jinja : fix quadratic cost in gather_string_parts (#27034)

* jinja : fix quadratic cost in gather_string_parts

* fix some comments

* remove test

common/jinja/runtime.cpp

@@ -263,7 +263,7 @@ value binary_expression::execute_impl(context & ctx) {
             return res;
         }
         for (int64_t i = 0; i < repeat; ++i) {
-            res->val_str = res->val_str.append(str);
+            res->val_str.append(str);
         }
         return res;
     }

common/jinja/runtime.h

@@ -763,14 +763,22 @@ struct runtime {
         gather_string_parts_recursive(val, parts);
         // join consecutive parts with the same type
         auto & p = parts->val_str.parts;
-        for (size_t i = 1; i < p.size(); ) {
-            if (p[i].is_input == p[i - 1].is_input) {
-                p[i - 1].val += p[i].val;
-                p.erase(p.begin() + i);
+        if (p.empty()) {
+            return parts;
+        }
+        size_t w = 0;
+        for (size_t r = 1; r < p.size(); r++) {
+            if (p[w].is_input == p[r].is_input) {
+                p[w].val += p[r].val;
             } else {
-                i++;
+                w++;
+                if (w != r) {
+                    // the guard is needed, self-move leaves the string in an unspecified state
+                    p[w] = std::move(p[r]);
+                }
             }
         }
+        p.resize(w + 1);
         return parts;
     }
 

common/jinja/string.cpp

@@ -103,7 +103,7 @@ void string::mark_input_based_on(const string & other) {
     }
 }
 
-string string::append(const string & other) {
+string & string::append(const string & other) {
     for (const auto & part : other.parts) {
         parts.push_back(part);
     }

common/jinja/string.h

@@ -47,7 +47,7 @@ struct string {
     // mark this string as input if other has ALL parts as input
     void mark_input_based_on(const string & other);
 
-    string append(const string & other);
+    string & append(const string & other);
 
     // in-place transformations