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/*
* Modular Reducer
* (C) 1999-2011 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
#include <botan/reducer.h>
#include <botan/internal/mp_core.h>
namespace Botan {
/*
* Modular_Reducer Constructor
*/
Modular_Reducer::Modular_Reducer(const BigInt& mod)
{
if(mod <= 0)
throw Invalid_Argument("Modular_Reducer: modulus must be positive");
modulus = mod;
mod_words = modulus.sig_words();
modulus_2 = Botan::square(modulus);
mu = BigInt(BigInt::Power2, 2 * MP_WORD_BITS * mod_words) / modulus;
}
/*
* Barrett Reduction
*/
BigInt Modular_Reducer::reduce(const BigInt& x) const
{
if(mod_words == 0)
throw Invalid_State("Modular_Reducer: Never initalized");
if(x.cmp(modulus, false) < 0)
{
if(x.is_negative())
return x + modulus; // make positive
return x;
}
else if(x.cmp(modulus_2, false) < 0)
{
BigInt t1 = x;
t1.set_sign(BigInt::Positive);
t1 >>= (MP_WORD_BITS * (mod_words - 1));
t1 *= mu;
t1 >>= (MP_WORD_BITS * (mod_words + 1));
t1 *= modulus;
t1.mask_bits(MP_WORD_BITS * (mod_words + 1));
BigInt t2 = x;
t2.set_sign(BigInt::Positive);
t2.mask_bits(MP_WORD_BITS * (mod_words + 1));
t2 -= t1;
if(t2.is_negative())
{
BigInt b_to_k1(BigInt::Power2, MP_WORD_BITS * (mod_words + 1));
t2 += b_to_k1;
}
while(t2 >= modulus)
t2 -= modulus;
if(x.is_positive())
return t2;
else
return (modulus - t2);
}
else
{
// too big, fall back to normal division
return (x % modulus);
}
}
}
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