power and modulo on the fly for big numbers

Question

I raise some basis b to the power p and take the modulo m of that.

Let\'s assume b=55170 or 55172 and m=3043839241 (which happens to be the square of 55171). The li

Answer 1

Not familiar with Scala, but...

def powMod (b: Long, pot: Int, mod: Long) : Long = {  
      if (pot == 1) b % mod else { 
          val pot2 = pot/2 
          val pm1 = powMod (b, pot, mod)              
          val pm2 = powMod (b, pot-pot2, mod)            
          (pm1 * pm2) % mod  
      }  
} 

Did you mean

          val pm1 = powMod (b, pot2, mod) 

Notice the pot2 instead of pot.

Strangely, it seems that this should loop forever/overflow the stack, but who knows what Scala is doing.