class Trachtenberg
{
public:
void multiply(const char* a, const char* b)
{
size_t large = strlen(a);
size_t small = strlen(b);
if (large == 0 || small == 0) {
answer = solution = new char[2]{ '0', '\0' };
return;
}
if (large < small) {
std::swap(a, b);
std::swap(large, small);
}
solution = new char[large + small];
answer = solution + large + small - 1;
*answer = '\0'; // zero terminate
size_t digits = 0;
const char* starta = a + large;
const char* startb = b + small;
intermediate = 0;
while (digits != small) {
multiplystep(++digits, --starta, startb);
}
for (size_t d = digits; d != large; ++d) {
multiplystep(digits, --starta, startb);
}
while (digits != 1) {
multiplystep(--digits, starta, --startb);
}
while (intermediate) {
*--answer = '0' + intermediate % 10;
intermediate /= 10;
}
}
const char* result() const { return answer; }
private:
size_t intermediate;
char* solution = nullptr;
char* answer = nullptr;
void multiplystep(size_t digits, const char* starta, const char* startb)
{
const char* digita = starta;
const char* digitb = startb;
for (size_t n = 0; n != digits; ++n) {
--digitb;
intermediate += (*digita - '0') * (*digitb - '0'); // convert from ASCII string
++digita;
}
*--answer = '0' + intermediate % 10; // convert back to string
intermediate /= 10;
}
};
