/**
 * Little helper to create a circle
 * @group control-points
 * @access private
 * @param {Number} $size - Circle size
 * @output Dimensions, margin and border-radius
 */
@mixin circle($size) {
  width: $size;
  height: $size;
  margin: -($size/2);
  border-radius: 50%;
}

/**
 * Helper to position an item in absolute context
 * @param {Number} $top (null) - Top offset
 * @param {Number} $right (null) - Right offset
 * @param {Number} $bottom (null) - Bottom offset
 * @param {Number} $left (null) - Left offset
 * @output `position: absolute` and offsets
 */
@mixin absolute($top: null, $right: null, $bottom: null, $left: null) {
  position: absolute;
  top: $top;
  right: $right;
  bottom: $bottom;
  left: $left;
}

/**
 * Linear interpolation
 * @author Tim Severien
 * @param {Number} $a
 * @param {Number} $b
 * @param {Number} $p
 * @return {Number}
 */
@function lerp($a, $b, $p) {
  @return ($b - $a) * $p + $a;
}


/**
 * Linear interpolation points
 * @author Tim Severien
 * @param {Number} $a
 * @param {Number} $b
 * @param {Number} $p
 * @return {List}
 */
@function lerp-point($a, $b, $p) {
  @return lerp(nth($a, 1), nth($b, 1), $p), lerp(nth($a, 2), nth($b, 2), $p);
}


/**
 * Bezier Reduce
 * @author Tim Severien
 * @param {List} $points
 * @param {Number} $p
 * @return {Number}
 */
@function bezier-reduce($points, $p) {
  @while length($points) > 1 {
    $tmp: ();
    
    @for $i from 1 to length($points) {
      $tmp: append($tmp, lerp-point(nth($points, $i), nth($points, $i + 1), $p));
    }
    
    $points: $tmp;
  }
  
  @return nth($points, 1);
}


/**
 * Bezier shadow
 * @param {List} $points - List of points from Bezier
 * @param {Number} $detail - Number of particles
 * @output box-shadow
 * @author Tim Severien
 */
@mixin bezier-shadow($points, $detail) {
  $shadow: ();
  
  @for $i from 0 to $detail {
    $point: bezier-reduce($points, $i / $detail);
    $shadow: append($shadow, nth($point, 1) nth($point, 2), comma);
  }
  
  box-shadow: $shadow;
}


/**
 * Curve wrapper
 * @access private
 * @param {Map} $conf - Curve configuration
 * @requires {mixin} absolute
 * @output Contexte
 */
@mixin draw-curve-wrapper($conf) {
  @include absolute($top: 0, $right: 0, $bottom: 0, $left: 0);
  color: map-get($conf, 'color');
}


/**
 * Display dots with `::after` pseudo-element
 * @access private
 * @param {Map} $conf - Curve configuration
 * @requires {mixin} absolute
 * @requires {mixin} circle
 * @output Dots
 */
@mixin draw-dots($conf) {
  $args: map-get($conf, 'points');
  $size: map-get($conf, 'size');
  
  &::after {
    content: '';
    @include circle(4px);
    @include absolute($left: 0, $top: 0); 
    @include bezier-shadow((
      0 $size,
      (nth($args, 1) * $size) ((1 - nth($args, 2)) * $size),
      (nth($args, 3) * $size) ((1 - nth($args, 4)) * $size),
      $size 0
    ), map-get($conf, 'detail'));
  }
}


/**
 * Draw control-points as well as lines getting to those
 * @access private
 * @param {Map} $conf - Curve configuration
 * @requires {mixin} absolute
 * @requires {mixin} circle
 * @output control-points
 */
@mixin draw-control-points($conf) {
  $args: map-get($conf, 'points');
  $size: map-get($conf, 'size');
  $color: map-get($conf, 'color');
  
  $x1: nth($args, 1);
  $y1: nth($args, 2);
  $x2: nth($args, 3);
  $y2: nth($args, 4);
  
  &::before {
    content: '';
    @include absolute;
    @include circle(10px);
    box-shadow:
      0 $size 0 0 tomato,
      $size 0 0 0 deepskyblue,
      ($size * $x1) $size - ($size * $y1) 0 0 tomato,
      ($size * $x2) $size - ($size * $y2) 0 0 deepskyblue; 
  } 
  
  > * {
    @include absolute($top: 0, $right: 0, $bottom: 0, $left: 0);

    &::before {
      @include absolute($left: 0, $bottom: 0);
      content: '';
      width:  $x1 * 100%;
      height: $y1 * 100%; 
      background: linear-gradient(
        if($x1 == 1, 90deg, 0 - atan($y1 / $x1) * 1rad), 
        transparent calc(50% - 1px), 
        $color calc(50% - 1px), 
        $color calc(50% + 1px), 
        transparent calc(50% + 1px)
      ); 
    }

    &::after {
      $v: abs($x2 * 100 - 100);
      $w: abs($y2 * 100 - 100);
      
      @include absolute($top: 0, $right: 0);
      width:  if($v == 0, 1px, $v * 1%);
      height: if($w == 0, 1px, $w * 1%);
      content: '';
      background: linear-gradient(
        if($x2 == 1, 90deg, 0 - atan(($y2 - 1) / ($x2 - 1)) * 1rad), 
        transparent calc(50% - 1px), 
        $color calc(50% - 1px), 
        $color calc(50% + 1px), 
        transparent calc(50% + 1px)
      ); 
    }
  }
}


/**
 * Main function, drawing a Bezier function
 * @access private
 * @param {Map} $conf - Curve configuration
 * @output Dots and possibly control-points
 */
@mixin draw-curve($conf) {
  // Print the wrapper
  @include draw-curve-wrapper($conf);
  
  // Print the dots
  @include draw-dots($conf);
  
  // Print the control-points
  @if map-get($conf, 'control-points') != false {
    @include draw-control-points($conf);
  }
}

/**
 * Draw coords system
 * @access private
 * @param {Map} $conf - Curve configuration
 * @param {Bool} $display-name (false) - Enable/disable name display
 */
@mixin draw-system($conf) {
  width: map-get($conf, 'size');
  height: map-get($conf, 'size');
  position: relative;
  color: map-get($conf, 'color');
  border-left: 2px solid;
  border-bottom: 2px solid;
  border-top: 1px dashed;
  border-right: 1px dashed;
  
  &::after,
  &::before {
    position: absolute;
    bottom: -1.75em;
    text-transform: uppercase;
    font-size: .75em;
  }
  
  @if map-get($conf, 'informations') {
    @if map-has-key($conf, 'name') {
      // Display name
      &::before {
        content: "#{map-get($conf, 'name')}";
        left: 0;
      }
    }

    // Display values
    &::after {
      content: "#{map-get($conf, 'points')}";
      right: 0;
    }
  }
  // Print the curve
  > * {
    @include draw-curve($conf); 
  }
}


/**
 * Draw a cubic bezier function.
 * Also initialize a global variable holding the configuration.
 * @access public
 * @param {Number} $x1 - X of first pair
 * @param {Number} $y1 - Y of first pair
 * @param {Number} $x2 - X of second paid
 * @param {Number} $y2 - Y of second pair
 * @param {Map} $options - Map of options
 */
@mixin cubic-bezier($x1, $y1, $x2, $y2, $options: ()) {
  // Default options
  $options: map-merge((
    // Enable/disable control-points
    'control-points': true,
    // Extra informations
    'informations': true,
    // Size of the grid
    'size': 300px,
    // Color scheme
    'color': #999, 
    // Points from the curve
    'points': ($x1, $y1, $x2, $y2),
    // Number of dots on the curve
    'detail': 30
  ), $options);
  
  // Start printing things
  @include draw-system($options);
}



/**
 * DEMO *
       **/

.grid {
  margin: 1em;
  float: left;
}

.grid-1 {
  @include cubic-bezier(.42, 0, 1, 1, ( 
    'name': 'ease-in'
  ));
} 

.grid-2 {
  @include cubic-bezier(.42, 0, .58, 1, (
    'name': 'ease-in-out'
  ));
}

.grid-3 {
  @include cubic-bezier(.13, .88, .5, .42); 
}