Baby step , Giant step

/*<br />a ^ x = n (mod p)<br />計算x,該代碼僅解決p為素數<br />*/<br />#include <string.h><br />#include <stdio.h><br />#include <math.h><br />#include <algorithm><br />#include <iostream><br />using namespace std;<br />typedef struct { int res, times; } rem;<br />typedef __int64 LL;<br />rem r[1000001];<br />bool cmp(rem r1 , rem r2)<br />{<br />if (r1.res == r2.res)<br />return r1.times < r2.times;<br />return r1.res < r2.res;<br />}<br />int binary(int x , int e)<br />{<br />int s = 0 , mid , ans = -1;<br />while (s <= e)<br />{<br />mid = (s + e) >> 1;<br />if (r[mid].res > x)<br />e = mid - 1;<br />else<br />s = mid + 1;<br /> if (r[mid].res == x)<br /> ans = r[mid].times;<br />}<br />return ans;<br />}<br />int power(LL a,int b,int p)<br />{<br />LL ans = 1;<br />while (b)<br />{<br />while (!(b&1))<br />{<br />b >>= 1;<br />a = a * a % p;<br />}<br />b --;<br />ans = ans * a % p;<br />}<br />return ans;<br />}<br />int main()<br />{<br />int i , j , n , m , sq , p , a , asq , tmp;<br />while (scanf("%d%d%d" , &a , &p , &n)!= EOF)<br />{<br />a %= p;<br />n %= p;<br />sq = (int)ceil(sqrt(double(p)));<br />j = n;<br />r[0].res = n;<br />r[0].times = 0;<br />for (i = 1;i <= sq;i ++)<br />{<br />j = (LL)j * a % p;<br />r[i].res = j;<br />r[i].times = i;<br />}<br />sort(r , r + sq + 1 , cmp);<br />asq = power(a , sq , p);<br />j = 1;<br />for (i = 1;i <= sq;i ++)<br />{<br />j = (LL)j * asq % p;<br />tmp = binary(j , sq);<br />if (tmp + 1) break;<br />}<br />if (i > sq) printf("NONE/n");<br />else printf("%d/n" , i * sq - tmp);<br />}<br />return 0;<br />}