workup for "sass-calc" library

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

// str-lastindex -- get last index of substr in string
@function str-last-index($str, $substr) {
  $length: str-length($str);
  @for $n from $length - str-length($substr) through 1 {
    $index: str-index(str-slice($str, $n, $length), $substr);
    @if $index { @return $index + $n - 1; }
  }
  @return null;
}

// calc-pre -- prepare value for calc
@function calc-pre($value){
  @if type-of($value) == 'string' {
    $is-calc: str-index($value, 'calc') == 1;
    $start: str-index($value, '(');
    $end: str-last-index($value, ')');
    @if $is-calc and $start and $end {
      @return unquote(str-slice($value, $start, $end));
    } @else {
      @return unquote('(#{$value})');
    }
  } @else {
    @return unquote('#{$value}');
  }
}

// re-calc... ?
@function re-calc($calc1, $calc2, $op: '+'){ 
  @return unquote('calc(#{calc-pre($calc1)} #{$op} #{calc-pre($calc2)})'); 
}

// add/sub -- both args can be calc()
@function calc-add($calc1, $calc2){ @return unquote('calc(#{calc-pre($calc1)} + #{calc-pre($calc2)})'); }
@function calc-sub($calc1, $calc2){ @return unquote('calc(#{calc-pre($calc1)} - #{calc-pre($calc2)})'); }

// mul/div -- second arg must be number
@function calc-mul($calc, $n){ @return unquote('calc(#{calc-pre($calc)} * #{$n})'); }
@function calc-div($calc, $n){ @return unquote('calc(#{calc-pre($calc)} / #{$n})'); }

$t: calc((5 - 2) * 3);
$start: str-index($t, 'calc');
$start2: str-index($t, '(');
$end: str-index($t, ')');

.test {
  test: calc-pre($t);
  test: calc-pre(calc((5 + 7) / 100));
  test: calc-pre(calc(5));
  test: calc-pre('3rem - 5em');
  test: calc-pre(calc(3rem - 5em));
  test: calc-pre(2);
  test: calc-add(calc(3rem / 4), calc(10em - 2px));
  test: calc-add(2rem, calc(10em - 2px));
  test: calc-div(calc(3rem / 4), 4%);
  test: re-calc(calc(3rem / 4), 4%, '-');
  $width: calc(100vw - 100%);
  margin-right: calc-add($width, 2rem);
  margin-right: calc-sub($width, 2rem);
  margin-right: calc-div($width, 2 - 4);
  margin-right: calc-add(calc-mul($width, 2), 10em);
}

SassMeister-output.css

.test {
  test: ((5 - 2) * 3);
  test: ((5 + 7) / 100);
  test: (5);
  test: (3rem - 5em);
  test: (3rem - 5em);
  test: 2;
  test: calc((3rem / 4) + (10em - 2px));
  test: calc(2rem + (10em - 2px));
  test: calc((3rem / 4) / 4%);
  test: calc((3rem / 4) - 4%);
  margin-right: calc((100vw - 100%) + 2rem);
  margin-right: calc((100vw - 100%) - 2rem);
  margin-right: calc((100vw - 100%) / -2);
  margin-right: calc(((100vw - 100%) * 2) + 10em);
}