/// Directional-property mixins are shorthands for writing properties like the following
///
/// @ignore You can also use `false` instead of `null`.
///
/// @param {List} $vals
///   List of directional values
///
/// @example scss - Usage
///   .element {
///     @include border-style(dotted null);
///     @include margin(null 0 10px);
///   }
///
/// @example css - CSS Output
///   .element {
///     border-bottom-style: dotted;
///     border-top-style: dotted;
///     margin-bottom: 10px;
///     margin-left: 0;
///     margin-right: 0;
///   }
///
/// @require {function} contains-falsy
///
/// @return {List}

@function collapse-directionals($vals) {
  $output: null;

  $a: nth($vals, 1);
  $b: if(length($vals) < 2, $a, nth($vals, 2));
  $c: if(length($vals) < 3, $a, nth($vals, 3));
  $d: if(length($vals) < 2, $a, nth($vals, if(length($vals) < 4, 2, 4)));

  @if $a == 0 { $a: 0; }
  @if $b == 0 { $b: 0; }
  @if $c == 0 { $c: 0; }
  @if $d == 0 { $d: 0; }

  @if $a == $b and $a == $c and $a == $d { $output: $a;          }
  @else if $a == $c and $b == $d         { $output: $a $b;       }
  @else if $b == $d                      { $output: $a $b $c;    }
  @else                                  { $output: $a $b $c $d; }

  @return $output;
}

/// Output directional properties, for instance `margin`.
///
/// @access private
///
/// @param {String} $pre
///   Prefix to use
/// @param {String} $suf
///   Suffix to use
/// @param {List} $vals
///   List of values
///
/// @require {function} collapse-directionals
/// @require {function} contains-falsy

@mixin directional-property($pre, $suf, $vals) {
  // Property Names
  $top:    $pre + "-top"    + if($suf, "-#{$suf}", "");
  $bottom: $pre + "-bottom" + if($suf, "-#{$suf}", "");
  $left:   $pre + "-left"   + if($suf, "-#{$suf}", "");
  $right:  $pre + "-right"  + if($suf, "-#{$suf}", "");
  $all:    $pre +             if($suf, "-#{$suf}", "");

  $vals: collapse-directionals($vals);

  @if contains-falsy($vals) {
    @if nth($vals, 1) { #{$top}: nth($vals, 1); }

    @if length($vals) == 1 {
      @if nth($vals, 1) { #{$right}: nth($vals, 1); }
    } @else {
      @if nth($vals, 2) { #{$right}: nth($vals, 2); }
    }

    @if length($vals) == 2 {
      @if nth($vals, 1) { #{$bottom}: nth($vals, 1); }
      @if nth($vals, 2) { #{$left}:   nth($vals, 2); }
    } @else if length($vals) == 3 {
      @if nth($vals, 3) { #{$bottom}: nth($vals, 3); }
      @if nth($vals, 2) { #{$left}:   nth($vals, 2); }
    } @else if length($vals) == 4 {
      @if nth($vals, 3) { #{$bottom}: nth($vals, 3); }
      @if nth($vals, 4) { #{$left}:   nth($vals, 4); }
    }
  } @else {
    #{$all}: $vals;
  }
}
