separate vao for each kind of slice

vis/include/combo_iterator.hpp

@@ -0,0 +1,72 @@
+#pragma once
+
+#include <vector>
+#include <algorithm>
+#include <stdexcept>
+
+template<class T>
+struct ComboIterator {
+    const std::vector<T> &vals;
+    const size_t k;
+    size_t n;
+    std::vector<bool> bits;
+    std::vector<T> curr;
+
+    ComboIterator(ComboIterator &) = default;
+
+    ComboIterator(const std::vector<T> &vals, const size_t k, const size_t n)
+        : vals(vals), k(k), n(n), curr(k), bits(vals.size()) {
+        for (size_t i = 0; i < vals.size(); ++i) {
+            bits[i] = i < k;
+        }
+        std::reverse(bits.begin(), bits.end());
+        set_curr();
+    }
+
+    void set_curr() {
+        for (size_t i = 0, j = 0; i < vals.size(); ++i) {
+            if (bits[i]) curr[j++] = vals[i];
+        }
+    }
+
+    const std::vector<T> &operator*() const {
+        return curr;
+    }
+
+    void operator++() {
+        std::next_permutation(bits.begin(), bits.end());
+        set_curr();
+        ++n;
+    }
+
+    [[nodiscard]] bool operator!=(const ComboIterator<T> &o) const {
+        return n != o.n;
+    }
+
+    [[nodiscard]] bool operator==(const ComboIterator<T> &o) const {
+        return n == o.n;
+    }
+};
+
+size_t choose(size_t n, size_t k) {
+    if (k == 0) return 1;
+    return n * choose(n - 1, k - 1) / k;
+}
+
+template<class T>
+struct Combos {
+    const std::vector<T> &vals;
+    size_t k;
+
+    Combos(const std::vector<T> &vals, size_t k) : vals(vals), k(k) {
+    }
+
+    [[nodiscard]] ComboIterator<T> begin() const {
+        return ComboIterator(vals, k, 0);
+    }
+
+    [[nodiscard]] ComboIterator<T> end() const {
+        int j = choose(vals.size(), k);
+        return ComboIterator(vals, k, j);
+    }
+};