public function set_spacing_sizes() {
    $spacing_scale = _wp_array_get( $this->theme_json, array( 'settings', 'spacing', 'spacingScale' ), array() );

    if ( ! isset( $spacing_scale['steps'] )
      || ! is_numeric( $spacing_scale['steps'] )
      || ! isset( $spacing_scale['mediumStep'] )
      || ! isset( $spacing_scale['unit'] )
      || ! isset( $spacing_scale['operator'] )
      || ! isset( $spacing_scale['increment'] )
      || ! isset( $spacing_scale['steps'] )
      || ! is_numeric( $spacing_scale['increment'] )
      || ! is_numeric( $spacing_scale['mediumStep'] )
      || ( '+' !== $spacing_scale['operator'] && '*' !== $spacing_scale['operator'] ) ) {
      if ( ! empty( $spacing_scale ) ) {
        trigger_error( __( 'Some of the theme.json settings.spacing.spacingScale values are invalid' ), E_USER_NOTICE );
      }
      return null;
    }

    // If theme authors want to prevent the generation of the core spacing scale they can set their theme.json spacingScale.steps to 0.
    if ( 0 === $spacing_scale['steps'] ) {
      return null;
    }

    $unit            = '%' === $spacing_scale['unit'] ? '%' : sanitize_title( $spacing_scale['unit'] );
    $current_step    = $spacing_scale['mediumStep'];
    $steps_mid_point = round( $spacing_scale['steps'] / 2, 0 );
    $x_small_count   = null;
    $below_sizes     = array();
    $slug            = 40;
    $remainder       = 0;

    for ( $below_midpoint_count = $steps_mid_point - 1; $spacing_scale['steps'] > 1 && $slug > 0 && $below_midpoint_count > 0; $below_midpoint_count-- ) {
      if ( '+' === $spacing_scale['operator'] ) {
        $current_step -= $spacing_scale['increment'];
      } elseif ( $spacing_scale['increment'] > 1 ) {
        $current_step /= $spacing_scale['increment'];
      } else {
        $current_step *= $spacing_scale['increment'];
      }

      if ( $current_step <= 0 ) {
        $remainder = $below_midpoint_count;
        break;
      }

      $below_sizes[] = array(
        /* translators: %s: Digit to indicate multiple of sizing, eg. 2X-Small. */
        'name' => $below_midpoint_count === $steps_mid_point - 1 ? __( 'Small' ) : sprintf( __( '%sX-Small' ), (string) $x_small_count ),
        'slug' => (string) $slug,
        'size' => round( $current_step, 2 ) . $unit,
      );

      if ( $below_midpoint_count === $steps_mid_point - 2 ) {
        $x_small_count = 2;
      }

      if ( $below_midpoint_count < $steps_mid_point - 2 ) {
        $x_small_count++;
      }

      $slug -= 10;
    }

    $below_sizes = array_reverse( $below_sizes );

    $below_sizes[] = array(
      'name' => __( 'Medium' ),
      'slug' => '50',
      'size' => $spacing_scale['mediumStep'] . $unit,
    );

    $current_step  = $spacing_scale['mediumStep'];
    $x_large_count = null;
    $above_sizes   = array();
    $slug          = 60;
    $steps_above   = ( $spacing_scale['steps'] - $steps_mid_point ) + $remainder;

    for ( $above_midpoint_count = 0; $above_midpoint_count < $steps_above; $above_midpoint_count++ ) {
      $current_step = '+' === $spacing_scale['operator']
        ? $current_step + $spacing_scale['increment']
        : ( $spacing_scale['increment'] >= 1 ? $current_step * $spacing_scale['increment'] : $current_step / $spacing_scale['increment'] );

      $above_sizes[] = array(
        /* translators: %s: Digit to indicate multiple of sizing, eg. 2X-Large. */
        'name' => 0 === $above_midpoint_count ? __( 'Large' ) : sprintf( __( '%sX-Large' ), (string) $x_large_count ),
        'slug' => (string) $slug,
        'size' => round( $current_step, 2 ) . $unit,
      );

      if ( 1 === $above_midpoint_count ) {
        $x_large_count = 2;
      }

      if ( $above_midpoint_count > 1 ) {
        $x_large_count++;
      }

      $slug += 10;
    }