit, 'amount');
+  var hsl = color.hsl;
+  return new nodes.HSLA(
+      hsl.h
+    , hsl.s
+    , hsl.l * amount.val / 100
+    , hsl.a);
+};
+
 /**
  * Assign `type` to the given `unit` or return `unit`'s type.
  *

test/cases/bifs.darken-by.css

@@ -0,0 +1,5 @@
+body {
+  background: #808080;
+  background: #404040;
+  background: #202020;
+}

test/cases/bifs.darken-by.styl

@@ -0,0 +1,4 @@
+body
+  background: darken-by(white, 50%)
+  background: darken-by(darken-by(white, 50%), 50%)
+  background: darken-by(darken-by(darken-by(white, 50%), 50%), 50%)

test/cases/bifs.lighten-by.css

@@ -0,0 +1,5 @@
+body {
+  background: #808080;
+  background: #bfbfbf;
+  background: #fff;
+}

test/cases/bifs.lighten-by.styl

@@ -0,0 +1,4 @@
+body
+  background: lighten-by(black, 50%)
+  background: lighten-by(lighten-by(black, 50%), 50%)
+  background: lighten-by(lighten-by(lighten-by(black, 50%), 50%), 50%)