jinja : fix quadratic cost in gather_string_parts (#27034)

* jinja : fix quadratic cost in gather_string_parts

* fix some comments

* remove test

tests/test-jinja.cpp

@@ -33,6 +33,7 @@ static void test_array_methods(testing & t);
 static void test_object_methods(testing & t);
 static void test_hasher(testing & t);
 static void test_stats(testing & t);
+static void test_string_parts(testing & t);
 static void test_fuzzing(testing & t);
 
 static bool g_python_mode = false;
@@ -72,6 +73,7 @@ int main(int argc, char *argv[]) {
     if (!g_python_mode) {
         t.test("hasher", test_hasher);
         t.test("stats", test_stats);
+        t.test("string parts", test_string_parts);
         t.test("fuzzing", test_fuzzing);
     }
 
@@ -2057,6 +2059,36 @@ static void test_stats(testing & t) {
     });
 }
 
+static void test_string_parts(testing & t) {
+    static auto render = [](const std::string & tmpl, const json & vars) -> jinja::string {
+        jinja::lexer lexer;
+        auto lexer_res = lexer.tokenize(tmpl);
+
+        jinja::program ast = jinja::parse_from_tokens(lexer_res);
+
+        jinja::context ctx(tmpl);
+        jinja::global_from_json(ctx, vars, true);
+
+        jinja::runtime runtime(ctx);
+        return runtime.gather_string_parts(runtime.execute(ast))->as_string();
+    };
+
+    t.test("merge joins only the neighbours with the same type", [](testing & t) {
+        // "AB" comes from the input and merges, "-" comes from the template and must not
+        jinja::string res = render("{{ val.a }}{{ val.b }}-{{ val.c }}",
+                                   json{{"val", json{{"a", "A"}, {"b", "B"}, {"c", "C"}}}});
+
+        if (t.assert_true("3 parts after the merge", res.parts.size() == 3)) {
+            t.assert_true("part 0 is the merged input", res.parts[0].val == "AB" && res.parts[0].is_input);
+            t.assert_true("part 1 is from the template", res.parts[1].val == "-" && !res.parts[1].is_input);
+            t.assert_true("part 2 is input",             res.parts[2].val == "C" && res.parts[2].is_input);
+        } else {
+            t.log("parts: " + std::to_string(res.parts.size()) + ", rendered: " + json(res.str()).dump());
+        }
+    });
+
+}
+
 static void test_template_cpp(testing & t, const std::string & name, const std::string & tmpl, const json & vars, const std::string & expect) {
     t.test(name, [&tmpl, &vars, &expect](testing & t) {
         jinja::lexer lexer;