fusc_function.sf

#!/usr/bin/ruby

# Simple (and efficient) implementation of the "fusc" function.

# See also:
#   https://oeis.org/A002487
#   https://en.wikipedia.org/wiki/Calkin–Wilf_tree

func fusc(n) {

     var (a=1, b=0)

     if (n.is_even) {
        n.remove!(2)
     }

     for bit in (n.as_bin.chars) {
        if (bit) {
            b += a
        }
        else {
            a += b
        }
     }

     return b
}

say 30.of(fusc)

__END__
[0, 1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4, 1, 5, 4, 7, 3, 8, 5, 7, 2, 7, 5, 8, 3, 7]