ilog.c

int
ilog ( int b , int n )
{
    int lo=0 , b_lo=1 , hi=1 , mid , b_mid ;
    
    long long b_hi=b ;
  
    while ( b_hi < n )
    { lo = hi ; b_lo = b_hi ; hi = hi*2 ; b_hi = b_hi * b_hi ; }
  
    while( hi-lo > 1 )
    {
      mid = (lo + hi) / 2 ;
      
      b_mid = b_lo * (int) pow( b , mid - lo ) ;

      if ( n < b_mid  ) { hi = mid ; b_hi = b_mid ; }
      if ( n > b_mid  ) { lo = mid ; b_lo = b_mid ; }
      if ( n == b_mid ) return mid ;
    }

    return b_hi == n ? hi : lo ;
}