/**
 * Return the specified or calculated value of the given stack and size
 * depending on the settings.sizes ratios stack
 *
 * @param 		{Map} 			$stack 			The stack from which to interpolate the value
 * @param 		{String} 		$size 			The size wanted (has to exist in the settings.sizes stack)
 * @return 		{Number} 						The $stack.$size value or the interpolated one depending on the settings.sizes ratios
 *
 * @example
 *
 * // settings.sizes
 * \@include s-setup((
 * 		sizes : (
 * 			small : 0.5,
 * 			default : 1, // default is ALWAYS 1
 * 			big : 1.5
 * 		)
 * ));
 *
 * $myStack : (
 * 		default : 3rem,
 * 		big : 6rem
 * )
 *
 * s-interpolate($myStack, big) // 6rem cause specified in the $myStack value
 * s-interpolate($myStack, small) // 1.5rem => calculated like : 3rem * 0.5 = 1.5rem
 *
 * @author 			Olivier Bossel <olivier.bossel@gmail.com> (https://olivierbossel.com)
 */
@function s-interpolate($stack, $size) {
  @if not $stack or type-of($stack) != string {
    @error ('The value "#{$stack}" is not a settings path');
  }

  // get the value
  $value: s("#{$stack}.#{$size}");

  // if we have a value with units, return
  @if $value and not unitless($value) {
    @return $value;
  }

  // get what we need to interpolate the value
  $ratio: s-setting("sizes.#{$size}");
  $default-size: s("#{$stack}.default");

  // if the getted value is a unitless one, mean that it's the ratio to use
  // to interpolate the default stack value
  @if $value and unitless($value) {
    $ratio: $value;
  }

  // protect
  @if not $default-size {
    @error ("You need to specify a #{$stack}.default setting...");
  }
  @if not unit($default-size) {
    @error ("Your #{$stack}.default has to have a unit like rem, px, etc...");
  }
  @if not $ratio {
    @error ("You need to have the sizes.#{$size} specified in order to interpolate the value to this size...");
  }

  // if no value, interpolate the default one with the size ratio
  @if not $value or unitless($value) {
    @return $default-size * $ratio;
  }

  // if not number
  @if type-of($value) != number {
    @error ('You cannot interpolate the value "#{$value}" cause it is not a number...');
  }

  // default return the value
  @return $value;
}