State the exact input size limit of json_document

The check rejects inputs of 0xFFFFFFF0 bytes or more, but the exception
message and the documentation said 4 GiB. Name the limit once
(max_input_size), and state 4 GiB minus 16 bytes in the message and the
documentation. Test the limit with a container that only claims the size.

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

include/nlohmann/detail/view/errors.hpp

@@ -55,9 +55,8 @@ template<typename BasicJsonType>
 {
     if (f.code == error_code::input_too_large)
     {
-        // LCOV_EXCL_START (4 GiB)
-        NLOHMANN_VIEW_THROW(out_of_range::create(416, "input of 4 GiB or more is not supported by json_document", nullptr));
-        // LCOV_EXCL_STOP
+        // (the limit is detail::view::max_input_size: 4 GiB minus 16 bytes)
+        NLOHMANN_VIEW_THROW(out_of_range::create(416, "input of 4294967280 bytes or more is not supported by json_document", nullptr));
     }
     const BasicJsonType accepted = BasicJsonType::parse(src, src + size, nullptr, true, ignore_comments, ignore_trailing_commas);
     // LCOV_EXCL_START (only if parse() accepts what the view rejects: a bug)

include/nlohmann/detail/view/node.hpp

@@ -29,6 +29,11 @@ static_assert(static_cast<std::uint8_t>(value_t::null) == 0 && static_cast<std::
               && static_cast<std::uint8_t>(value_t::number_unsigned) == 6 && static_cast<std::uint8_t>(value_t::number_float) == 7,
               "the node format depends on the numbering of value_t");
 
+/// The largest input a document accepts, in bytes. Offsets and node counts are
+/// 32 bits wide; the limit keeps 16 bytes (the width of the scanner's steps)
+/// below 2^32, so that a position one step past the end of the text fits.
+static constexpr std::size_t max_input_size = 0xFFFFFFEFu;
+
 /// node flags
 struct node_flags
 {

include/nlohmann/json_view.hpp

@@ -475,9 +475,9 @@ class basic_json_document
         d.discarded = true;
         detail::view::parse_failure failure;
         bool ok = false;
-        if (NLOHMANN_VIEW_UNLIKELY(size >= 0xFFFFFFF0u))
+        if (NLOHMANN_VIEW_UNLIKELY(size > detail::view::max_input_size))
         {
-            failure.code = detail::view::error_code::input_too_large; // LCOV_EXCL_LINE (4 GiB)
+            failure.code = detail::view::error_code::input_too_large;
         }
         else
         {