Add variations

math/variations.11.cpp

@@ -0,0 +1,53 @@
+// Variations with repetitions
+#include <vector>
+#include <tuple>
+using namespace std;
+
+/*snippet-begin*/
+template<class T = int>
+struct variations {
+    struct iterator {
+        T l, h;
+        vector<T> a;
+
+        const vector<T>& operator* () const { return a; }
+        bool operator== (const iterator& b) const { return tie(l, h, a) == tie(b.l, b.h, b.a); }
+        bool operator!= (const iterator& b) const { return !(*this == b); }
+        iterator& operator++ () {
+            int i = (int)a.size() - 1;
+            a[i]++;
+            while (i && a[i] > h) {
+                a[i] = l;
+                a[--i]++;
+            }
+            return *this;
+        }
+    };
+
+    int n, k; T l;
+    variations(int n, int k, T l = 0) : n(n), k(k), l(l) {}
+
+    iterator begin() {
+        return {l, l+k-1, vector<T>(n, l)};
+    }
+
+    iterator end() {
+        vector<T> e(n, l);
+        e[0] = l+k;
+        return {l, l+k-1, move(e)};
+    }
+};
+/*snippet-end*/
+
+int main() {
+    variations<> v(2,3,1);
+
+    int ind = 0;
+    vector<vector<int>> a = {{1,1},{1,2},{1,3},{2,1},{2,2},{2,3},{3,1},{3,2},{3,3}};
+    for (auto it = v.begin(); it != v.end(); ++it){
+        if (*it != a[ind]) return 1;
+        ++ind;
+    }
+
+    return (ind != (int)a.size());
+}