number of permutations of string A in string B

#include <iostream>
#include <unordered_map>

using namespace std;

int main() {
    string a, b;
    cin >> a >> b;
    unordered_map<char, int> alf_a;
    for (auto el: a) {
        ++alf_a[el];
    }
    int cur_diff = alf_a.size();
    unordered_map<char, int> cur_alf;
    int ans = 0;
    for (int i = 0; i < a.size(); ++i) {
        ++cur_alf[b[i]];
        if (cur_alf[b[i]] == alf_a[b[i]]) {
            --cur_diff;
        } else if (cur_alf[b[i]] - 1 == alf_a[b[i]]) {
            ++cur_diff;
        }
        if (cur_diff == 0) ++ans;
    }
    for (int i = 0; i + a.size() < b.size(); ++i) {
        --cur_alf[b[i]];
        if (cur_alf[b[i]] == alf_a[b[i]]) {
            --cur_diff;
        } else if (cur_alf[b[i]] + 1 == alf_a[b[i]]) {
            ++cur_diff;
        }
        ++cur_alf[b[i + a.size()]];
        if (cur_alf[b[i + a.size()]] == alf_a[b[i + a.size()]]) {
            --cur_diff;
        } else if (cur_alf[b[i + a.size()]] - 1 == alf_a[b[i + a.size()]]) {
            ++cur_diff;
        }
        if (cur_diff == 0) ++ans;
    }
    cout << ans;
}
//abbac