base62.pl

use v6;

my @CHARS = '0'..'9', 'a'..'z', 'A'..'Z';
my %MAP = @CHARS.pairs.map(*.invert);

sub encode($string as Str, :$escape = '_', *@skip) {
	my multi encode-char($_ where any(@CHARS, @skip)) { $_ }
	my multi encode-char($_) {
		my $cp = .ord;
		$escape, reverse gather for ^4 {
			NEXT $cp div= 62;
			take @CHARS[$cp % 62];
		}
	}

	$string.comb.map(&encode-char).join
}

sub decode($string as Str, :$escape = '_') {
	my sub decode-char($_) {
		chr [+] .comb[$escape.chars..*].kv.map: {
			%MAP{$^v} * 62**(3 - $^k)
		}
	}

	$string.subst(:g, / $escape <[0..9a..zA..Z]> ** 4 /, &decode-char)
}