Walk the index of json_view through a navigation policy

Preparation for editable documents, without a change in behavior: views and
documents get a template parameter Editable (false by default), and every
walk over the index (iterators, lookups, dump(), materialize()) goes
through detail::view::navigation<Editable>. For read-only documents it is
the plain node array, as before, so they compile without any of the edit
handling. For editable documents it also follows the representation of
edits, which this commit defines:

- node flags `edited` (a string or number token in the edit arena),
  `moved` (the elements of an array/object live in a separate sequence),
  and `is_new` (no source position), and link nodes (kind_link) that
  stand for a value stored elsewhere
- document_data::edit_state: the moved sequences, the storage of new
  values, and the edit arena

materialize() now keeps a frame per open container instead of returning
to the end of a closed one, as the serializer does, so that it can
follow moved sequences. Floats whose token lives in the edit arena (also
"nan", "inf", "-inf") are converted out of line. dump() copies only
strings of the source without escaping, and shrink_to_fit() leaves the
node array in place once there are edits, as they link into it.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/materialize.hpp

@@ -34,17 +34,24 @@ iterative, so the nesting depth is limited by memory only, as for parse().
 Without a lexer the handler records no source positions
 (JSON_DIAGNOSTIC_POSITIONS).
 */
-template<typename BasicJsonType>
+template<typename BasicJsonType, bool Editable>
 BasicJsonType materialize(const document_data& d, const node* n)
 {
     using string_t = typename BasicJsonType::string_t;
     using sax_t = json_sax_dom_parser<BasicJsonType, iterator_input_adapter<const char*>>;
+    using nav = navigation<Editable>;
+
+    struct frame
+    {
+        const node* pos; ///< next element, or key of the next member
+        const node* end;
+        bool object;
+    };
 
     BasicJsonType result;
     sax_t sax(result, true);
     const string_t no_token{};
-    // the ends of the open containers, and whether they are objects
-    std::vector<std::pair<const node*, bool>> open;
+    std::vector<frame> open;
     for (;;)
     {
         switch (static_cast<value_t>(n->kind))
@@ -61,67 +68,66 @@ BasicJsonType materialize(const document_data& d, const node* n)
                 {
                     sax.start_array(n->len);
                 }
-                open.emplace_back(document_data::child_end(n), object);
-                n = document_data::first_child(n);
+                open.push_back(frame{nav::first(d, n), nav::end(d, n), object});
                 break;
             }
             case value_t::string:
             {
                 string_t s(d.str(*n), n->len);
                 sax.string(s);
-                ++n;
                 break;
             }
             case value_t::number_integer:
                 sax.number_integer(static_cast<typename BasicJsonType::number_integer_t>(static_cast<std::int64_t>(integer_bits(*n))));
-                ++n;
                 break;
             case value_t::number_unsigned:
                 sax.number_unsigned(static_cast<typename BasicJsonType::number_unsigned_t>(integer_bits(*n)));
-                ++n;
                 break;
             case value_t::number_float:
                 sax.number_float(float_value<typename BasicJsonType::number_float_t>(d, *n), no_token);
-                ++n;
                 break;
             case value_t::boolean:
                 sax.boolean((n->flags & node_flags::is_true) != 0);
-                ++n;
                 break;
             case value_t::null:
             case value_t::binary:
             case value_t::discarded:
             default:
                 sax.null();
-                ++n;
                 break;
         }
+
+        // the next value: close finished containers, then read the key
         for (;;)
         {
             if (open.empty())
             {
                 return result;
             }
-            if (n != open.back().first)
+            frame& f = open.back();
+            if (f.pos == f.end)
             {
-                break;
+                if (f.object)
+                {
+                    sax.end_object();
+                }
+                else
+                {
+                    sax.end_array();
+                }
+                open.pop_back();
+                continue;
             }
-            if (open.back().second)
+            const node* entry = f.pos;
+            if (f.object)
             {
-                sax.end_object();
+                string_t key(d.str(*entry), entry->len);
+                sax.key(key);
+                ++entry;
             }
-            else
-            {
-                sax.end_array();
-            }
-            open.pop_back();
-        }
-        if (open.back().second)
-        {
-            // the key of the next member
-            string_t key(d.str(*n), n->len);
-            sax.key(key);
-            ++n;
+            n = nav::value(entry);
+            f.pos = document_data::after(entry);
+            break;
         }
     }
 }