feat(util/list): improved filter that reuses list cells

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

src/tests/util/list.cpp

@@ -209,6 +209,15 @@ static void tst16() {
     lean_assert_eq(map(list<int>(), [](int i) { return i + 10; }), list<int>());
 }
 
+static void tst17() {
+    list<int> l({1, 2, 3, 4});
+    lean_assert(is_eqp(filter(l, [](int v) { return v < 10; }), l));
+    list<int> l2(filter(l, [](int v) { return v != 1; }));
+    lean_assert(is_eqp(tail(l), l2));
+    list<int> l3(filter(l, [](int v) { return v != 2; }));
+    lean_assert(is_eqp(tail(tail(l)), tail(l2)));
+}
+
 int main() {
     tst1();
     tst2();
@@ -226,5 +235,6 @@ int main() {
     tst14();
     tst15();
     tst16();
+    tst17();
     return has_violations() ? 1 : 0;
 }

src/util/list_fn.h

@@ -158,7 +158,7 @@ list<T> map(list<T> const & l, F && f) {
 
 /**
    \brief Filter/Remove elements from the list
-   that satisfy the given predicate.
+   that do not satisfy the given predicate.
 */
 template<typename T, typename P>
 list<T> filter(list<T> const & l, P && p) {
@@ -168,7 +168,10 @@ list<T> filter(list<T> const & l, P && p) {
         buffer<typename list<T>::cell*> tmp;
         to_buffer(l, tmp);
         unsigned i = tmp.size();
-        list<T> r;
+        while (i > 0) {
+            --i;
+            if (!p(tmp[i]->head())) {
+                list<T> r = tmp[i]->tail();
                 while (i > 0) {
                     --i;
                     if (p(tmp[i]->head()))
@@ -177,6 +180,9 @@ list<T> filter(list<T> const & l, P && p) {
                 return r;
             }
         }
+        return l; // not element was removed
+    }
+}
 
 /**
    \brief Similar to \c map but \c f has signature