Codeforces 472G Design Tutorial: Increase the Constraints(分块+FFT)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <bits/stdc++.h>
using namespace std;

#define N 550020
#define LL long long
#define MOD 1000000007
#define K 3
#define G 3

const int B = 6000;

int n, m;
char s
, t
;
int len;
int rev
;
const double Pi = acos(-1.0);

struct cd {
double r, i;
cd() {}
cd(double r, double i):r(r), i(i) {}

cd operator + (const cd &b) const {
return cd(r + b.r, i + b.i);
}
cd operator - (const cd &b) const {
return cd(r - b.r, i - b.i);
}
cd operator * (const cd &b) const {
return cd(r * b.r - i * b.i, i * b.r + r * b.i);
}
}x1
, x2
;

void change(cd y[], int len) {
for(int i = 1; i < len; ++i) {
rev[i] = (rev[i>>1] >> 1) + (i & 1) * (len >> 1);
if(i < rev[i]) swap(y[i], y[rev[i]]);
}
}
void FFT(cd y[], int len, int on) {
change(y, len);
for(int h = 2; h <= len; h <<= 1) {
cd wn(cos(on * 2 * Pi / h), sin(on * 2 * Pi / h));
for(int j = 0; j < len; j += h) {
cd w(1, 0);
for(int k = j; k < j + h / 2; ++k) {
cd u = y[k];
cd t = w * y[k+h/2];
y[k] = u + t;
y[k+h/2] = u - t;
w = w * wn;
}
}
}
if(on == -1) {
for(int i = 0; i < len; ++i) y[i].r /= len;
}
}

void debug() {
cd x1[4] = {cd(-1, 0), cd(-1, 0)};
cd x2[4] = {cd(1, 0), cd(1, 0)};
FFT(x1, 4, 1);
FFT(x2, 4, 1);
for(int i = 0; i < 4; ++i) x1[i] = x1[i] * x2[i];
FFT(x1, 4, -1);
for(int i = 0; i < 4; ++i) printf("%.3lf ", floor(x1[i].r + 0.5));
puts("");
}

short int cnt[200020/B]
;

int main() {
scanf("%s%s", s, t);
n = strlen(s);
m = strlen(t);
len = 1;
while(len <= n + m) len <<= 1;

for(int i = 0; i + B < n; i += B) {
for(int j = 0; j < len; ++j) {
if(j < m) {
int v = t[j] == '1'? 1: -1;
x2[m-j-1] = cd(v, 0);
}
else x2[j] = cd(0, 0);
if(j >= i && j < i + B) {
int v = s[j] == '1'? 1: -1;
x1[j] = cd(v, 0);
}
else x1[j] = cd(0, 0);
}
FFT(x1, len, 1);
FFT(x2, len, 1);
for(int j = 0; j < len; ++j) x1[j] = x1[j] * x2[j];
FFT(x1, len, -1);
for(int j = 0; j < len; ++j) cnt[i/B][j] = (int)floor(x1[j].r + 0.5);
}
int q;
scanf("%d", &q);
while(q--) {
int a, b, x;
scanf("%d%d%d", &a, &b, &x);
int l = a, r = a + x - 1;
int u = l / B, v = r / B;

int ans = 0;
if(u == v) {
for(int i = 0; i < x; ++i) {
if(s[i+a] != t[b+i]) ans++;
}
}
else {
for(int i = l % B, j = 0; i < B; ++i, ++j) {
if(s[a+j] != t[b+j]) ++ans;
}
for(int i = 0, j = r % B; i <= r % B; ++i, --j)
if(s[r-j] != t[b+x-1-j]) ++ans;
for(int k = u + 1; k < v; ++k) {
ans += (B - cnt[k][a+m-b-1]) / 2;
}
}
printf("%d\n", ans);
}
return 0;
}