Poly Fluid Sizing using linear equations, viewport units and calc()

_poly-fluid-sizing.scss

/// poly-fluid-sizing
/// Generate linear interpolated size values through multiple break points
/// @param $property - A string CSS property name
/// @param $map - A SASS map of viewport unit and size value pairs
/// @requires function linear-interpolation
/// @requires function map-sort
/// @example
///   @include poly-fluid-sizing('font-size', (576px: 22px, 768px: 24px, 992px: 34px));
/// @author Jake Wilson <jake.e.wilson@gmail.com>
@mixin poly-fluid-sizing($property, $map) {
  // Get the number of provided breakpoints
  $length: length(map-keys($map));
  
  // Error if the number of breakpoints is < 2
  @if ($length < 2) {
    @error "poly-fluid-sizing() $map requires at least values"
  }

  // Sort the map by viewport width (key)
  $map: map-sort($map);
  $keys: map-keys($map);

  // Minimum size
  #{$property}: map-get($map, nth($keys,1));
  
  // Interpolated size through breakpoints
  @for $i from 1 through ($length - 1) {
    @media (min-width:nth($keys,$i)) {
      $value1: map-get($map, nth($keys,$i));
      $value2: map-get($map, nth($keys,($i + 1)));
      // If values are not equal, perform linear interpolation
      @if ($value1 != $value2) {
        #{$property}: linear-interpolation((nth($keys,$i): $value1, nth($keys,($i+1)): $value2));
      } @else {
        #{$property}: $value1;
      }
    }
  }
  
  // Maxmimum size
  @media (min-width:nth($keys,$length)) {
    #{$property}: map-get($map, nth($keys,$length));
  }
}