#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

sub multiply {
    my ( $x, $y, $implementation ) = @_;
    $implementation->( $x, $y );
}

sub whateverthedefaultis {
    my ( $x, $y ) = @_;
    return $x * $y;
}

# https://blogs.msdn.microsoft.com/matthew_van_eerde/2009/07/23/bad-perl-russian-peasant-multiplication-algorithm/

sub peasant {
    my ( $a, $b ) = @_;
    my $c;
    map { $c += $_ * $b }
      grep { $a & $_ }
      map { 1 << $_ } ( 0 .. log($a) / log 2 );
    return $c;
}

say multiply( 7, 3, \&whateverthedefaultis );
say multiply( 7, 3, \&peasant );