扩展BSGS学习笔记

Attention

在哈希表中查询的时候答案不能取模，可以证明是错的。

#include <bits/stdc++.h>
#define ll long long
#define xx first
#define yy second
using namespace std;
int a, p, b;
inline int gcd (int m, int n) {return (!n) ? m : gcd (n, m % n);}
unordered_map<int, int> mp;
inline int Pow (int u, int v, int mod) {
	int tot = 1, base = u % mod;
	while (v) {
		if (v & 1) {tot = 1LL * tot * base % mod;}
		base = 1LL * base * base % mod; v /= 2;
	}
	return tot;
}
inline void solve () {
	if (b == 1) {printf ("0
"); return ;}
	int d = gcd (a, p), k = 1, cnt = 0;
	while (d > 1) {
		if (b % d) {printf ("No Solution
"); return ;}
		cnt++, k = 1LL * k * (a / d) % p;
		p /= d, b /= d;
		if (b == k) {printf ("%d
", cnt); return ;}
		d = gcd (a, p);
	}
	mp.clear ();
	int sz = sqrt (p) + 1, step = Pow (a, sz, p);
	for (int i = 1; i <= sz; ++i) {
		k = 1LL * k * step % p;
		if (!mp.count (k) || mp[k] > i * sz) {mp[k] = i * sz;}
	}
	int cdt = 1;
	bool flag = false; int ans = INT_MAX;
	for (int i = 0; i <= sz; ++i) {
		if (i) {cdt = 1LL * cdt * a % p;}
		if (mp[1LL * cdt * b % p]) {
			int tans = mp[1LL * cdt * b % p] - i + cnt;
//			tans = (tans % p + p) % p;
			ans = min (ans, tans);
			flag = true;
		}
	}
	if (flag) {printf ("%d
", ans);return ;}
	printf ("No Solution
");
}
int main () {
	while (scanf ("%d%d%d", &a, &p, &b) == 3) {
		if (a == 0 && b == 0 && p == 0) {return 0;}
		a %= p;
		b %= p;
		solve ();
	}
	return 0;
}

这是洛谷的模板题，注释掉的地方不能要。