What operation does the following code implement?

public static int mystery$($int a$,$ int b$)$

$\{$

if $($b $==0)$ return $0$;

if $($b$\%2==0)$ return mystery$($a$+$a$,$b$/2)$;

return mystery$($a$+$a$,$b$/2)+$a;

$\}$