_linear-interpolation.scss

## _linear-interpolation.scss
/// linear-interpolation
/// Calculate the definition of a line between two points
/// @param $map - A SASS map of viewport widths and size value pairs
/// @returns A linear equation as a calc() function
/// @example
///   font-size: linear-interpolation((320px: 18px, 768px: 26px));
/// @author Jake Wilson <jake.e.wilson@gmail.com>
@function linear-interpolation($map) {
  $keys: map-keys($map);
  @if (length($keys) != 2) {
    @error "linear-interpolation() $map must be exactly 2 values";
  }
  // The slope
  $m: (map-get($map, nth($keys, 2)) - map-get($map, nth($keys, 1)))/(nth($keys, 2) - nth($keys,1));

  // The y-intercept
  $b: map-get($map, nth($keys, 1)) - $m * nth($keys, 1);

  // Determine if the sign should be positive or negative
  $sign: "+";
  @if ($b < 0) {
    $sign: "-";
    $b: abs($b);
  }

  @return calc(#{$m*100}vw #{$sign} #{$b});
}