friend BigInt operator - (const BigInt& a, const BigInt& b) {
// precondition: a >= b
assert(b <= a);
const int N = a.splits.size();
const int M = b.splits.size();
BigInt ret;
ret.splits.resize(N);
int borrow = 0;
for (int i = 0; i < N; i++) {
int d = a.splits[i] - (i < M ? b.splits[i] : 0) - borrow;
if (d < 0) {
d += SPLIT_OVERFLOW;
borrow = 1;
} else {
borrow = 0;
}
ret.splits[i] = d;
}
assert(borrow == 0);
ret.canonicalize();
return ret;
}
friend BigInt operator / (const BigInt &a, int d) {
const int N = a.splits.size();
BigInt ret;
ret.splits.resize(N);
int64_t carry = 0;
for (int i = N-1; i >= 0; i--) {
int64_t x = carry * SPLIT_OVERFLOW + a.splits[i];
ret.splits[i] = x / d;
carry = x % d;
}
ret.canonicalize();
return ret;
}
static BigInt mul(const BigInt& a, const BigInt& b) {
const int n = a.splits.size();
const int m = b.splits.size();
vector<int64_t> h(n+m+10);
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
h[i+j] += 1LL * a.splits[i] * b.splits[j];
h[i+j+1] += h[i+j] / SPLIT_OVERFLOW;
h[i+j] %= SPLIT_OVERFLOW;
}
}
for (int i = 0; i < n+m+9; i++) {
h[i+1] += h[i]/SPLIT_OVERFLOW;
h[i] %= SPLIT_OVERFLOW;
}
BigInt c;
c.splits.resize(n+m+10);
for (int i = 0; i < n+m+10; i++) {
c.splits[i] = h[i];
}
c.canonicalize();
return c;
}
