Set up sass functions folder, add new functions for color contrast (#628

)

What this PR does:

1. Separates Sass functions into a new `/global_styling/functions/` folder so that they don't dirty up the rest of the files. These get imported along with `global_styling` genetically like our other mixins.
2. Adds new math and color functions:
    * A bunch of math functions for the single purpose of calculating luminosity is Sass
    * Using luminosity, we now have functions to `checkContrast`, `chooseLightOrDarkText` and `makeHighContrastColor`.

CHANGELOG.md

@@ -1,6 +1,7 @@
 # [`master`](https://github.com/elastic/eui/tree/master)
 
-- Modified drop shadow intensities and color. ([#607](https://github.com/elastic/eui/pull/607))
+- Modifying drop shadow intensities and color. ([607](https://github.com/elastic/eui/pull/607))
+- Add Sass color functions. Make `$euiColorWarning` color usage more accessible while still being "yellow". ([628](https://github.com/elastic/eui/pull/628))
 - Removed extraneous `global_styling/mixins/_forms.scss` file and importing the correct files in the `filter_group.scss` and `combo_box.scss` files. ([#609](https://github.com/elastic/eui/pull/609))
 
 **Bug fixes**

src/components/button/_button.scss

@@ -78,21 +78,24 @@ $buttonTypes: (
 // Create button modifiders based upon the map.
 @each $name, $color in $buttonTypes {
   .euiButton--#{$name} {
-    color: $color;
+
+    @if ($name == 'ghost') {
+      // Ghost is unique and ALWAYS sits against a dark background.
+      color: $color;
+    } @else {
+      // Other colors need to check their contrast against the page background color.
+      color: makeHighContrastColor($color, $euiColorEmptyShade);
+    }
+
     border-color: $color;
 
     &.euiButton--fill {
       background-color: $color;
       border-color: $color;
 
-      $textColor: #FFF;
-      @if ($name == 'ghost') {
-        $textColor: #000;
-      } @else if (lightness($euiTextColor) > 50) {
-        $textColor: $euiTextColor;
-      }
+      $fillTextColor: chooseLightOrDarkText($color, #FFF, #000);
 
-      color: $textColor;
+      color: $fillTextColor;
 
       &:enabled {
         &:hover, &:focus {
@@ -102,7 +105,7 @@ $buttonTypes: (
       }
 
       &:disabled .euiButton__spinner {
-        border-color: euiLoadingSpinnerBorderColors(transparentize($textColor, .3));
+        border-color: euiLoadingSpinnerBorderColors(transparentize($fillTextColor, .3));
       }
     }
 

src/components/call_out/_call_out.scss

@@ -11,17 +11,18 @@
 $callOutTypes: (
   primary: $euiColorPrimary,
   success: $euiColorSecondary,
-  warning: adjust-hue($euiColorWarning, 20%),
+  warning: $euiColorWarning,
   danger: $euiColorDanger,
 );
 
 // Create button modifiders based upon the map.
 @each $name, $color in $callOutTypes {
   .euiCallOut--#{$name} {
-    border-color: $color;
-    background-color: tintOrShade($color, 90%, 70%);
+    $backgroundColor: tintOrShade($color, 90%, 70%);
+    $textColor: makeHighContrastColor($color, $backgroundColor);
 
-    $textColor: shadeOrTint($color, 30%, 70%);
+    border-color: $color;
+    background-color: $backgroundColor;
 
     .euiCallOutHeader__icon {
       fill: $textColor;

src/components/text/_text_color.scss

@@ -12,11 +12,13 @@ $textColors: (
 // Create color modifiers based on the map
 @each $name, $color in $textColors {
   .euiTextColor.euiTextColor--#{$name} {
-    color: $color !important;
+
+    // The below function makes sure the color is accessible on our default background.
+    color: makeHighContrastColor($color, $euiColorEmptyShade) !important;
 
     // We need a blanket approach for coloring. It should overule everything.
     * {
-      color: $color !important;
+      color: makeHighContrastColor($color, $euiColorEmptyShade) !important;
     }
   }
 }

src/global_styling/functions/_colors.scss

@@ -0,0 +1,88 @@
+// Mixes a provided color with white.
+@function tint($color, $percent) {
+  @return mix(#fff, $color, $percent);
+}
+
+// Mixes a provided color with black.
+@function shade($color, $percent) {
+  @return mix(#000, $color, $percent);
+}
+
+// For theming. Checks the text color and tells us whether it's light or dark.
+// Based on that we either tint (add white) or shade (add black).
+@function tintOrShade($color, $tint, $shade) {
+  @if (lightness($euiTextColor) > 50) {
+    @return shade($color, $shade);
+  } @else {
+    @return tint($color, $tint);
+  }
+}
+
+// The reverse of the above
+@function shadeOrTint($color, $shade, $tint) {
+  @if (lightness($euiTextColor) < 50) {
+    @return shade($color, $shade);
+  } @else {
+    @return tint($color, $tint);
+  }
+}
+
+// Calculates luminance, which is better than brightness for checking colors
+// pow, nth functions come from the _math.scss functions
+@function luminance($color) {
+  // Formula: http://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
+  $rgba: red($color), green($color), blue($color);
+  $rgba2: ();
+
+  @for $i from 1 through 3 {
+    $rgb: nth($rgba, $i);
+    $rgb: $rgb / 255;
+
+    $rgb: if($rgb < .03928, $rgb / 12.92, pow(($rgb + .055) / 1.055, 2.4));
+
+    $rgba2: append($rgba2, $rgb);
+  }
+
+  @return .2126 * nth($rgba2, 1) + .7152 * nth($rgba2, 2) + 0.0722 * nth($rgba2, 3);
+}
+
+// Calculate contrast
+@function contrastRatio($background, $foreground) {
+  $backgroundLum: luminance($background) + .05;
+  $foregroundLum: luminance($foreground) + .05;
+
+  @return max($backgroundLum, $foregroundLum) / min($backgroundLum, $foregroundLum);
+}
+
+// Given $color, decide whether $lightText or $darkText should be used as the text color
+// ex: chooseLightOrDarkText(#EEE, #FFF, #000) would return #000 because it has
+//     a higher contrast than #FFF against a #EEE background.
+@function chooseLightOrDarkText($background, $lightText, $darkText) {
+  $lightContrast: contrastRatio($background, $lightText);
+  $darkContrast: contrastRatio($background, $darkText);
+
+  @if ($lightContrast > $darkContrast) {
+    @return $lightText;
+  }
+  @else {
+    @return $darkText;
+  }
+}
+
+// Given a $foreground and a $background, make the $foreground AA accessibility by slightly
+// adjusting it till the contrast is high enough
+
+// ex: makeContrastColor($lightPink, #FFF) would continually shade the pink until
+//     it had higher than 4.5 contrast on a white background.
+@function makeHighContrastColor($foreground, $background) {
+  $contrast: contrastRatio($foreground, $background);
+
+  $highContrastTextColor: $foreground;
+
+  @while ($contrast < 4.5) {
+    $highContrastTextColor: shadeOrTint($highContrastTextColor, 5%, 5%);
+    $contrast: contrastRatio($highContrastTextColor, $euiColorEmptyShade);
+  }
+
+  @return $highContrastTextColor;
+}

src/global_styling/functions/_index.scss

@@ -0,0 +1,5 @@
+// Math needs to be first in the load order
+@import 'math';
+
+// Using math, we have functions to manipulate contrast / luminosity for accessibility
+@import 'colors';

src/global_styling/functions/_math.scss

@@ -0,0 +1,44 @@
+// Greatest common devisor
+// From: http://rosettacode.org/wiki/Greatest_common_divisor#JavaScript
+@function gcd($a, $b) {
+  @if ($b != 0) {
+    @return gcd($b, $a % $b);
+  } @else {
+    @return abs($a);
+  }
+}
+
+// Handles decimal exponents by trying to convert them into a fraction and then
+// use a nthRoot-algorithm for parts of the calculation
+@function pow($base, $exponent, $prec: 12) {
+  @if (floor($exponent) != $exponent) {
+    $prec2 : pow(10, $prec);
+    $exponent: round($exponent * $prec2);
+    $denominator: gcd($exponent, $prec2);
+    @return nthRoot(pow($base, $exponent / $denominator), $prec2 / $denominator, $prec);
+  }
+
+  $value: $base;
+  @if $exponent > 1 {
+    @for $i from 2 through $exponent {
+      $value: $value * $base;
+    }
+  } @else if $exponent < 1 {
+    @for $i from 0 through -$exponent {
+      $value: $value / $base;
+    }
+  }
+
+  @return $value;
+}
+
+// From: http://rosettacode.org/wiki/Nth_root#JavaScript
+@function nthRoot($num, $n: 2, $prec: 12) {
+  $x: 1;
+
+  @for $i from 0 through $prec {
+    $x: 1 / $n * (($n - 1) * $x + ($num / pow($x, $n - 1)));
+  }
+
+  @return $x;
+}

src/global_styling/index.scss

@@ -1,4 +1,13 @@
 // Core
+
+// Functions need to be first, since we use them in our variables and mixin definitions
+@import 'functions/index';
+
+// Variables come next, and are used in some mixins
 @import 'variables/index';
+
+// Mixins provide generic code expansion through helpers
 @import 'mixins/index';
+
+// The reset file makes use of variables and mixins
 @import 'reset/index';

src/global_styling/variables/_colors.scss

@@ -1,31 +1,3 @@
-// Mixes a provided color with white.
-@function tint($color, $percent) {
-  @return mix(#fff, $color, $percent);
-}
-
-// Mixes a provided color with black.
-@function shade($color, $percent) {
-  @return mix(#000, $color, $percent);
-}
-
-// For theming. Checks the text color and tells us whether it's light or dark.
-// Based on that we either tint or shade.
-@function tintOrShade($color, $tint, $shade) {
-  @if (lightness($euiTextColor) > 50) {
-    @return shade($color, $shade);
-  } @else {
-    @return tint($color, $tint);
-  }
-}
-
-@function shadeOrTint($color, $shade, $tint) {
-  @if (lightness($euiTextColor) < 50) {
-    @return shade($color, $shade);
-  } @else {
-    @return tint($color, $tint);
-  }
-}
-
 // Core
 
 $euiColorPrimary: #0079a5 !default;
@@ -37,7 +9,7 @@ $euiColorGhost: #FFF !default;
 // Status
 $euiColorSuccess: $euiColorSecondary !default;
 $euiColorDanger: #A30000 !default;
-$euiColorWarning: #CF3800 !default;
+$euiColorWarning: #E5830E !default;
 
 // Grays
 $euiColorEmptyShade: #FFF !default;

src/themes/eui/eui_colors_dark.scss

@@ -9,7 +9,7 @@ $euiColorDarkestShade: #F5F5F5;
 $euiColorFullShade: #FFF;
 $euiColorSlightHue: #494E51;
 $euiColorPrimary: #4da1c0;
-$euiColorWarning: #e45c29;
+$euiColorWarning: #c06c4c;
 $euiColorDanger: #bf4d4d;
 $euiTextColor: #DDD;