merge(3p/immer): Subtree merge at 'ad3e3556d' as 'third_party/immer'

Change-Id: I9636a41ad44b4218293833fd3e9456d9b07c731b

third_party/immer/test/transient_tester.hpp

@@ -0,0 +1,54 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include "dada.hpp"
+
+namespace {
+
+template <typename VP, typename VT>
+struct transient_tester
+{
+    VP vp;
+    VT vt;
+    dadaism d      = {};
+    bool transient = false;
+
+    transient_tester(VP vp)
+        : vp{vp}
+        , vt{vp.transient()}
+    {}
+
+    bool step()
+    {
+        auto s = d.next();
+        if (soft_dada()) {
+            auto new_transient = !transient;
+            try {
+                if (new_transient)
+                    vt = vp.transient();
+                else
+                    vp = vt.persistent();
+            } catch (const dada_error&) {
+                return false;
+            }
+            transient = new_transient;
+            return true;
+        } else
+            return false;
+    }
+};
+
+template <typename VP>
+transient_tester<VP, typename VP::transient_type> as_transient_tester(VP p)
+{
+    return {std::move(p)};
+}
+
+} // anonymous namespace