QFlatMap: add remove_if

The existing API of QFlatMap did not allow efficient removal of
elements:

- std::remove_if does not apply, because it works by moving elements
  back in the range onto those that need to be removed, which doesn't
  work in flat_map's case, because, like for all associative
  containers, the key in value_type is const.

- The node-based erase-loop (over it = cond ? c.erase(it) :
  std::next(it)) works, but, unlike in traditional associative
  containers, is quadratic, because flat_map::erase is a linear
  operation.

According to Stepanov's principle of Efficient Computational Basis
(Elements of Programming, Section 1.4), we're therefore missing API.

Add it.

I couldn't make up my mind about the calling convention for the
predicate and, despite having authored a merged paper about erase_if,
can never remember what the predicate is supposed to take, so be fancy
and accept all: (*it), (it.key(), it.value()), (it.key()). This means
that unary predicates can either not be generic or must be properly
constrained to distinguish between pair<const K, V> and K, but that's
not necessarily a bad thing.

There's no reason to supply a Qt-ified removeIf on top of the standard
name, because this is private API and doubling the names would do
nothing except double the testing overhead.

Fixes: QTBUG-100983
Change-Id: I12545058958fc5d620baa770f92193c8de8b2d26
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 75 insertions, 0 deletions

diff --git a/src/corelib/tools/qflatmap_p.h b/src/corelib/tools/qflatmap_p.h
index 18d5dec652..7500cfb42d 100644
--- a/src/corelib/tools/qflatmap_p.h
+++ b/src/corelib/tools/qflatmap_p.h
@@ -863,6 +863,81 @@ public:
         return it;
     }
 
+    template <typename Predicate>
+    size_type remove_if(Predicate pred)
+    {
+        const auto indirect_call_to_pred = [pred = std::move(pred)](iterator it) {
+            auto dependent_false = [](auto &&...) { return false; };
+            using Pair = decltype(*it);
+            using K = decltype(it.key());
+            using V = decltype(it.value());
+            using P = Predicate;
+            if constexpr (std::is_invocable_v<P, K, V>) {
+                return pred(it.key(), it.value());
+            } else if constexpr (std::is_invocable_v<P, Pair> && !std::is_invocable_v<P, K>) {
+                return pred(*it);
+            } else if constexpr (std::is_invocable_v<P, K> && !std::is_invocable_v<P, Pair>) {
+                return pred(it.key());
+            } else {
+                static_assert(dependent_false(pred),
+                    "Don't know how to call the predicate.\n"
+                    "Options:\n"
+                    "- pred(*it)\n"
+                    "- pred(it.key(), it.value())\n"
+                    "- pred(it.key())");
+            }
+        };
+
+        auto first = begin();
+        const auto last = end();
+
+        // find_if prefix loop
+        while (first != last && !indirect_call_to_pred(first))
+            ++first;
+
+        if (first == last)
+            return 0; // nothing to do
+
+        // we know that we need to remove *first
+
+        auto kdest = toKeysIterator(first);
+        auto vdest = toValuesIterator(first);
+
+        ++first;
+
+        auto k = std::next(kdest);
+        auto v = std::next(vdest);
+
+        // Main Loop
+        // - first is used only for indirect_call_to_pred
+        // - operations are done on k, v
+        // Loop invariants:
+        // - first, k, v are pointing to the same element
+        // - [begin(), first[, [c.keys.begin(), k[, [c.values.begin(), v[: already processed
+        // - [first, end()[,   [k, c.keys.end()[,   [v, c.values.end()[:   still to be processed
+        // - [c.keys.begin(), kdest[ and [c.values.begin(), vdest[ are keepers
+        // - [kdest, k[, [vdest, v[ are considered removed
+        // - kdest is not c.keys.end()
+        // - vdest is not v.values.end()
+        while (first != last) {
+            if (!indirect_call_to_pred(first)) {
+                // keep *first, aka {*k, *v}
+                *kdest = std::move(*k);
+                *vdest = std::move(*v);
+                ++kdest;
+                ++vdest;
+            }
+            ++k;
+            ++v;
+            ++first;
+        }
+
+        const size_type r = std::distance(kdest, c.keys.end());
+        c.keys.erase(kdest, c.keys.end());
+        c.values.erase(vdest, c.values.end());
+        return r;
+    }
+
     key_compare key_comp() const noexcept
     {
         return static_cast<key_compare>(*this);