LA 4998 Simple Encryption

题意：输入正整数$K_1(K_1 \leq 50000)$, 找一个$12$位正整数$K_2$（不能含有前导零）使得${K_1}^{K_2}\equiv K_2(mod10^{12})$。

例如，$K_1=78$和$99$时，$K_2$分别为$308646916096$和$817245479899$。

分析：这题还是需要搜索来寻找答案的，直接构造很困难。事实上，${K_1}^{K_2}\equiv K_2(mod10^{12})$，同时意味着：

${K_1}^{K_2}\equiv K_2(mod10^{i}), i \leq 12$。

并且有$\varphi(10^i)={10^i}(1-\frac{1}{2})(1-\frac{1}{5})=4\cdot10^{i-1}$

于是${K_1}^{K_2}\mod {10^i}={K_1}^{K_2\mod{\varphi(10^i)}+\varphi(10^i)}\mod {10^i}$

于是${K_1}^{K_2}\mod {10^i}={K_1}^{K_3}\mod {10^i}\iff\varphi(10^i) | (K_3 - K_2)$

满足上式的一个充分条件是$10^{i+1} | (K_3 - K_2)$

也就是说，我们从低到高枚举$K_2$第$i$位的值，同时枚举其第$i+1$位的值，那么只要从最低位到第$i+1$位关于分解后对应模方程是合法的，

在以后的构造过程中始终不会破坏当前的性质，因为高位的那些值对当前模取模值都是相同的。

那么我们就可以边枚举边验证进行dfs了。

交了之后发现TLE了。为了保证快速幂时乘法不溢出，又用了一个快速乘法操作，这个操作实际上是可以从$O(log(n))$优化到$O(1)$的。

首先考虑模数最大为$10^{12}$，不超过$2$进制中的$40$位，那么我们可以将待乘值拆分成高$20$位与低$20$位，确保在乘法时始终不超过$60$位，

用long long就不会溢出，具体如下：

$a\cdot b \mod n = a \cdot ((b>>20)\cdot(1<<20) + b \& ((1 <<20) - 1)\mod n$

$= ((a\cdot(b>>20)\mod n)\cdot(1<<20)\mod n) + (a\cdot(b \& ((1 <<20) - 1))))\mod n$

这样优化之后就可以通过了。

 #include <algorithm>
 #include <cstdio>
 #include <cstring>
 #include <string>
 #include <queue>
 #include <map>
 #include <set>
 #include <ctime>
 #include <cmath>
 #include <iostream>
 #include <assert.h>
 #define PI acos(-1.)
 #pragma comment(linker, "/STACK:102400000,102400000")
 #define max(a, b) ((a) > (b) ? (a) : (b))
 #define min(a, b) ((a) < (b) ? (a) : (b))
 #define mp make_pair
 #define st first
 #define nd second
 #define keyn (root->ch[1]->ch[0])
 #define lson (u << 1)
 #define rson (u << 1 | 1)
 #define pii pair<int, int>
 #define pll pair<ll, ll>
 #define pb push_back
 #define type(x) __typeof(x.begin())
 #define foreach(i, j) for(type(j)i = j.begin(); i != j.end(); i++)
 #define FOR(i, s, t) for(int i = (s); i <= (t); i++)
 #define ROF(i, t, s) for(int i = (t); i >= (s); i--)
 #define dbg(x) cout << x << endl
 #define dbg2(x, y) cout << x << " " << y << endl
 #define clr(x, i) memset(x, (i), sizeof(x))
 #define maximize(x, y) x = max((x), (y))
 #define minimize(x, y) x = min((x), (y))
 using namespace std;
 typedef long long ll;
 const int int_inf = 0x3f3f3f3f;
 const ll ll_inf = 0x3f3f3f3f3f3f3f3f;
 const int INT_INF = (int)((1ll << ) - );
 const double double_inf = 1e30;
 const double eps = 1e-;
 typedef unsigned long long ul;
 inline int readint(){
     int x;
     scanf("%d", &x);
     return x;
 }
 inline int readstr(char *s){
     scanf("%s", s);
     return strlen(s);
 }
 //Here goes 2d geometry templates
 struct Point{
     double x, y;
     Point(double x = , double y = ) : x(x), y(y) {}
 };
 typedef Point Vector;
 Vector operator + (Vector A, Vector B){
     return Vector(A.x + B.x, A.y + B.y);
 }
 Vector operator - (Point A, Point B){
     return Vector(A.x - B.x, A.y - B.y);
 }
 Vector operator * (Vector A, double p){
     return Vector(A.x * p, A.y * p);
 }
 Vector operator / (Vector A, double p){
     return Vector(A.x / p, A.y / p);
 }
 bool operator < (const Point& a, const Point& b){
     return a.x < b.x || (a.x == b.x && a.y < b.y);
 }
 int dcmp(double x){
     if(abs(x) < eps) return ;
     return x <  ? - : ;
 }
 bool operator == (const Point& a, const Point& b){
     return dcmp(a.x - b.x) ==  && dcmp(a.y - b.y) == ;
 }
 double Dot(Vector A, Vector B){
     return A.x * B.x + A.y * B.y;
 }
 double Len(Vector A){
     return sqrt(Dot(A, A));
 }
 double Angle(Vector A, Vector B){
     return acos(Dot(A, B) / Len(A) / Len(B));
 }
 double Cross(Vector A, Vector B){
     return A.x * B.y - A.y * B.x;
 }
 double Area2(Point A, Point B, Point C){
     return Cross(B - A, C - A);
 }
 Vector Rotate(Vector A, double rad){
     //rotate counterclockwise
     return Vector(A.x * cos(rad) - A.y * sin(rad), A.x * sin(rad) + A.y * cos(rad));
 }
 Vector Normal(Vector A){
     double L = Len(A);
     return Vector(-A.y / L, A.x / L);
 }
 void Normallize(Vector &A){
     double L = Len(A);
     A.x /= L, A.y /= L;
 }
 Point GetLineIntersection(Point P, Vector v, Point Q, Vector w){
     Vector u = P - Q;
     double t = Cross(w, u) / Cross(v, w);
     return P + v * t;
 }
 double DistanceToLine(Point P, Point A, Point B){
     Vector v1 = B - A, v2 = P - A;
     return abs(Cross(v1, v2)) / Len(v1);
 }
 double DistanceToSegment(Point P, Point A, Point B){
     if(A == B) return Len(P - A);
     Vector v1 = B - A, v2 = P - A, v3 = P - B;
     if(dcmp(Dot(v1, v2)) < ) return Len(v2);
     else if(dcmp(Dot(v1, v3)) > ) return Len(v3);
     else return abs(Cross(v1, v2)) / Len(v1);
 }
 Point GetLineProjection(Point P, Point A, Point B){
     Vector v = B - A;
     return A + v * (Dot(v, P - A) / Dot(v, v));
 }
 bool SegmentProperIntersection(Point a1, Point a2, Point b1, Point b2){
     //Line1:(a1, a2) Line2:(b1,b2)
     double c1 = Cross(a2 - a1, b1 - a1), c2 = Cross(a2 - a1, b2 - a1),
            c3 = Cross(b2 - b1, a1 - b1), c4 = Cross(b2 - b1, a2 - b1);
     return dcmp(c1) * dcmp(c2) <  && dcmp(c3) * dcmp(c4) < ;
 }
 bool OnSegment(Point p, Point a1, Point a2){
     return dcmp(Cross(a1 - p, a2 - p)) ==  && dcmp(Dot(a1 - p, a2 -p)) < ;
 }
 Vector GetBisector(Vector v, Vector w){
     Normallize(v), Normallize(w);
     return Vector((v.x + w.x) / , (v.y + w.y) / );
 }

 bool OnLine(Point p, Point a1, Point a2){
     Vector v1 = p - a1, v2 = a2 - a1;
     double tem = Cross(v1, v2);
     return dcmp(tem) == ;
 }
 struct Line{
     Point p;
     Vector v;
     Point point(double t){
         return Point(p.x + t * v.x, p.y + t * v.y);
     }
     Line(Point p, Vector v) : p(p), v(v) {}
 };
 struct Circle{
     Point c;
     double r;
     Circle(Point c, double r) : c(c), r(r) {}
     Circle(int x, int y, int _r){
         c = Point(x, y);
         r = _r;
     }
     Point point(double a){
         return Point(c.x + cos(a) * r, c.y + sin(a) * r);
     }
 };
 int GetLineCircleIntersection(Line L, Circle C, double &t1, double& t2, vector<Point>& sol){
     double a = L.v.x, b = L.p.x - C.c.x, c = L.v.y, d = L.p.y - C.c.y;
     double e = a * a + c * c, f =  * (a * b + c * d), g = b * b + d * d - C.r * C.r;
     double delta = f * f -  * e * g;
     if(dcmp(delta) < ) return ;
     if(dcmp(delta) == ){
         t1 = t2 = -f / ( * e); sol.pb(L.point(t1));
         return ;
     }
     t1 = (-f - sqrt(delta)) / ( * e); sol.pb(L.point(t1));
     t2 = (-f + sqrt(delta)) / ( * e); sol.pb(L.point(t2));
     return ;
 }
 double angle(Vector v){
     return atan2(v.y, v.x);
     //(-pi, pi]
 }
 int GetCircleCircleIntersection(Circle C1, Circle C2, vector<Point>& sol){
     double d = Len(C1.c - C2.c);
     if(dcmp(d) == ){
         if(dcmp(C1.r - C2.r) == ) return -; //two circle duplicates
         return ; //two circles share identical center
     }
     if(dcmp(C1.r + C2.r - d) < ) return ; //too close
     if(dcmp(abs(C1.r - C2.r) - d) > ) return ; //too far away
     double a = angle(C2.c - C1.c); // angle of vector(C1, C2)
     double da = acos((C1.r * C1.r + d * d - C2.r * C2.r) / ( * C1.r * d));
     Point p1 = C1.point(a - da), p2 = C1.point(a + da);
     sol.pb(p1);
     if(p1 == p2) return ;
     sol.pb(p2);
     return ;
 }
 int GetPointCircleTangents(Point p, Circle C, Vector* v){
     Vector u = C.c - p;
     double dist = Len(u);
     if(dist < C.r) return ;//p is inside the circle, no tangents
     else if(dcmp(dist - C.r) == ){
         // p is on the circles, one tangent only
         v[] = Rotate(u, PI / );
         return ;
     }else{
         double ang = asin(C.r / dist);
         v[] = Rotate(u, -ang);
         v[] = Rotate(u, +ang);
         return ;
     }
 }
 int GetCircleCircleTangents(Circle A, Circle B, Point* a, Point* b){
     //a[i] store point of tangency on Circle A of tangent i
     //b[i] store point of tangency on Circle B of tangent i
     //six conditions is in consideration
     int cnt = ;
     if(A.r < B.r) { swap(A, B); swap(a, b); }
     int d2 = (A.c.x - B.c.x) * (A.c.x - B.c.x) + (A.c.y - B.c.y) * (A.c.y - B.c.y);
     int rdiff = A.r - B.r;
     int rsum = A.r + B.r;
     if(d2 < rdiff * rdiff) return ; // one circle is inside the other
     double base = atan2(B.c.y - A.c.y, B.c.x - A.c.x);
     if(d2 ==  && A.r == B.r) return -; // two circle duplicates
     if(d2 == rdiff * rdiff){ // internal tangency
         a[cnt] = A.point(base); b[cnt] = B.point(base); cnt++;
         return ;
     }
     double ang = acos((A.r - B.r) / sqrt(d2));
     a[cnt] = A.point(base + ang); b[cnt++] = B.point(base + ang);
     a[cnt] = A.point(base - ang); b[cnt++] = B.point(base - ang);
     if(d2 == rsum * rsum){
         //one internal tangent
         a[cnt] = A.point(base);
         b[cnt++] = B.point(base + PI);
     }else if(d2 > rsum * rsum){
         //two internal tangents
         double ang = acos((A.r + B.r) / sqrt(d2));
         a[cnt] = A.point(base + ang); b[cnt++] = B.point(base + ang + PI);
         a[cnt] = A.point(base - ang); b[cnt++] = B.point(base - ang + PI);
     }
     return cnt;
 }
 Point ReadPoint(){
     double x, y;
     scanf("%lf%lf", &x, &y);
     return Point(x, y);
 }
 Circle ReadCircle(){
     double x, y, r;
     scanf("%lf%lf%lf", &x, &y, &r);
     return Circle(x, y, r);
 }
 //Here goes 3d geometry templates
 struct Point3{
     double x, y, z;
     Point3(double x = , double y = , double z = ) : x(x), y(y), z(z) {}
 };
 typedef Point3 Vector3;
 Vector3 operator + (Vector3 A, Vector3 B){
     return Vector3(A.x + B.x, A.y + B.y, A.z + B.z);
 }
 Vector3 operator - (Vector3 A, Vector3 B){
     return Vector3(A.x - B.x, A.y - B.y, A.z - B.z);
 }
 Vector3 operator * (Vector3 A, double p){
     return Vector3(A.x * p, A.y * p, A.z * p);
 }
 Vector3 operator / (Vector3 A, double p){
     return Vector3(A.x / p, A.y / p, A.z / p);
 }
 double Dot3(Vector3 A, Vector3 B){
     return A.x * B.x + A.y * B.y + A.z * B.z;
 }
 double Len3(Vector3 A){
     return sqrt(Dot3(A, A));
 }
 double Angle3(Vector3 A, Vector3 B){
     return acos(Dot3(A, B) / Len3(A) / Len3(B));
 }
 double DistanceToPlane(const Point3& p, const Point3 &p0, const Vector3& n){
     return abs(Dot3(p - p0, n));
 }
 Point3 GetPlaneProjection(const Point3 &p, const Point3 &p0, const Vector3 &n){
     return p - n * Dot3(p - p0, n);
 }
 Point3 GetLinePlaneIntersection(Point3 p1, Point3 p2, Point3 p0, Vector3 n){
     Vector3 v = p2 - p1;
     double t = (Dot3(n, p0 - p1) / Dot3(n, p2 - p1));
     return p1 + v * t;//if t in range [0, 1], intersection on segment
 }
 Vector3 Cross(Vector3 A, Vector3 B){
     return Vector3(A.y * B.z - A.z * B.y, A.z * B.x - A.x * B.z, A.x * B.y - A.y * B.x);
 }
 double Area3(Point3 A, Point3 B, Point3 C){
     return Len3(Cross(B - A, C - A));
 }
 class cmpt{
 public:
     bool operator () (const int &x, const int &y) const{
         return x > y;
     }
 };

 int Rand(int x, int o){
     //if o set, return [1, x], else return [0, x - 1]
     if(!x) return ;
     int tem = (int)((double)rand() / RAND_MAX * x) % x;
     return o ? tem +  : tem;
 }
 ////////////////////////////////////////////////////////////////////////////////////
 ////////////////////////////////////////////////////////////////////////////////////
 void data_gen(){
     srand(time());
     freopen("in.txt", "w", stdout);
     int times = ;
     printf("%d\n", times);
     while(times--){
         int r = Rand(, ), a = Rand(, ), c = Rand(, );
         int b = Rand(r, ), d = Rand(r, );
         int m = Rand(, ), n = Rand(m, );
         printf("%d %d %d %d %d %d %d\n", n, m, a, b, c, d, r);
     }
 }

 struct cmpx{
     bool operator () (int x, int y) { return x > y; }
 };
 int debug = ;
 int dx[] = {, , , };
 int dy[] = {, , , };
 //-------------------------------------------------------------------------
 ll multi(ll x, ll y, ll mod){
     ll lhs = x * (y >> ) % mod * (1ll << ) % mod;
     ll rhs = x * (y & ((1ll << ) - )) % mod;
     return (lhs + rhs) % mod;
 }

 ll __power(ll a, ll p, ll mod){
     ll ans = ;
     a %= mod;
     while(p){
         if(p & ) ans = (ans * a) % mod;
         p >>= ;
         a = (a * a) % mod;
     }
     return ans;
 }

 ll power(ll a, ll p, ll mod){
     ll ans = ;
     while(p){
         if(p & ) ans = multi(ans, a, mod);
         p >>= ;
         a = multi(a, a, mod);
     }
     return ans % mod;
 }

 ll n;
 ll ans[ + ];
 bool ok;
 void dfs(int pos, ll pre, ll mod){
     if(pos == ){
         if(pre < 1e11 || power(n, pre, mod) != pre) return;
         ans[n] = pre;
         ok = ;
         return;
     }
     FOR(i, , ){
         ll nex = mod * i + pre;
         //try to set next position as i
         ll lhs = power(n, nex, mod);
         ll rhs = nex % mod;
         if(lhs != rhs) continue;
         dfs(pos + , nex, mod * );
         if(ok) return;
     }
 }
 void solve(){
     if(ans[n] != -) return;
     ok = ;
     dfs(, , );
 }

 //-------------------------------------------------------------------------
 int main(){
     //data_gen(); return 0;
     //C(); return 0;
     debug = ;
     ///////////////////////////////////////////////////////////////////////////////////////////////////////////////
     if(debug) freopen("in.txt", "r", stdin);
     //freopen("in.txt", "w", stdout);
     int kase = ;
     clr(ans, -);
     while(~scanf("%lld", &n) && n){
         solve();
         printf("Case %d: Public Key = %lld Private Key = %lld\n", ++kase, n, ans[n]);
     }
     //////////////////////////////////////////////////////////////////////////////////////////////////////////////
     return ;
 }

code：