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+////
+/// Smoother background gradients.
+///
+/// The default background gradients produced by any browser have a quite harsh transition between
+/// colors. This is especially apparent if you, for example, use a strong fade-out gradient to make
+/// text in front of a background more readable.
+///
+/// The function in this file generates smoother gradients by using easing functions of the user's
+/// choice.
+/// It's essentially a more flexible alternative to the PostCSS plugin "PostCSS Easing Gradients":
+/// https://github.com/larsenwork/postcss-easing-gradients
+///
+/// @group Background gradients
+///
+/// @access public
+////
+
+///
+/// Number of intermediate color stops generated to achieve easing.
+/// A higher value results in better quality, but also much more generated code.
+///
+/// @type number
+///
+$iro-easing-gradient-steps: 10 !default;
+
+/// 
+/// Generate a new easing background gradient.
+/// This function is intended to be as similar as possible to the newly proposed syntax for
+/// linear-gradient and radial-gradient which includes easing hints.
+///
+/// @param {string}       $type  - Either 'linear' or 'radial', which means the gradient will be either a linear-gradient or a radial-gradient.
+/// @param {string}       $dir   - The direction of the gradient. Depending on $type, this value must be a valid direction for linear-gradient or radial-gradient.
+/// @param {color | list} $stop  - A color stop as used for linear-gradient or radial-gradient.
+/// @param {arglist}      $stops - More color stops as used for linear-gradient or radial-gradient. Between two color stops, you may also define an easing hint such as `ease-in-out`, `cubic-bezier 0.42 0 0.58 1`, `steps 3 jump-end`, and so on.
+///
+/// @return {string} A linear-gradient or radial-gradient with an alternative transitioning behavior.
+///
+/// @throw If $type is invalid
+///
+/// @link https://github.com/w3c/csswg-drafts/issues/1332 The new CSSWG proposal
+///
+/// @example scss - A smoother linear gradient
+///   .background {
+///     background-image: iro-easing-gradient(
+///       linear,
+///       to top,
+///       #000,
+///       in-out-sine,
+///       transparent
+///     );
+///   }
+///   
+///   // Generates:
+///   
+///   .background {
+///     background-image: linear-gradient(
+///       to top,
+///       black                        0%,
+///       rgba(0, 0, 0, 0.975528)     10%,
+///       rgba(0, 0, 0, 0.904508)     20%,
+///       rgba(0, 0, 0, 0.793893)     30%,
+///       rgba(0, 0, 0, 0.654508)     40%,
+///       rgba(0, 0, 0, 0.5)          50%,
+///       rgba(0, 0, 0, 0.345492)     60%,
+///       rgba(0, 0, 0, 0.206107)     70%,
+///       rgba(0, 0, 0, 0.0954915)    80%,
+///       rgba(0, 0, 0, 0.0244717)    90%,
+///       rgba(0, 0, 0, 3.78257e-11) 100%
+///     );
+///   }
+///
+/// @example scss - A smoother radial gradient
+///   .background {
+///     background-image: iro-easing-gradient(
+///       radial,
+///       50em 16em at 0 0,
+///       #000,
+///       in-out-sine,
+///       transparent
+///     );
+///   }
+///   
+///   // Generates:
+///   
+///   .background {
+///     background-image: radial-gradient(
+///       50em 16em at 0 0,
+///       black                        0%,
+///       rgba(0, 0, 0, 0.975528)     10%,
+///       rgba(0, 0, 0, 0.904508)     20%,
+///       rgba(0, 0, 0, 0.793893)     30%,
+///       rgba(0, 0, 0, 0.654508)     40%,
+///       rgba(0, 0, 0, 0.5)          50%,
+///       rgba(0, 0, 0, 0.345492)     60%,
+///       rgba(0, 0, 0, 0.206107)     70%,
+///       rgba(0, 0, 0, 0.0954915)    80%,
+///       rgba(0, 0, 0, 0.0244717)    90%,
+///       rgba(0, 0, 0, 3.78257e-11) 100%
+///     );
+///   }
+///
+/// @example scss - A smoother linear gradient with complex color positions
+///   .background {
+///     background-image: iro-easing-gradient(
+///       linear,
+///       to top,
+///       #000 20%,
+///       in-out-sine,
+///       transparent calc(20% + 25em)
+///     );
+///   }
+///   
+///   // Generates:
+///   
+///   .background {
+///     background-image: linear-gradient(
+///       to top,
+///       black                    20%,
+///       rgba(0, 0, 0, 0.975528)  calc(20% + (20% + 25em - 20%) * 0.1),
+///       rgba(0, 0, 0, 0.904508)  calc(20% + (20% + 25em - 20%) * 0.2),
+///       rgba(0, 0, 0, 0.793893)  calc(20% + (20% + 25em - 20%) * 0.3),
+///       rgba(0, 0, 0, 0.654508)  calc(20% + (20% + 25em - 20%) * 0.4),
+///       rgba(0, 0, 0, 0.5)       calc(20% + (20% + 25em - 20%) * 0.5),
+///       rgba(0, 0, 0, 0.345492)  calc(20% + (20% + 25em - 20%) * 0.6),
+///       rgba(0, 0, 0, 0.206107)  calc(20% + (20% + 25em - 20%) * 0.7),
+///       rgba(0, 0, 0, 0.0954915) calc(20% + (20% + 25em - 20%) * 0.8),
+///       rgba(0, 0, 0, 0.0244717) calc(20% + (20% + 25em - 20%) * 0.9),
+///       transparent              calc(20% + 25em))
+///     );
+///   }
+///
+@function iro-easing-gradient($type, $dir, $stop, $stops...) {
+    $pos-template: null;
+    $stops:        iro-list-prepend($stops, $stop);
+
+    $last-positioned-stop: 1;
+    $generated-stops:      ();
+
+    //
+    // Generate gradient
+    //
+
+    @for $i from 1 through length($stops) {
+        $stop: nth($stops, $i);
+
+        @if $i == 1 {
+            @if not iro-easing-gradient-is-color-stop($stop) {
+                @error 'The first color stop argument must be a color stop.';
+            }
+
+            @if type-of($stop) == color {
+                //
+                // The first color stop is unpositioned. The default position for the first
+                // color stop is 0, which is explicitly added for easier calculations.
+                //
+
+                $stop:  $stop 0;
+                $stops: set-nth($stops, $i, $stop);
+            }
+
+            $generated-stops: append($generated-stops, iro-str-implode($stop, ' '));
+        } @else if iro-easing-gradient-is-positioned-color-stop($stop) or ($i == length($stops)) {
+            @if not iro-easing-gradient-is-color-stop($stop) {
+                @error 'The last color stop argument must be a color stop.';
+            }
+
+            //
+            // Either the current stops list item is a positioned color stop, or the end of
+            // the stops list has been reached.
+            //
+
+            @if (type-of($stop) == color) and ($i == length($stops)) {
+                //
+                // The current stop is an unpositioned color stop, which means this is the end
+                // of the stops list. The default position for the last color stop is 100%, which
+                // is explicitly added for easier calculations.
+                //
+
+                $stop:  $stop 100%;
+                $stops: set-nth($stops, $i, $stop);
+            }
+
+            //
+            // Now the current color stop is guaranteed to be a positioned color stop.
+            //
+
+            @if $i > $last-positioned-stop + 1 {
+                //
+                // There is at least one stops list item (unpositioned color stop or easing function)
+                // between the last positioned color stop and the current stops list item. Interpolate
+                // the positions of all stops list items that are color stops.
+                //
+
+                $interpolated-stops: iro-easing-gradient-interpolate-stop-positions(
+                    nth($stops, $last-positioned-stop),
+                    iro-list-slice($stops, $last-positioned-stop + 1, $i - 1),
+                    $stop
+                );
+
+                $new-stops: join(
+                    iro-list-slice($stops, 1, $last-positioned-stop),
+                    $interpolated-stops
+                );
+                $new-stops: join(
+                    $new-stops,
+                    iro-list-slice($stops, $i)
+                );
+                $stops: $new-stops;
+            }
+
+            //
+            // Now all color stops between this one and the last positioned one have
+            // interpolated positions.
+            // Next task is to perform an easing transition between all color stops that
+            // have an easing function specified. The rest can be left alone since the
+            // browser will automatically apply a linear transition between them.
+            //
+
+            $j: $last-positioned-stop + 1;
+            @while $j <= $i {
+                $easing:    null;
+                $prev-stop: nth($stops, $j - 1);
+                $next-stop: nth($stops, $j);
+
+                @if not iro-easing-gradient-is-color-stop($next-stop) {
+                    $j: $j + 1;
+
+                    $easing:    $next-stop;
+                    $next-stop: nth($stops, $j);
+
+                    @if not iro-easing-gradient-is-color-stop($next-stop) {
+                        @error 'There can be at most one interpolation hint between to color stops.';
+                    }
+                }
+
+                @if $easing != null {
+                    @if type-of($easing) == number {
+                        @error 'Midpoint shifts are not supported.';
+                    }
+
+                    $easing-func: null;
+                    $easing-args: ();
+
+                    @if type-of($easing) == list {
+                        $easing-args: iro-list-slice($easing, 2);
+                        $easing:      nth($easing, 1);
+                    }
+
+                    $generated-stops: join(
+                        $generated-stops,
+                        iro-easing-gradient-ease-stops($prev-stop, $next-stop, $easing, $easing-args)
+                    );
+                } @else {
+                    $generated-stops: append($generated-stops, iro-str-implode($next-stop, ' '));
+                }
+
+                $j: $j + 1;
+            }
+
+            $last-positioned-stop: $i;
+        }
+    }
+
+    @if $type == 'linear' {
+        @return linear-gradient($dir, unquote(iro-str-implode($generated-stops, ', ')));
+    } @else if $type == 'radial' {
+        @return radial-gradient($dir, unquote(iro-str-implode($generated-stops, ', ')));
+    } @else {
+        @error 'Invalid gradient type: #{inspect($type)}.';
+    }
+}
+
+///
+/// Alias for iro-easing-gradient('linear',...)
+///
+/// @see {function} iro-easing-gradient
+///
+@function iro-easing-linear-gradient($dir, $stop, $stops...) {
+    @return iro-easing-gradient('linear', $dir, $stop, $stops...);
+}
+
+///
+/// Alias for iro-easing-gradient('radial',...)
+///
+/// @see {function} iro-easing-gradient
+///
+@function iro-easing-radial-gradient($dir, $stop, $stops...) {
+    @return iro-easing-gradient('radial', $dir, $stop, $stops...);
+}
+
+///
+/// Generate a smooth transition from one color stop to another using the provided easing function.
+///
+/// @access private
+///
+@function iro-easing-gradient-ease-stops($prev-stop, $next-stop, $easing, $easing-args: ()) {
+    @if $easing == 'steps' {
+        $steps: null;
+        $jump:  null;
+
+        @if length($easing-args) > 1 {
+            $steps: nth($easing-args, 1);
+            $jump:  nth($easing-args, 2);
+        } @else {
+            $steps: nth($easing-args, 1);
+            $jump:  jump-end;
+        }
+
+        @return iro-easing-gradient-steps-stops($prev-stop, $next-stop, $steps, $jump);
+    } @else {
+        $easing-func: null;
+        @if function-exists('iro-' + $easing) {
+            $easing-func: get-function('iro-' + $easing);
+        } @else {
+            $easing-func: get-function($easing);
+        }
+
+        @return iro-easing-gradient-bezier-stops($prev-stop, $next-stop, $easing-func, $easing-args);
+    }
+}
+
+///
+/// Generate a smooth transition from one color stop to another using the provided cubic-bezier function.
+///
+/// @access private
+///
+@function iro-easing-gradient-bezier-stops($prev-stop, $next-stop, $easing-func, $easing-args: ()) {
+    $prev-stop-color: nth($prev-stop, 1);
+    $prev-stop-pos:   nth($prev-stop, 2);
+    $next-stop-color: nth($next-stop, 1);
+    $next-stop-pos:   nth($next-stop, 2);
+
+    $stops: ();
+
+    @if ((type-of($prev-stop-pos) == number) and (type-of($next-stop-pos) == number) and (unit($prev-stop-pos) == unit($next-stop-pos))) or ($prev-stop-pos == 0) or ($next-stop-pos == 0) {
+        //
+        // The transition color stop positions can be statically calculated.
+        //
+
+        $distance: $next-stop-pos - $prev-stop-pos;
+
+        @for $i from 1 through $iro-easing-gradient-steps {
+            $perc:  $i / $iro-easing-gradient-steps;
+
+            $color: null;
+            $pos:   $prev-stop-pos + $perc * $distance;
+            @if $perc == 1 {
+                $color: $next-stop-color;
+            } @else {
+                $color: mix($next-stop-color, $prev-stop-color, call($easing-func, append($easing-args, $perc)...) * 100%);
+            }
+
+            $stops: append($stops, $color + ' ' + $pos);
+        }
+    } @else {
+        //
+        // The transition color stop positions have to be dynamically calculated with the calc() function.
+        //
+
+        @if type-of($prev-stop-pos) != number {
+            // must be calc()
+            @if (type-of($prev-stop-pos) != string) or (str-index($prev-stop-pos, 'calc(') != 1) {
+                @error 'Invalid color stop position: #{inspect($prev-stop-pos)}';
+            }
+
+            $prev-stop-pos: str-slice($prev-stop-pos, 6, str-length($prev-stop-pos) - 1);
+        }
+
+        @if type-of($next-stop-pos) != number {
+            // must be calc()
+            @if (type-of($next-stop-pos) != string) or (str-index($next-stop-pos, 'calc(') != 1) {
+                @error 'Invalid color stop position: #{inspect($next-stop-pos)}';
+            }
+
+            $next-stop-pos: str-slice($next-stop-pos, 6, str-length($next-stop-pos) - 1);
+        }
+
+        @for $i from 1 through $iro-easing-gradient-steps {
+            $perc:  $i / $iro-easing-gradient-steps;
+
+            $color: null;
+            $pos:   null;
+            @if $perc == 1 {
+                $color: $next-stop-color;
+                $pos:   calc(#{$next-stop-pos});
+            } @else {
+                $color: mix($next-stop-color, $prev-stop-color, call($easing-func, append($easing-args, $perc)...) * 100%);
+                $pos:   calc(#{$prev-stop-pos} + (#{$next-stop-pos} - #{$prev-stop-pos}) * #{$perc});
+            }
+
+            $stops: append($stops, $color + ' ' + $pos);
+        }
+    }
+
+    @return $stops;
+}
+
+///
+/// Generate a step transition from one color stop to another.
+///
+/// @access private
+///
+@function iro-easing-gradient-steps-stops($prev-stop, $next-stop, $steps, $jump: jump-end) {
+    $prev-stop-color: nth($prev-stop, 1);
+    $prev-stop-pos:   nth($prev-stop, 2);
+    $next-stop-color: nth($next-stop, 1);
+    $next-stop-pos:   nth($next-stop, 2);
+
+    $stops: ();
+
+    @if ((type-of($prev-stop-pos) == number) and (type-of($next-stop-pos) == number) and (unit($prev-stop-pos) == unit($next-stop-pos))) or ($prev-stop-pos == 0) or ($next-stop-pos == 0) {
+        //
+        // The transition color stop positions can be statically calculated.
+        //
+
+        $distance: $next-stop-pos - $prev-stop-pos;
+
+        @for $i from 1 through $steps {
+            $x1: ($i - 1) / $steps;
+            $x2: $i / $steps;
+            $y:  null;
+
+            @if $jump == jump-start {
+                $y: $i / $steps;
+            } @else if $jump == jump-end {
+                $y: ($i - 1) / $steps;
+            } @else if $jump == jump-both {
+                $y: $i / ($steps + 1);
+            } @else if $jump == jump-none {
+                $y: ($i - 1) / ($steps - 1);
+            } @else {
+                @error 'Invalid $jump: #{inspect($jump)}';
+            }
+
+            $color: null;
+            $pos1:  if($x1 == 0, $prev-stop-pos, $prev-stop-pos + $x1 * $distance);
+            $pos2:  if($x2 == 1, $next-stop-pos, $prev-stop-pos + $x2 * $distance);
+
+            @if $y == 0 {
+                $color: $prev-stop-color;
+            } @else if $y == 1 {
+                $color: $next-stop-color;
+            } @else {
+                $color: mix($next-stop-color, $prev-stop-color, $y * 100%);
+            }
+
+            $stops: append($stops, $color + ' ' + $pos1);
+            $stops: append($stops, $color + ' ' + $pos2);
+        }
+    } @else {
+        //
+        // The transition color stop positions have to be dynamically calculated with the calc() function.
+        //
+
+        @if type-of($prev-stop-pos) != number {
+            // must be calc()
+            @if (type-of($prev-stop-pos) != string) or (str-index($prev-stop-pos, 'calc(') != 1) {
+                @error 'Invalid color stop position: #{inspect($prev-stop-pos)}';
+            }
+
+            $prev-stop-pos: str-slice($prev-stop-pos, 6, str-length($prev-stop-pos) - 1);
+        }
+
+        @if type-of($next-stop-pos) != number {
+            // must be calc()
+            @if (type-of($next-stop-pos) != string) or (str-index($next-stop-pos, 'calc(') != 1) {
+                @error 'Invalid color stop position: #{inspect($next-stop-pos)}';
+            }
+
+            $next-stop-pos: str-slice($next-stop-pos, 6, str-length($next-stop-pos) - 1);
+        }
+
+        @for $i from 1 through $steps {
+            $x1: ($i - 1) / $steps;
+            $x2: $i / $steps;
+            $y:  null;
+
+            @if $jump == jump-start {
+                $y: $i / $steps;
+            } @else if $jump == jump-end {
+                $y: ($i - 1) / $steps;
+            } @else if $jump == jump-both {
+                $y: $i / ($steps + 1);
+            } @else if $jump == jump-none {
+                $y: ($i - 1) / ($steps - 1);
+            } @else {
+                @error 'Invalid $jump: #{inspect($jump)}';
+            }
+
+            $color: null;
+            $pos1:  if($x1 == 0, $prev-stop-pos, calc(#{$prev-stop-pos} + (#{$next-stop-pos} - #{$prev-stop-pos}) * #{$x1}));
+            $pos2:  if($x2 == 1, $next-stop-pos, calc(#{$prev-stop-pos} + (#{$next-stop-pos} - #{$prev-stop-pos}) * #{$x2}));
+
+            @if $y == 0 {
+                $color: $prev-stop-color;
+            } @else if $y == 1 {
+                $color: $next-stop-color;
+            } @else {
+                $color: mix($next-stop-color, $prev-stop-color, $y * 100%);
+            }
+
+            $stops: append($stops, $color + ' ' + $pos1);
+            $stops: append($stops, $color + ' ' + $pos2);
+        }
+    }
+
+    @return $stops;
+}
+
+///
+/// Interpolate the positions of multiple color stops between two color stops whose positions are set.
+///
+/// @access private
+///
+@function iro-easing-gradient-interpolate-stop-positions($prev-stop, $stops, $next-stop) {
+    $prev-stop-pos: nth($prev-stop, 2);
+    $next-stop-pos: nth($next-stop, 2);
+
+    $stops-num: 0;
+    @for $i from 1 through length($stops) {
+        $stop: nth($stops, $i);
+        @if iro-easing-gradient-is-color-stop($stop) {
+            $stops-num: $stops-num + 1;
+        }
+    }
+
+    $i: 1;
+    $cur-stop-num: 1;
+
+    @if ((type-of($prev-stop-pos) == number) and (type-of($next-stop-pos) == number) and (unit($prev-stop-pos) == unit($next-stop-pos))) or ($prev-stop-pos == 0) or ($next-stop-pos == 0) {
+        //
+        // The color stop positions can be statically calculated.
+        //
+
+        $distance: $next-stop-pos - $prev-stop-pos;
+
+        @for $i from 1 through length($stops) {
+            $stop: nth($stops, $i);
+            @if iro-easing-gradient-is-color-stop($stop) {
+                $pos:   $prev-stop-pos + $distance / ($stops-num + 1) * $cur-stop-num;
+                $stops: set-nth($stops, $i, $stop $pos);
+
+                $cur-stop-num: $cur-stop-num + 1;
+            }
+        }
+    } @else {
+        //
+        // The color stop positions have to be dynamically calculated with the calc() function.
+        //
+
+        @if type-of($prev-stop-pos) != number {
+            // must be calc()
+            @if (type-of($prev-stop-pos) != string) or (str-index($prev-stop-pos, 'calc(') != 1) {
+                @error 'Invalid color stop position: #{inspect($prev-stop-pos)}';
+            }
+
+            $prev-stop-pos: str-slice($prev-stop-pos, 6, str-length($prev-stop-pos) - 1);
+        }
+
+        @if type-of($next-stop-pos) != number {
+            // must be calc()
+            @if (type-of($next-stop-pos) != string) or (str-index($next-stop-pos, 'calc(') != 1) {
+                @error 'Invalid color stop position: #{inspect($next-stop-pos)}';
+            }
+
+            $next-stop-pos: str-slice($next-stop-pos, 6, str-length($next-stop-pos) - 1);
+        }
+
+        @for $i from 1 through length($stops) {
+            $stop: nth($stops, $i);
+            @if iro-easing-gradient-is-color-stop($stop) {
+                $perc:  $cur-stop-num / ($stops-num + 1);
+                $pos:   calc(#{$prev-stop-pos} + (#{$next-stop-pos} - #{$prev-stop-pos}) * #{$perc});
+                $stops: set-nth($stops, $i, $stop $pos);
+
+                $cur-stop-num: $cur-stop-num + 1;
+            }
+        }
+    }
+
+    @return $stops;
+}
+
+///
+/// Check if the input is a valid color stop.
+///
+/// @access private
+///
+@function iro-easing-gradient-is-color-stop($input) {
+    @return (type-of($input) == color) or iro-easing-gradient-is-positioned-color-stop($input);
+}
+
+///
+/// Check if the input is a valid positioned color stop.
+///
+/// @access private
+///
+@function iro-easing-gradient-is-positioned-color-stop($input) {
+    @return (type-of($input) == list) and (type-of(nth($input, 1)) == color);
+}