nestedIndex.p6

my @tree =
  <<"Forum 1"      1  20>>,
  <<"Forum 1.1"    2  7>>,
  <<"Forum1.1.1"   3  4>>,
  <<"Forum1.1.2"   5  6>>,
  <<"Forum1.2"     8  9>>,
  <<"Forum1.3"     10 11>>,
  <<"Forum1.4"     12 19>>,
  <<"Forum1.4.1"   13 14>>,
  <<"Forum1.4.2"   15 18>>,
  <<"Forum1.4.2.1" 16 17>>,
;

my $i = 0;
my $d = 0;
sub tree() {
  my $node = @tree[$i];
  my $left = left($node);
  my $right = right($node);
  my %result = name => name($node);

  while $left + 1 != $right and @tree[++$i]:exists {
    my $newRight = right(@tree[$i]);
    %result<children>.push: tree();
    # only check if we need to break out *AFTER* we added the row
    if $newRight + 1 == $right {
      last;
    }
  }
  return %result;
};

sub name($a) { $a[0]; }
sub left($a) { $a[1]; }
sub right($a) { $a[2] }

say to-json(tree());