模板 BSGS
EXBSGS
用于求解形如\(a^x≡b~(mod~p)\)的方程
ll exbsgs(ll a, ll b, ll p)
{
if (b == 1LL) return 0;
ll t, d = 1, k = 0;
while ((t = gcd(a, p)) != 1)
{
if (b % t) return -1;
++k, b /= t, p /= t, d = d * (a / t) % p;
if (b == d) return k;
}
unordered_map<ll, ll> dic;
ll m = ceil(sqrt(p));
ll a_m = ksm(a, m, p), mul = b;
for (ll j = 1; j <= m; ++j) mul = mul * a % p, dic[mul] = j;
for (ll i = 1; i <= m; ++i)
{
d = d * a_m % p;
if (dic[d]) return i * m - dic[d] + k;
}
return -1;
}