feat(util/list): add map_filter template

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/tests/util/list.cpp

@@ -161,6 +161,23 @@ static void tst12() {
     lean_assert(sum == 30);
 }
 
+static void tst13() {
+    list<int> l({1, 2, 3, 4, 5, 6, 7});
+    list<int> l2 = map_filter(l, [](int in, int & out) {
+            if (in % 2 == 0) {
+                out = in + 10;
+                return true;
+            } else {
+                return false;
+            }
+        });
+    std::cout << l2 << "\n";
+    list<int> l3({12, 14, 16});
+    lean_assert_eq(l2, l3);
+    lean_assert(empty(map_filter(l, [](int, int &) { return false; })));
+    lean_assert(empty(map_filter(list<int>(), [](int in, int & out) { out = in; return true; })));
+}
+
 int main() {
     tst1();
     tst2();
@@ -174,5 +191,6 @@ int main() {
     tst10(1000, 5);
     tst11(1000, 5);
     tst12();
+    tst13();
     return has_violations() ? 1 : 0;
 }

src/util/list_fn.h

@@ -156,6 +156,32 @@ list<T> map(list<T> const & l, F && f) {
     }
 }
 
+/**
+   \brief Similar to \c map but \c f has signature
+
+       <tt>bool f(T const & in, T & out)</tt>
+
+   If \c out becomes part of the result iff \c f returns true.
+*/
+template<typename T, typename F>
+list<T> map_filter(list<T> const & l, F && f) {
+    if (is_nil(l)) {
+        return l;
+    } else {
+        buffer<typename list<T>::cell*> tmp;
+        to_buffer(l, tmp);
+        unsigned i = tmp.size();
+        list<T> r;
+        while (i > 0) {
+            --i;
+            T out;
+            if (f(tmp[i]->head(), out))
+                r = cons(out, r);
+        }
+        return r;
+    }
+}
+
 /**
    \brief Semantically equivalent to \c map, but it tries to reuse
    list cells. The elements are compared using the predicate \c eq.