// These functions and mixins taken from:
//
// "Building a linear-gradient Mixin in Sass" by Hugo Giraudel
//    http://www.sitepoint.com/building-linear-gradient-mixin-sass/
//    http://sassmeister.com/gist/b58f6e2cc3160007c880
//

/// Convert angle
/// @author Chris Eppstein
/// @param {Number} $value - Value to convert
/// @param {String} $unit - Unit to convert to
/// @return {Number} Converted angle
@function convert-angle($value, $unit) {
  $convertable-units: deg grad turn rad;
  $conversion-factors: 1 (10grad/9deg) (1turn/360deg) (3.1415926rad/180deg);
  @if index($convertable-units, unit($value)) and index($convertable-units, $unit) {
    @return $value
             / nth($conversion-factors, index($convertable-units, unit($value)))
             * nth($conversion-factors, index($convertable-units, $unit));
  }

  @warn "Cannot convert `#{unit($value)}` to `#{$unit}`.";
}

/// Test if `$value` is an angle
/// @param {*} $value - Value to test
/// @return {Bool}
@function is-direction($value) {
  $is-direction: index((
    'to top',
    'to top right',
    'to right top',
    'to right',
    'to bottom right',
    'to right bottom',
    'to bottom',
    'to bottom left',
    'to left bottom',
    'to left',
    'to left top',
    'to top left'
  ), $value);
  $is-angle: type-of($value) == 'number' and index('deg' 'grad' 'turn' 'rad', unit($value));

  @return $is-direction or $is-angle;
}

/// Convert a direction to legacy syntax
/// @param {Keyword | Angle} $value - Value to convert
/// @require {function} is-direction
/// @require {function} convert-angle
@function legacy-direction($value) {
  @if is-direction($value) == false {
    @warn "Cannot convert `#{$value}` to legacy syntax because it doesn't seem to be an angle or a direction";
  }

  $conversion-map: (
    'to top'          : 'bottom',
    'to top right'    : 'bottom left',
    'to right top'    : 'left bottom',
    'to right'        : 'left',
    'to bottom right' : 'top left',
    'to right bottom' : 'left top',
    'to bottom'       : 'top',
    'to bottom left'  : 'top right',
    'to left bottom'  : 'right top',
    'to left'         : 'right',
    'to left top'     : 'right bottom',
    'to top left'     : 'bottom right'
  );

  @if map-has-key($conversion-map, $value) {
    @return map-get($conversion-map, $value);
  }

  @return 90deg - convert-angle($value, 'deg');
}

/// Mixin printing a linear-gradient
/// as well as a plain color fallback
/// and the `-webkit-` prefixed declaration
/// @access public
/// @param {String | List | Angle} $direction - Linear gradient direction
/// @param {Arglist} $color-stops - List of color-stops composing the gradient
@mixin linear-gradient($direction, $color-stops...) {
  @if is-direction($direction) == false {
    $color-stops: ($direction, $color-stops);
    $direction: 180deg;
  }

  background: nth(nth($color-stops, 1), 1);
  background: -webkit-linear-gradient(legacy-direction($direction), $color-stops);
  background: linear-gradient($direction, $color-stops);
}
