This introduces a reset functionality for object and array containers (#1639)

* This introduces a reset functionality.

* Minor simplification.

* Tweaking further.

* This should fix the tests.

.vscode/settings.json

@@ -5,6 +5,9 @@
     ],
     "files.trimTrailingWhitespace": true,
     "files.associations": {
-        "array": "cpp"
+        "array": "cpp",
+        "iterator": "cpp",
+        "chrono": "cpp",
+        "optional": "cpp"
     }
 }

include/simdjson/generic/ondemand/array-inl.h

@@ -73,9 +73,6 @@ simdjson_really_inline simdjson_result<array_iterator> array::end() noexcept {
 }
 
 simdjson_really_inline simdjson_result<size_t> array::count_elements() & noexcept {
-  // If the array is empty (i.e., we already scanned past it), then we use a
-  // fast path and return 0.
-  if(!iter.is_open()) { return 0; }
   size_t count{0};
   // Important: we do not consume any of the values.
   for(simdjson_unused auto v : *this) { count++; }
@@ -83,8 +80,7 @@ simdjson_really_inline simdjson_result<size_t> array::count_elements() & noexcep
   if(iter.error()) { return iter.error(); }
   // We need to move back at the start because we expect users to iterate through
   // the array after counting the number of elements.
-  // enter_at_container_start is safe here because we know that we do not have an empty array.
-  iter.enter_at_container_start(); // sets the depth to indicate that we are inside the container and accesses the first element
+  iter.reset_array();
   return count;
 }
 

include/simdjson/generic/ondemand/value_iterator-inl.h

@@ -194,9 +194,8 @@ simdjson_warn_unused simdjson_really_inline simdjson_result<bool> value_iterator
     // this object iterator will blithely scan that object for fields.
     if (_json_iter->depth() < depth() - 1) { return OUT_OF_ORDER_ITERATION; }
 #endif
-    _json_iter->reenter_child(_start_position + 1, _depth);
+    has_value = reset_object();
     at_first = true;
-    has_value = started_object();
   // 3. When a previous search found a field or an iterator yielded a value:
   //
   //    ```
@@ -288,8 +287,7 @@ simdjson_warn_unused simdjson_really_inline simdjson_result<bool> value_iterator
   // beginning of the object.
   // (We have already run through the object before, so we've already validated its structure. We
   // don't check errors in this bit.)
-  _json_iter->reenter_child(_start_position + 1, _depth);
-  has_value = started_object();
+  has_value = reset_object();
   while (true) {
     SIMDJSON_ASSUME(has_value); // we should reach search_start before ever reaching the end of the object
     SIMDJSON_ASSUME( _json_iter->_depth == _depth ); // We must be at the start of a field
@@ -628,15 +626,24 @@ inline void value_iterator::assert_at_next() const noexcept {
   SIMDJSON_ASSUME( _depth > 0 );
 }
 
+simdjson_really_inline void value_iterator::move_at_start() noexcept {
+  _json_iter->_depth = _depth;
+  _json_iter->token.index = _start_position;
+}
 
-simdjson_really_inline void value_iterator::enter_at_container_start() noexcept {
-  _json_iter->_depth = _depth + 1;
+simdjson_really_inline void value_iterator::move_at_container_start() noexcept {
+  _json_iter->_depth = _depth;
   _json_iter->token.index = _start_position + 1;
 }
 
-simdjson_really_inline void value_iterator::move_at_start() noexcept {
-    _json_iter->_depth = _depth;
-  _json_iter->token.index = _start_position;
+simdjson_really_inline bool value_iterator::reset_array() noexcept {
+  move_at_container_start();
+  return started_array();
+}
+
+simdjson_really_inline bool value_iterator::reset_object() noexcept {
+  move_at_container_start();
+  return started_object();
 }
 
 inline void value_iterator::assert_at_child() const noexcept {

include/simdjson/generic/ondemand/value_iterator.h

@@ -281,24 +281,32 @@ public:
 
   /** @} */
 protected:
+  /**
+   * Restarts an array iteration.
+   * @returns Whether the array has any elements (returns false for empty).
+   */
+  simdjson_really_inline bool reset_array() noexcept;
+  /**
+   * Restarts an object iteration.
+   * @returns Whether the object has any fields (returns false for empty).
+   */
+  simdjson_really_inline bool reset_object() noexcept;
   /**
    * move_at_start(): moves us so that we are pointing at the beginning of
    * the container. It updates the index so that at_start() is true and it
    * syncs the depth. The user can then create a new container instance.
    *
-   * Usage: used with value::count_elements()
+   * Usage: used with value::count_elements().
    **/
-   simdjson_really_inline void move_at_start() noexcept;
+  simdjson_really_inline void move_at_start() noexcept;
+
   /**
-   * enter_at_container_start moves at the beginning of the container
-   * and sets the depth to indicate that we are inside the
-   * container and ready to access the first element. It is only
-   * safely used with non-empty containers. The caller is responsible
-   * to ensure that the container is not empty!
+   * move_at_container_start(): moves us so that we are pointing at the beginning of
+   * the container so that assert_at_container_start() passes.
    *
-   * Usage: used with array::count_elements().
+   * Usage: used with reset_array() and reset_object().
    **/
-  simdjson_really_inline void enter_at_container_start() noexcept;
+   simdjson_really_inline void move_at_container_start() noexcept;
   /* Useful for debugging and logging purposes. */
   inline std::string to_string() const noexcept;
   simdjson_really_inline value_iterator(json_iterator *json_iter, depth_t depth, token_position start_index) noexcept;