Fix pow function in order to fix luminance test

lumninance uses float exponents which our current pow implmentation doens't support. Replaced with a powerfull math pow function provide by Tobias Bengfort.

Write some more assertions specifient all variations of input values that a pow function can have.

scss/tools/functions/__tests__/_fn.luminance.spec.scss

@@ -4,6 +4,6 @@
 
 @include describe("luminance()") {
   @include it("Returns luminance of color") {
-    @include assert-equal(luminance(#555), 0.04987, $inspect: true);
+    @include assert-equal(luminance(#555), 0.0908417112, $inspect: true);
   }
 }

scss/tools/functions/__tests__/_fn.pow.spec.scss

@@ -2,7 +2,29 @@
 @import "../_fn.pow";
 
 @include describe("pow()") {
-  @include it("Returns value powered to exponent") {
-    @include assert-equal(pow(2, 2), 4);
+  @include it("Returns result of position integer number powered to positive integer exponent") {
+    @include assert-equal(pow(2, 2, 2), 4);
+  }
+
+  @include it("Returns result of positive integer number powered to negative integer exponent") {
+    @include assert-equal(pow(2, -2, 2), 0.25);
+  }
+
+  @include it("Returns result of positive integer number powered to a float exponent") {
+    @include assert-equal(pow(2, 2.5), 5.6568542495);
+  }
+
+  @include it("Returns result of positive float number powered to a positive integer exponent") {
+    @include assert-equal(pow(2.5, 2), 6.25);
+  }
+
+  @include it("Returns result of positive float number powered to a positive integer exponent") {
+    @include assert-equal(pow(2.5, -2, 2), 0.16);
+  }
+
+  /// hack despite the number output is the same the test still fails
+  /// probably related to sass precision, requires investigation
+  @include it("Returns result of positive float number powered to a positive float exponent") {
+    @include assert-equal("#{pow(1.5, 1.5)}", "1.8371173071");
   }
 }

scss/tools/functions/_fn.pow.scss

@@ -1,17 +1,106 @@
+$PI: 3.141592653589793;
+
+@function -math-exp-taylor-0($x, $steps) {
+    $item: 1;
+    $result: 1;
+
+    @for $i from 1 to $steps {
+        $item: $item * $x / $i;
+        $result: $result + $item;
+    }
+
+    @return $result;
+}
+
+@function -math-ln-taylor-1($x, $steps) {
+    $z: ($x - 1) / ($x + 1);
+
+    $power: $z;
+    $result: $z;
+
+    @for $i from 1 to $steps {
+        $power: $power * $z *$z;
+        $result: $result + $power / (2 * $i + 1);
+    }
+
+    @return 2 * $result;
+}
+
+@function -math-sin-taylor-0($x, $steps) {
+    $item: $x;
+    $result: $x;
+
+    @for $i from 1 to $steps {
+        $item: -$item * $x * $x / (2 * $i) / (2 * $i + 1);
+        $result: $result + $item;
+    }
+
+    @return $result;
+}
+
+@function -math-pow-int($base, $exponent) {
+    @if $exponent < 0 {
+        @return 1 / -math-pow-int($base, -$exponent);
+    } @else if $exponent == 0 {
+        @return 1;
+    } @else if $exponent == 1 {
+        @return $base;
+    } @else {
+        $exp: floor($exponent / 2);
+        $pow: -math-pow-int($base, $exp);
+        @if $exp * 2 == $exponent {
+            @return $pow * $pow;
+        } @else {
+            @return $pow * $pow * $base;
+        }
+    }
+}
+
+@function -math-log-approx($x) {
+    @if $x <= 0 {
+        @error "cannot calculate log of #{$x}";
+    } @else if $x >= 1 {
+        // choose the smaller option (-1) because it yield better
+        // results in ln().
+        @return str-length(inspect(round($x))) - 1;
+    } @else {
+        @return -1 * str-length(inspect(round(1 / $x)));
+    }
+}
+
+@function ln($x, $steps: 32) {
+    $ln10: 2.302585092994046;
+    $approx: -math-log-approx($x);
+    // $y is in range [1, 10]
+    $y: $x / -math-pow-int(10, $approx);
+    @return $approx * $ln10 + -math-ln-taylor-1($y, $steps);
+}
+
 ///
-/// power number to exponent
+/// power a number to exponent, that can be positiv or negative int or float
+/// when we get sass dependencies right, consider moving this to the
+/// external package dependency like https://github.com/xi/sass-planifolia or
+/// https://www.npmjs.com/package/sass-math-pow
 ///
 /// @group Tools
-/// @author Hugo Giraudel
+/// @author Tobias Bengfort
+/// @link https://www.sassmeister.com/gist/5bbe8480c48e2fc10ab5
+/// @link https://gist.github.com/davidkpiano/ad6e6771df050ff3727f
+/// @link https://css-tricks.com/snippets/sass/power-function/
 ///
-@function pow($number, $exponent) {
-  $value: 1;
+/// @param {Number} $x - integer or float
+/// @param {Number} $exponent - integer or float
+/// @param {Number} $steps [32] - Number of iterations, more equals more precision, less perf
 
-  @if $exponent > 0 {
-    @for $i from 1 through $exponent {
-      $value: $value * $number;
+@function pow($x, $exponent, $steps: 32) {
+    $exp1: round($exponent);
+    $exp2: $exponent - $exp1;
+    $pow1: -math-pow-int($x, $exp1);
+    @if $exp2 == 0 {
+        @return $pow1;
+    } @else {
+        $y: ln($x, $steps) * $exp2;
+        $pow2: -math-exp-taylor-0($y, $steps);
+        @return $pow1 * $pow2;
     }
-  }
-
-  @return $value;
 }