Add insert() and erase() to editable documents

- insert(array, index, value): insert before an element (index <= size)
- erase(object, key): remove all members with the key; returns their
  number
- erase(array, index): remove an element
- erase(json_pointer): remove the member or element a pointer names

The errors are those of basic_json (type_error.307/309,
out_of_range.401/403/405). A view of an erased value keeps its last value,
and views of other values keep referring to them when elements move.

Tests: the differential test now also inserts and erases members and
elements, directly and through JSON pointers; plus the errors, views
across inserts and erasures, duplicate keys, and large objects.

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

include/nlohmann/detail/view/edit.hpp

@@ -227,6 +227,62 @@ class editor
         return View(&m_doc, slot);
     }
 
+    /// insert into an array before position idx (idx <= size()); returns a
+    /// view of the new element
+    template<typename V>
+    View insert(const View& array, std::size_t idx, V&& value)
+    {
+        node* const a = own(array);
+        if (a->kind != static_cast<std::uint8_t>(value_t::array))
+        {
+            throw_type_error(309, "cannot use insert() with ", array.type_name());
+        }
+        check_index(idx, a->len + 1);
+        const encoded e = encode(std::forward<V>(value));
+        node* const slot = new_slot(e);
+        node* const h = block_of(m_doc, a, 1);
+        std::memmove(h + 2 + idx, h + 1 + idx, (h->next - 1 - idx) * sizeof(node));
+        make_link(h[1 + idx], slot);
+        ++h->next;
+        ++h->len;
+        ++a->len;
+        return View(&m_doc, slot);
+    }
+
+    /// remove all members with this key; returns their number
+    std::size_t erase(const View& object, string_view_t key)
+    {
+        node* const o = own(object);
+        if (o->kind != static_cast<std::uint8_t>(value_t::object))
+        {
+            throw_type_error(307, "cannot use erase() with ", object.type_name());
+        }
+        for (const node* k = nav::first(m_doc, o), *end = nav::end(m_doc, o); k != end; k = document_data::after(k + 1))
+        {
+            if (key_equals(*k, key))
+            {
+                return erase_members(o, key, false);
+            }
+        }
+        return 0;
+    }
+
+    /// remove an array element
+    void erase(const View& array, std::size_t idx)
+    {
+        node* const a = own(array);
+        if (a->kind != static_cast<std::uint8_t>(value_t::array))
+        {
+            throw_type_error(307, "cannot use erase() with ", array.type_name());
+        }
+        check_index(idx, a->len);
+        node* const h = block_of(m_doc, a, 0);
+        std::memmove(h + 1 + idx, h + 2 + idx, (h->next - 2 - idx) * sizeof(node));
+        --h->next;
+        --h->len;
+        --a->len;
+    }
+
   private:
     /// an encoded value: a scalar node, or the root of a new array/object
     struct encoded

include/nlohmann/json_view.hpp

@@ -1061,6 +1061,45 @@ class basic_json_document
         return editor().push_back(array, std::forward<V>(value));
     }
 
+    /// insert into an array before position idx (idx <= size()); returns a
+    /// view of the new element
+    template < typename I, typename V, typename std::enable_if < std::is_integral<I>::value && !std::is_same<I, bool>::value, int >::type = 0 >
+    view_type insert(view_type array, I idx, V && value)
+    {
+        return editor().insert(array, index(idx), std::forward<V>(value));
+    }
+
+    /// remove all members with this key; returns their number
+    std::size_t erase(view_type object, string_view_t key)
+    {
+        return editor().erase(object, key);
+    }
+
+    /// remove an array element
+    template < typename I, typename std::enable_if < std::is_integral<I>::value && !std::is_same<I, bool>::value, int >::type = 0 >
+    void erase(view_type array, I idx)
+    {
+        editor().erase(array, index(idx));
+    }
+
+    /// remove the value at a JSON pointer; returns the number of removed
+    /// values
+    std::size_t erase(const json_pointer& ptr)
+    {
+        if (ptr.empty())
+        {
+            detail::view::throw_out_of_range(405, "JSON pointer has no parent");
+        }
+        const view_type parent = root().at(ptr.parent_pointer());
+        const auto& token = ptr.back();
+        if (parent.is_array())
+        {
+            erase(parent, pointer_index(token));
+            return 1;
+        }
+        return erase(parent, string_view_t(token.data(), token.size()));
+    }
+
   private:
     using input_kind = detail::view::input_kind;