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fixed addition/subtraction edge cases, improved modPow #4

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fixed addition/subtraction edge cases, improved modPow #4

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26 changes: 19 additions & 7 deletions include/BigNumber.h
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Original file line number Diff line number Diff line change
Expand Up @@ -216,7 +216,7 @@ class BigNumber {
if (this->isPositive() && !number.isPositive()) {
return *this - number.absoluteValue();
} else if (!this->isPositive() && number.isPositive()) {
return -(number - this->absoluteValue());
return number - this->absoluteValue();
}

if (number.isZero()) {
Expand Down Expand Up @@ -258,7 +258,7 @@ class BigNumber {
return *this;
}
if (this->isZero()) {
return number;
return -number;
}

// At this point, both signs are equal.
Expand Down Expand Up @@ -409,11 +409,23 @@ class BigNumber {
if (this->isZero()) {
return BigNumber(0);
}
BigNumber result(1);
for (BigNumber i = 0; i < power; i++) { // repeat power times.
result = (*this * result) % mod;
if (power.isZero()) {
return BigNumber(1);
}
return result;

std::string pow_bin = power.asBinary();
int bin_indx = pow_bin.length()-2;
BigNumber mod_result = *this % mod;
BigNumber mod_sum = mod_result;

while (bin_indx > 0) {
mod_result = (mod_result * mod_result) % mod;
if (pow_bin.at(bin_indx) == '1') {
mod_sum *= mod_result;
}
bin_indx--;
}
return mod_sum % mod;
}


Expand Down Expand Up @@ -881,4 +893,4 @@ class BigNumber {
} // Closing namespace pr0crustes.


#endif // BIGNUMBER_H
#endif // BIGNUMBER_H