/*

* BigInt Random Generation

* (C) 1999-2007 Jack Lloyd

*

* Botan is released under the Simplified BSD License (see license.txt)

*/

#include <botan/bigint.h>

#include <botan/rng.h>

#include <botan/internal/rounding.h>

namespace Botan {

/*

* Randomize this number

*/

void BigInt::randomize(RandomNumberGenerator& rng, size_t bitsize, bool set_high_bit) {

   set_sign(Positive);

   if(bitsize == 0) {

      clear();

   } else {

      secure_vector<uint8_t> array = rng.random_vec(round_up(bitsize, 8) / 8);

      // Always cut unwanted bits

      if(bitsize % 8 > 0) {

         array[0] &= 0xFF >> (8 - (bitsize % 8));

      }

      // Set the highest bit if wanted

      if(set_high_bit) {

         array[0] |= 0x80 >> ((bitsize % 8) > 0 ? (8 - bitsize % 8) : 0);

      }

      assign_from_bytes(array);

   }

}

/*

* Generate a random integer within given range

*/

BigInt BigInt::random_integer(RandomNumberGenerator& rng, const BigInt& min, const BigInt& max) {

   if(min.is_negative() || max.is_negative() || max <= min) {

      throw Invalid_Argument("BigInt::random_integer invalid range");

   }

   /*

   If min is > 1 then we generate a random number `r` in [0,max-min)

   and return min + r.

   This same logic could also be reasonably chosen for min == 1, but

   that breaks certain tests which expect stability of this function

   when generating within [1,n)

   */

   if(min > 1) {

      const BigInt diff = max - min;

      // This call is recursive, but will not recurse further

      return min + BigInt::random_integer(rng, BigInt::zero(), diff);

   }

   BOTAN_DEBUG_ASSERT(min <= 1);

   const size_t bits = max.bits();

   for(;;) {

      BigInt r;

      r.randomize(rng, bits, false);

      if(r >= min && r < max) {

         return r;

      }

   }

}

}  // namespace Botan