I'm not sure if they teach you CS types this stuff in school but I read it long ago and recently come across it again, and I decided to implement the algorythm in perl for s&g.

It's called ethiopian multiplication. The premise is that peasants are not capable of performing multiplication proper but commerce has required them to implement a substitute. 

#Division eq fractions eq multiplication...
sub mul{
  #If peasants can't multiply they certainly can't grok fractions
  use integer;
  my ($i, $j) = @_;

  my @results = ([$i, $j]);

  #Halve until we can halve no more
  while($i > 1){
    my @z = ($i=halve($i), $j=double($j));
    push @results, [@z];
  }
  my $total;
  foreach( @results ){
    #Even halves are evil
    $total += $_->[1] if $_->[0] % 2;
  }
  return $total;

  sub double{
    #Cloning err doubling is easy;
    $_[0]+=$_[0];
  }
  sub halve{
    #Halving is a bit more tedious;
    my $count=0;
    while($_[0]>1){$_[0]-=2; $count++};
    return $count;
  }
}

[download]
And to think, it's almost 1/500_000th the speed of CORE multiplication. There are of course, overflow issues.

--
perl -pe "s/\b;([st])/'\1/mg"

In reply to Binary multiplication by belg4mit

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