SASS function to generate nth formulas based on repeating patterns

example.css

// example output CSS
.container :nth-of-type(5n + 1), .container :nth-of-type(5n + 2), .container :nth-of-type(5n + 3) {
  background: orange;
}
.container :nth-of-type(5n + 4), .container :nth-of-type(5n + 5) {
  background: blue;
}

example.scss

// example SASS

// first three elements are orange, next 2 elements are blue, next 3 elements are orange, etc. repeating in perpetuity
.container {
  // 3 rows on, 2 rows off
  #{nth-pattern(':nth-of-type()', 3 2)} {
    background: orange;
  }
  
  // 2 rows on, 3 rows off, with a starting offset of 3 rows
  #{nth-pattern(':nth-of-type()', 2 3, 3)} {
    background: blue;
  }
}

nth-pattern.scss

@function nth-pattern($selector, $pattern, $offset: 0) {
  // add up the pattern values to get the total period length
  $period: 0;
  @each $value in $pattern {
    $period: $period + $value;
  }

  // determine where we will stick the parenthetical formula
  $insert-at: str-index($selector, "()");
  @if $insert-at == null {
    // default to tacking it on the end
    $insert-at: str-length($selector) + 1;
  } @else {
    // strip out the placeholder
    $selector: #{str-slice($selector, 0, $insert-at - 1) str-slice($selector, $insert-at + 2)};
  }
  
  // selector collector
  $selectors: ();
  // go through the pattern list
  @for $i from 1 through length($pattern) {
    // odd index is "on"
    @if $i % 2 == 1 {
      // add as many selectors as the value of this pattern item 
      @for $count from 1 through nth($pattern, $i) {
        $selectors: append(
          $selectors, 
          str-insert($selector, "(#{$period}n + #{$offset + $count})", $insert-at), 
          comma
        );
      }
    }
    // advance the offset
    $offset: $offset + nth($pattern, $i);
  }
  
  @return $selectors;
}