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+////
+/// responsive properties.
+///
+/// The mixins and functions in this file allow you to scale any px- or rem-based value depending on
+/// the available viewport width. One popular use case is the dynamic scaling of fonts.
+///
+/// The code in this file is based on an article by Niklas Postulart:
+/// http://niklaspostulart.de/2015/10/sass-responsive-type-mixin
+/// 
+/// The following adjustments were made:
+/// - Support any property passed by the user, not just font-size
+/// - Allow multiple target viewports / values
+/// - Provide a variant of the mixin which integrates include-media for media queries
+///
+/// @group Responsive
+///
+/// @access public
+////
+
+///
+/// If true, named viewports will be supported if a compatible $breakpoints map exists.
+/// This is the case for [include-media](https://include-media.com/), for example.
+///
+/// @type bool
+///
+$iro-responsive-support-named-viewports: true !default;
+
+///
+/// Context ID used for responsive environment-related mixins.
+///
+/// @type string
+///
+$iro-responsive-context-id: 'responsive' !default;
+
+/// 
+/// Scale a property uniformly between a specific set of target viewports / values.
+///
+/// @param {string | list} $props            - Property or list of properties to set
+/// @param {number}        $responsive-map   - A map with keys = viewports and values = target value
+/// @param {bool}          $fluid    [true]  - If enabled, property values will smoothly transition from one viewport to the next
+/// @param {bool}          $vertical [false] - If enabled, property viewport height will be used instead of viewport width
+///
+/// @example scss - Responsive font-size between 2 viewports
+///   .something {
+///     @include iro-responsive-property(font-size, ( 320px: 20px, 720px: 30px ));
+///   }
+///   
+///   // Generates:
+///   
+///   @media (min-width: 320px) and (max-width: 720px) {
+///     .something {
+///       font-size: calc(20px + 10 * ((100vw - 20px) / 400));
+///     }
+///   }
+///   
+///   @media (max-width: 320px) {
+///     .something {
+///       font-size: 20px;
+///     }
+///   }
+///   
+///   @media (min-width: 720px) {
+///     .something {
+///       font-size: 30px;
+///     }
+///   }
+///
+/// @example scss - Responsive font-size between 3 viewports
+///   .something {
+///     @include iro-responsive-property(font-size, ( 320px: 20px, 720px: 30px, 1280px: 40px ));
+///   }
+///   
+///   // Generates:
+///   
+///   @media (min-width: 320px) and (max-width: 720px) {
+///     .something {
+///       font-size: calc(20px + 10 * ((100vw - 20px) / 400));
+///     }
+///   }
+///   
+///   @media (min-width: 720px) and (max-width: 1280px) {
+///     .something {
+///       font-size: calc(30px + 10 * ((100vw - 30px) / 400));
+///     }
+///   }
+///   
+///   @media (max-width: 320px) {
+///     .something {
+///       font-size: 20px;
+///     }
+///   }
+///   
+///   @media (min-width: 720px) {
+///     .something {
+///       font-size: 30px;
+///     }
+///   }
+///
+@mixin iro-responsive-property($props, $responsive-map, $fluid: true, $vertical: false) {
+    @include iro-responsive-env(map-keys($responsive-map), $fluid, $vertical) {
+        @if type-of($props) == list {
+            @each $prop in $props {
+                #{$prop}: iro-responsive-set(map-values($responsive-map));
+            }
+        } @else {
+            #{$props}: iro-responsive-set(map-values($responsive-map));
+        }
+    }
+}
+
+/// 
+/// Create a new responsive environment by specifying a set of viewports.
+/// Inside a responsive environment, use the iro-responsive-set function to make a property scale automatically.
+///
+/// @param {list} $viewports        - Viewports sorted in ascending order
+/// @param {bool} $fluid [true]     - If enabled, property values will smoothly transition from one viewport to the next
+/// @param {bool} $vertical [false] - If enabled, property viewport height will be used instead of viewport width
+///
+/// @content
+///
+/// @see {function} iro-responsive-set
+///
+/// @example scss - Responsive font-size between 2 viewports
+///   .something {
+///     @include iro-responsive-env((320px, 720px)) {
+///       font-size: iro-responsive-set(20px, 30px);
+///     }
+///   }
+///   
+///   // Generates:
+///   
+///   @media (min-width: 320px) and (max-width: 720px) {
+///     .something {
+///       font-size: calc(20px + 10 * ((100vw - 20px) / 400));
+///     }
+///   }
+///   
+///   @media (max-width: 320px) {
+///     .something {
+///       font-size: 20px;
+///     }
+///   }
+///   
+///   @media (min-width: 720px) {
+///     .something {
+///       font-size: 30px;
+///     }
+///   }
+///
+@mixin iro-responsive-env($viewports, $fluid: true, $vertical: false) {
+    @if length($viewports) <= 1 {
+        @error '$viewports must contain at least two viewports.';
+    }
+
+    $new-viewports: ();
+
+    @each $viewport in $viewports {
+        @if $iro-responsive-support-named-viewports and global-variable-exists(breakpoints) {
+            @if map-has-key($breakpoints, $viewport) {
+                $viewport: map-get($breakpoints, $viewport);
+            }
+        }
+
+        @if (type-of($viewport) != number) or unitless($viewport) {
+            @error '$viewports contains invalid viewports.';
+        }
+
+        $new-viewports: append($new-viewports, $viewport);
+    }
+
+    $viewports: iro-quicksort($new-viewports);
+
+    @if $new-viewports != $viewports {
+        @error '$viewports was not sorted in ascending order.';
+    }
+
+    @if $fluid {
+        $first-vp: nth($viewports, 1);
+        $last-vp:  nth($viewports, length($viewports));
+
+        @include iro-context-push($iro-responsive-context-id, 'env', (
+                'viewports': $viewports,
+                'mode':      set,
+                'index':     1,
+                'fluid':     $fluid,
+                'vertical':  $vertical,
+        ));
+
+        @content;
+
+        @include iro-context-pop($iro-responsive-context-id);
+
+        @for $i from 1 to length($viewports) {
+            $prev-vp: nth($viewports, $i);
+            $next-vp: nth($viewports, $i + 1);
+
+            @if not $vertical {
+                @media (min-width: $prev-vp) and (max-width: $next-vp) {
+                    @include iro-context-push($iro-responsive-context-id, 'env', (
+                            'viewports': $viewports,
+                            'mode':      transition,
+                            'index':     $i,
+                            'fluid':     $fluid,
+                            'vertical':  $vertical,
+                    ));
+
+                    @content;
+
+                    @include iro-context-pop($iro-responsive-context-id);
+                }
+            } @else {
+                @media (min-height: $prev-vp) and (max-height: $next-vp) {
+                    @include iro-context-push($iro-responsive-context-id, 'env', (
+                            'viewports': $viewports,
+                            'mode':      transition,
+                            'index':     $i,
+                            'fluid':     $fluid,
+                            'vertical':  $vertical,
+                    ));
+
+                    @content;
+
+                    @include iro-context-pop($iro-responsive-context-id);
+                }
+            }
+        }
+
+        @if not $vertical {
+            @media (min-width: $last-vp) {
+                @include iro-context-push($iro-responsive-context-id, 'env', (
+                        'viewports': $viewports,
+                        'mode':      set,
+                        'index':     length($viewports),
+                        'fluid':     $fluid,
+                        'vertical':  $vertical,
+                ));
+
+                @content;
+
+                @include iro-context-pop($iro-responsive-context-id);
+            }
+        } @else {
+            @media (min-height: $last-vp) {
+                @include iro-context-push($iro-responsive-context-id, 'env', (
+                        'viewports': $viewports,
+                        'mode':      set,
+                        'index':     length($viewports),
+                        'fluid':     $fluid,
+                        'vertical':  $vertical,
+                ));
+
+                @content;
+
+                @include iro-context-pop($iro-responsive-context-id);
+            }
+        }
+    } @else {
+        @include iro-context-push($iro-responsive-context-id, 'env', (
+                'viewports': $viewports,
+                'mode':      set,
+                'index':     1,
+                'fluid':     $fluid,
+                'vertical':  $vertical,
+        ));
+
+        @content;
+
+        @include iro-context-pop($iro-responsive-context-id);
+
+        @for $i from 2 through length($viewports) {
+            $vp: nth($viewports, $i);
+
+            @if not $vertical {
+                @media (min-width: $vp) {
+                    @include iro-context-push($iro-responsive-context-id, 'env', (
+                            'viewports': $viewports,
+                            'mode':      set,
+                            'index':     $i
+                    ));
+
+                    @content;
+
+                    @include iro-context-pop($iro-responsive-context-id);
+                }
+            } @else {
+                @media (min-height: $vp) {
+                    @include iro-context-push($iro-responsive-context-id, 'env', (
+                            'viewports': $viewports,
+                            'mode':      set,
+                            'index':     $i
+                    ));
+
+                    @content;
+
+                    @include iro-context-pop($iro-responsive-context-id);
+                }
+            }
+        }
+    }
+}
+
+///
+/// Make a property scale automatically inside a responsive environment.
+///
+/// @param {list} $values - Value for each viewport in the responsive environment. The first value will be used for the first viewport, the second value for the second viewport, and so on.
+/// 
+/// @return {number|string}
+///
+@function iro-responsive-set($values, $without-calc: false) {
+    $noop: iro-context-assert-stack-must-contain($iro-responsive-context-id, 'env');
+
+    $data:      nth(iro-context-get($iro-responsive-context-id, 'env'), 2);
+    $viewports: map-get($data, 'viewports');
+    $mode:      map-get($data, 'mode');
+    $fluid:     map-get($data, 'fluid');
+    $vertical:  map-get($data, 'vertical');
+
+    @if length($values) != length($viewports) {
+        @error '$values must contain the same number of items as the responsive environment\'s $viewports.';
+    }
+
+    @if $mode == set {
+        @return nth($values, map-get($data, 'index'));
+    } @else {
+        $index:      map-get($data, 'index');
+        $prev-vp:    nth($viewports, $index);
+        $next-vp:    nth($viewports, $index + 1);
+        $prev-value: nth($values, $index);
+        $next-value: nth($values, $index + 1);
+
+        @return iro-responsive-fluid-calc($prev-value, $next-value, $prev-vp, $next-vp, $vertical, $without-calc);
+    }
+}
+
+/// 
+/// Generate the calc() function that uniformly scales a value from $min-value to $max-value depending
+/// on the viewport width.
+///
+/// @param {number} $min-value        - Minimum value
+/// @param {number} $max-value        - Maximum value
+/// @param {number} $min-viewport     - Minimum viewport size
+/// @param {number} $max-viewport     - Maximum viewport size
+/// @param {bool}   $vertical [false] - If enabled, property viewport height will be used instead of viewport width
+///
+/// @access private
+///
+@function iro-responsive-fluid-calc($min-value, $max-value, $min-viewport, $max-viewport, $vertical: false, $without-calc: false) {
+    $value-unit:        unit($min-value);
+    $max-value-unit:    unit($max-value);
+    $viewport-unit:     unit($min-viewport);
+    $max-viewport-unit: unit($max-viewport);
+
+    @if $min-value == 0 {
+        $value-unit: $max-value-unit;
+    }
+    @if $max-value == 0 {
+        $max-value-unit: $value-unit;
+    }
+    @if $min-viewport == 0 {
+        $viewport-unit: $max-viewport-unit;
+    }
+    @if $max-viewport == 0 {
+        $max-viewport-unit: $viewport-unit;
+    }
+
+    @if ($value-unit != $max-value-unit) or ($viewport-unit != $max-viewport-unit) {
+        @error 'Units of $min-value and $max-value, $min-viewport and $max-viewport must match.';
+    }
+
+    @if ($value-unit == rem) and ($viewport-unit == px) {
+        $min-viewport:  iro-px-to-rem($min-viewport);
+        $max-viewport:  iro-px-to-rem($max-viewport);
+        $viewport-unit: rem;
+    } @else if ($value-unit == px) and ($viewport-unit == rem) {
+        $min-value:  iro-px-to-rem($min-value);
+        $max-value:  iro-px-to-rem($max-value);
+        $value-unit: rem;
+    }
+
+    @if $value-unit != $viewport-unit {
+        @error 'This combination of units is not supported.';
+    }
+
+    $value-diff:    iro-strip-unit($max-value - $min-value);
+    $viewport-diff: iro-strip-unit($max-viewport - $min-viewport);
+
+    $calc: '';
+
+    @if $min-value != 0 {
+        $calc: '#{$min-value} + ';
+    }
+
+    @if not $vertical {
+        $calc: unquote('#{$calc}#{$value-diff} * ((100vw - #{$min-viewport}) / #{$viewport-diff})');
+    } @else {
+        $calc: unquote('#{$calc}#{$value-diff} * ((100vh - #{$min-viewport}) / #{$viewport-diff})');
+    }
+
+    @if $without-calc {
+        @return $calc;
+    } @else {
+        @return calc(#{$calc});
+    }
+}
+
+@include iro-context-stack-create($iro-responsive-context-id);