presidential-innovation-fellows/dap-guide

// Not function for Libsass compatibility
// https://github.com/sass/libsass/issues/368
@function is-not($value) {
  @return if($value, false, true);
}

// Checks if a number is even
@function is-even($int) {
  @return $int % 2 == 0;
}

// Checks if an element belongs to a list or not
@function belongs-to($tested-item, $list) {
  @return is-not(not-belongs-to($tested-item, $list));
}

@function not-belongs-to($tested-item, $list) {
  @return is-not(index($list, $tested-item));
}

// Contains display value
@function contains-display-value($query) {
  @return belongs-to(table, $query)
       or belongs-to(block, $query)
       or belongs-to(inline-block, $query)
       or belongs-to(inline, $query);
}

// Parses the first argument of span-columns()
@function container-span($span: $span) {
  @if length($span) == 3 {
    $container-columns: nth($span, 3);
    @return $container-columns;
  } @else if length($span) == 2 {
    $container-columns: nth($span, 2);
    @return $container-columns;
  }

  @return $grid-columns;
}

@function container-shift($shift: $shift) {
  $parent-columns: $grid-columns !default !global;

  @if length($shift) == 3 {
    $container-columns: nth($shift, 3);
    @return $container-columns;
  } @else if length($shift) == 2 {
    $container-columns: nth($shift, 2);
    @return $container-columns;
  }

  @return $parent-columns;
}

// Generates a striped background
@function gradient-stops($grid-columns, $color: $visual-grid-color) {
  $transparent: transparent;

  $column-width: flex-grid(1, $grid-columns);
  $gutter-width: flex-gutter($grid-columns);
  $column-offset: $column-width;

  $values: ($transparent 0, $color 0);

  @for $i from 1 to $grid-columns*2 {
    @if is-even($i) {
      $values: append($values, $transparent $column-offset, comma);
      $values: append($values, $color $column-offset, comma);
      $column-offset: $column-offset + $column-width;
    } @else {
      $values: append($values, $color $column-offset, comma);
      $values: append($values, $transparent $column-offset, comma);
      $column-offset: $column-offset + $gutter-width;
    }
  }

  @return $values;
}

// Layout direction
@function get-direction($layout, $default) {
  $direction: null;

  @if to-upper-case($layout) == "LTR" or to-upper-case($layout) == "RTL" {
    $direction: direction-from-layout($layout);
  } @else {
    $direction: direction-from-layout($default);
  }

  @return $direction;
}

@function direction-from-layout($layout) {
  $direction: null;

  @if to-upper-case($layout) == "LTR" {
    $direction: right;
  } @else {
    $direction: left;
  }

  @return $direction;
}

@function get-opposite-direction($direction) {
  $opposite-direction: left;

  @if $direction == "left" {
    $opposite-direction: right;
  }

  @return $opposite-direction;
}