Round-trip BJData ND-array annotations exactly (single precision, key order)

to_bjdata() encoded a JData-annotated object as a BJData ND-array in two
cases where from_bjdata() then returned a different value, breaking the
documented round-trip guarantee:

1. A "single" element that is finite and in range but not exactly
   representable as float (e.g. 0.1) or that underflows to 0 (e.g. 1e-300)
   was silently narrowed instead of falling back to a plain object, unlike
   out-of-range integer elements. write_bjdata_ndarray() now only accepts a
   "single" element if it survives the narrowing to float and back, the
   same criterion write_compact_float() already uses for CBOR/MessagePack.

2. from_bjdata() emitted the annotation keys as _ArraySize_, _ArrayType_,
   _ArrayData_ instead of the documented _ArrayType_, _ArraySize_,
   _ArrayData_, because the size key is written while the dimension vector
   is read, before the type key. For ordered_json, whose comparison takes
   key order into account, this made a round trip of the documented example
   compare unequal. The element type marker is known before the dimension
   vector is read (it precedes '#'), so it is now passed down and the
   "_ArrayType_" key is emitted first.

Fixes #5661.

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

docs/mkdocs/docs/features/binary_formats/bjdata.md

@@ -132,8 +132,14 @@ The library uses the following mapping from JSON values types to BJData types ac
       parsed back as a regular array,
     - every entry of `"_ArraySize_"` is a positive integer, and their product is representable as a `std::size_t`,
     - `"_ArrayData_"` is an array holding exactly that many elements, and
-    - every element of `"_ArrayData_"` is a number of the kind named by `"_ArrayType_"` (a floating-point number for
-      `single` and `double`, an integer otherwise).
+    - every element of `"_ArrayData_"` is a number of the kind named by `"_ArrayType_"`: for the integer types, a
+      value that fits the named width; for `double`, any value; for `single`, a value that survives narrowing to
+      `float` and back without change (for instance, `0.1` does not, since it is not exactly representable as
+      `float`).
+
+    An annotated object is always read back with its keys in the order shown above, `"_ArrayType_"`, `"_ArraySize_"`,
+    `"_ArrayData_"`, regardless of the order the ND-array's header stores them in on the wire. This matters for
+    `ordered_json`, whose comparison takes key order into account.
 
     The current version of this library does not yet support automatic detection of and conversion from a nested JSON
     array input to a BJData ND-array.

include/nlohmann/detail/input/binary_reader.hpp

@@ -2728,10 +2728,15 @@ class binary_reader
                                is_ndarray can only return `true` when its initial value
                                is `false`
     @param[in] prefix  type marker if already read, otherwise set to 0
+    @param[in] ndarray_dtype  the element type marker of the enclosing bjdata ndarray if
+                              already known (it precedes the dimension vector read here),
+                              otherwise 0; used to emit the "_ArrayType_" annotation key
+                              before "_ArraySize_" if a dimension vector turns out to
+                              describe an ndarray
 
     @return whether size determination completed
     */
-    bool get_ubjson_size_value(std::size_t& result, bool& is_ndarray, char_int_type prefix = 0)
+    bool get_ubjson_size_value(std::size_t& result, bool& is_ndarray, char_int_type prefix = 0, char_int_type ndarray_dtype = 0)
     {
         if (prefix == 0)
         {
@@ -2901,8 +2906,37 @@ class binary_reader
                         }
                     }
 
+                    if (JSON_HEDLEY_UNLIKELY(!sax->start_object(3)))
+                    {
+                        return false;
+                    }
+
+                    // the element type precedes the dimension vector (see get_ubjson_size_type)
+                    // and is passed down as ndarray_dtype; emit it here so the annotation keys
+                    // follow the documented _ArrayType_, _ArraySize_, _ArrayData_ order
+                    if (ndarray_dtype != 0)
+                    {
+                        auto it = std::lower_bound(bjd_types_map.begin(), bjd_types_map.end(), ndarray_dtype, [](const bjd_type & p, char_int_type t)
+                        {
+                            return p.first < t;
+                        });
+                        if (JSON_HEDLEY_UNLIKELY(it == bjd_types_map.end() || it->first != ndarray_dtype))
+                        {
+                            auto last_token = get_token_string();
+                            return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                                    exception_message(input_format, "invalid byte: 0x" + last_token, "type"), nullptr));
+                        }
+
+                        string_t type_key = "_ArrayType_";
+                        string_t type = it->second; // sax->string() takes a reference
+                        if (JSON_HEDLEY_UNLIKELY(!sax->key(type_key) || !sax->string(type)))
+                        {
+                            return false;
+                        }
+                    }
+
                     string_t key = "_ArraySize_";
-                    if (JSON_HEDLEY_UNLIKELY(!sax->start_object(3) || !sax->key(key) || !sax->start_array(dim.size())))
+                    if (JSON_HEDLEY_UNLIKELY(!sax->key(key) || !sax->start_array(dim.size())))
                     {
                         return false;
                     }
@@ -3003,7 +3037,7 @@ class binary_reader
                                         exception_message(input_format, concat("expected '#' after type information; last byte: 0x", last_token), "size"), nullptr));
             }
 
-            const bool is_error = get_ubjson_size_value(result.first, is_ndarray);
+            const bool is_error = get_ubjson_size_value(result.first, is_ndarray, 0, result.second);
             // an ndarray was read here only if the flag flipped; when it was
             // seeded true, get_ubjson_size_value() already rejected the nested
             // dimension vector
@@ -3239,30 +3273,17 @@ class binary_reader
         if (input_format == input_format_t::bjdata && size_and_type.first != npos && (size_and_type.second & (1 << 8)) != 0)
         {
             size_and_type.second &= ~(static_cast<char_int_type>(1) << 8);  // use bit 8 to indicate ndarray, here we remove the bit to restore the type marker
-            auto it = std::lower_bound(bjd_types_map.begin(), bjd_types_map.end(), size_and_type.second, [](const bjd_type & p, char_int_type t)
-            {
-                return p.first < t;
-            });
-            string_t key = "_ArrayType_";
-            if (JSON_HEDLEY_UNLIKELY(it == bjd_types_map.end() || it->first != size_and_type.second))
-            {
-                auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
-                                        exception_message(input_format, "invalid byte: 0x" + last_token, "type"), nullptr));
-            }
-
-            string_t type = it->second; // sax->string() takes a reference
-            if (JSON_HEDLEY_UNLIKELY(!sax->key(key) || !sax->string(type)))
-            {
-                return false;
-            }
 
+            // the "_ArrayType_" and "_ArraySize_" annotation keys were already emitted by
+            // get_ubjson_size_value() (the type marker is known before the dimension vector
+            // that determines size_and_type.first is read, so it is emitted first there to
+            // match the documented _ArrayType_, _ArraySize_, _ArrayData_ key order)
             if (size_and_type.second == 'C' || size_and_type.second == 'B')
             {
                 size_and_type.second = 'U';
             }
 
-            key = "_ArrayData_";
+            string_t key = "_ArrayData_";
             if (JSON_HEDLEY_UNLIKELY(!sax->key(key) || !sax->start_array(size_and_type.first) ))
             {
                 return false;

include/nlohmann/detail/output/binary_writer.hpp

@@ -1991,9 +1991,21 @@ class binary_writer
                 case 'd':
                 {
                     const auto dval = el.template get<double>();
-                    in_range = !std::isfinite(dval) ||
+#ifdef __GNUC__
+                    JSON_HEDLEY_DIAGNOSTIC_PUSH
+                    JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
+#endif
+                    // a value that would be rounded (rather than exactly represented) by the
+                    // narrowing to float is treated like an out-of-range integer element above;
+                    // this is the same criterion write_compact_float() uses for CBOR/MessagePack
+                    in_range = std::isnan(dval) ||
                                (dval >= static_cast<double>(std::numeric_limits<float>::lowest()) &&
-                                dval <= static_cast<double>((std::numeric_limits<float>::max)()));
+                                dval <= static_cast<double>((std::numeric_limits<float>::max)()) &&
+                                static_cast<double>(static_cast<float>(dval)) == dval) ||
+                               std::isinf(dval);
+#ifdef __GNUC__
+                    JSON_HEDLEY_DIAGNOSTIC_POP
+#endif
                     break;
                 }
                 default:

single_include/nlohmann/json.hpp

@@ -15495,10 +15495,15 @@ class binary_reader
                                is_ndarray can only return `true` when its initial value
                                is `false`
     @param[in] prefix  type marker if already read, otherwise set to 0
+    @param[in] ndarray_dtype  the element type marker of the enclosing bjdata ndarray if
+                              already known (it precedes the dimension vector read here),
+                              otherwise 0; used to emit the "_ArrayType_" annotation key
+                              before "_ArraySize_" if a dimension vector turns out to
+                              describe an ndarray
 
     @return whether size determination completed
     */
-    bool get_ubjson_size_value(std::size_t& result, bool& is_ndarray, char_int_type prefix = 0)
+    bool get_ubjson_size_value(std::size_t& result, bool& is_ndarray, char_int_type prefix = 0, char_int_type ndarray_dtype = 0)
     {
         if (prefix == 0)
         {
@@ -15668,8 +15673,37 @@ class binary_reader
                         }
                     }
 
+                    if (JSON_HEDLEY_UNLIKELY(!sax->start_object(3)))
+                    {
+                        return false;
+                    }
+
+                    // the element type precedes the dimension vector (see get_ubjson_size_type)
+                    // and is passed down as ndarray_dtype; emit it here so the annotation keys
+                    // follow the documented _ArrayType_, _ArraySize_, _ArrayData_ order
+                    if (ndarray_dtype != 0)
+                    {
+                        auto it = std::lower_bound(bjd_types_map.begin(), bjd_types_map.end(), ndarray_dtype, [](const bjd_type & p, char_int_type t)
+                        {
+                            return p.first < t;
+                        });
+                        if (JSON_HEDLEY_UNLIKELY(it == bjd_types_map.end() || it->first != ndarray_dtype))
+                        {
+                            auto last_token = get_token_string();
+                            return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                                    exception_message(input_format, "invalid byte: 0x" + last_token, "type"), nullptr));
+                        }
+
+                        string_t type_key = "_ArrayType_";
+                        string_t type = it->second; // sax->string() takes a reference
+                        if (JSON_HEDLEY_UNLIKELY(!sax->key(type_key) || !sax->string(type)))
+                        {
+                            return false;
+                        }
+                    }
+
                     string_t key = "_ArraySize_";
-                    if (JSON_HEDLEY_UNLIKELY(!sax->start_object(3) || !sax->key(key) || !sax->start_array(dim.size())))
+                    if (JSON_HEDLEY_UNLIKELY(!sax->key(key) || !sax->start_array(dim.size())))
                     {
                         return false;
                     }
@@ -15770,7 +15804,7 @@ class binary_reader
                                         exception_message(input_format, concat("expected '#' after type information; last byte: 0x", last_token), "size"), nullptr));
             }
 
-            const bool is_error = get_ubjson_size_value(result.first, is_ndarray);
+            const bool is_error = get_ubjson_size_value(result.first, is_ndarray, 0, result.second);
             // an ndarray was read here only if the flag flipped; when it was
             // seeded true, get_ubjson_size_value() already rejected the nested
             // dimension vector
@@ -16006,30 +16040,17 @@ class binary_reader
         if (input_format == input_format_t::bjdata && size_and_type.first != npos && (size_and_type.second & (1 << 8)) != 0)
         {
             size_and_type.second &= ~(static_cast<char_int_type>(1) << 8);  // use bit 8 to indicate ndarray, here we remove the bit to restore the type marker
-            auto it = std::lower_bound(bjd_types_map.begin(), bjd_types_map.end(), size_and_type.second, [](const bjd_type & p, char_int_type t)
-            {
-                return p.first < t;
-            });
-            string_t key = "_ArrayType_";
-            if (JSON_HEDLEY_UNLIKELY(it == bjd_types_map.end() || it->first != size_and_type.second))
-            {
-                auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
-                                        exception_message(input_format, "invalid byte: 0x" + last_token, "type"), nullptr));
-            }
-
-            string_t type = it->second; // sax->string() takes a reference
-            if (JSON_HEDLEY_UNLIKELY(!sax->key(key) || !sax->string(type)))
-            {
-                return false;
-            }
 
+            // the "_ArrayType_" and "_ArraySize_" annotation keys were already emitted by
+            // get_ubjson_size_value() (the type marker is known before the dimension vector
+            // that determines size_and_type.first is read, so it is emitted first there to
+            // match the documented _ArrayType_, _ArraySize_, _ArrayData_ key order)
             if (size_and_type.second == 'C' || size_and_type.second == 'B')
             {
                 size_and_type.second = 'U';
             }
 
-            key = "_ArrayData_";
+            string_t key = "_ArrayData_";
             if (JSON_HEDLEY_UNLIKELY(!sax->key(key) || !sax->start_array(size_and_type.first) ))
             {
                 return false;
@@ -22321,9 +22342,21 @@ class binary_writer
                 case 'd':
                 {
                     const auto dval = el.template get<double>();
-                    in_range = !std::isfinite(dval) ||
+#ifdef __GNUC__
+                    JSON_HEDLEY_DIAGNOSTIC_PUSH
+                    JSON_HEDLEY_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
+#endif
+                    // a value that would be rounded (rather than exactly represented) by the
+                    // narrowing to float is treated like an out-of-range integer element above;
+                    // this is the same criterion write_compact_float() uses for CBOR/MessagePack
+                    in_range = std::isnan(dval) ||
                                (dval >= static_cast<double>(std::numeric_limits<float>::lowest()) &&
-                                dval <= static_cast<double>((std::numeric_limits<float>::max)()));
+                                dval <= static_cast<double>((std::numeric_limits<float>::max)()) &&
+                                static_cast<double>(static_cast<float>(dval)) == dval) ||
+                               std::isinf(dval);
+#ifdef __GNUC__
+                    JSON_HEDLEY_DIAGNOSTIC_POP
+#endif
                     break;
                 }
                 default:

tests/src/unit-bjdata.cpp

@@ -11,6 +11,7 @@
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using nlohmann::json;
+using ordered_json = nlohmann::ordered_json;
 
 #include <algorithm>
 #include <climits>
@@ -2294,29 +2295,33 @@ TEST_CASE("BJData")
 
         SECTION("start_array() in ndarray _ArraySize_")
         {
+            // _ArrayType_ (2 events: key + string) is now emitted before
+            // _ArraySize_ (see GitHub issue #5661), which shifts the events
+            // below later by the same 2 events
             std::vector<uint8_t> const v = {'[', '$', 'i', '#', '[', '$', 'i', '#', 'i', 2, 2, 1, 1, 2};
-            SaxCountdown scp(2);
+            SaxCountdown scp(4);
             CHECK_FALSE(json::sax_parse(v, &scp, json::input_format_t::bjdata));
         }
 
         SECTION("number_integer() in ndarray _ArraySize_")
         {
             std::vector<uint8_t> const v = {'[', '$', 'U', '#', '[', '$', 'i', '#', 'i', 2, 2, 1, 1, 2};
-            SaxCountdown scp(3);
+            SaxCountdown scp(5);
             CHECK_FALSE(json::sax_parse(v, &scp, json::input_format_t::bjdata));
         }
 
         SECTION("key() in ndarray _ArrayType_")
         {
+            // _ArrayType_ is emitted right after start_object(), before _ArraySize_
             std::vector<uint8_t> const v = {'[', '$', 'U', '#', '[', '$', 'U', '#', 'i', 2, 2, 2, 1, 2, 3, 4};
-            SaxCountdown scp(6);
+            SaxCountdown scp(1);
             CHECK_FALSE(json::sax_parse(v, &scp, json::input_format_t::bjdata));
         }
 
         SECTION("string() in ndarray _ArrayType_")
         {
             std::vector<uint8_t> const v = {'[', '$', 'U', '#', '[', '$', 'U', '#', 'i', 2, 2, 2, 1, 2, 3, 4};
-            SaxCountdown scp(7);
+            SaxCountdown scp(2);
             CHECK_FALSE(json::sax_parse(v, &scp, json::input_format_t::bjdata));
         }
 
@@ -2919,6 +2924,22 @@ TEST_CASE("BJData")
                 CHECK(out_single.at(0) == '{');
                 CHECK(json::from_bjdata(out_single) == j_single);
 
+                // a double element that is finite and within the range of "single"
+                // but is not exactly representable as a float, so narrowing it would
+                // silently round it (0.1 is read back as 0.10000000149011612); this,
+                // like the overflow case above, falls back to a plain object (see
+                // GitHub issue #5661)
+                json const j_single_rounded = json({{"_ArrayType_", "single"}, {"_ArraySize_", {2, 1}}, {"_ArrayData_", {1.5, 0.1}}});
+                const auto out_single_rounded = json::to_bjdata(j_single_rounded);
+                CHECK(out_single_rounded.at(0) == '{');
+                CHECK(json::from_bjdata(out_single_rounded) == j_single_rounded);
+
+                // a double element that underflows to 0 when narrowed to "single"
+                json const j_single_underflow = json({{"_ArrayType_", "single"}, {"_ArraySize_", {2, 1}}, {"_ArrayData_", {1.5, 1e-300}}});
+                const auto out_single_underflow = json::to_bjdata(j_single_underflow);
+                CHECK(out_single_underflow.at(0) == '{');
+                CHECK(json::from_bjdata(out_single_underflow) == j_single_underflow);
+
                 // in-range boundary values still use the compact ndarray encoding
                 json const j_uint8_ok = json({{"_ArrayType_", "uint8"}, {"_ArraySize_", {2, 1}}, {"_ArrayData_", {0, 255}}});
                 CHECK(json::to_bjdata(j_uint8_ok) == std::vector<uint8_t>({'[', '$', 'U', '#', '[', 'i', 2, 'i', 1, ']', 0, 255}));
@@ -2932,6 +2953,23 @@ TEST_CASE("BJData")
                 CHECK(json::from_bjdata(out_single_ok) == json({{"_ArrayType_", "single"}, {"_ArraySize_", {2, 1}}, {"_ArrayData_", {1.5f, -1.5f}}}));
             }
 
+            SECTION("ndarray annotation keys are read back in the documented order")
+            {
+                // from_bjdata() must emit the annotation object's keys in the order
+                // used throughout the documentation, _ArrayType_, _ArraySize_,
+                // _ArrayData_: the type marker precedes the dimension vector on the
+                // wire (see get_ubjson_size_type()), so it is known, and emitted,
+                // before _ArraySize_. For a plain json this key order is invisible
+                // (its comparison ignores it), but for an ordered_json it is not (see
+                // GitHub issue #5661).
+                const ordered_json o = ordered_json::parse(R"({"_ArrayType_":"uint8","_ArraySize_":[2,2],"_ArrayData_":[1,2,3,4]})");
+                const auto packed = ordered_json::to_bjdata(o);
+                CHECK(packed.at(0) == '[');
+                const ordered_json o_back = ordered_json::from_bjdata(packed);
+                CHECK(o_back == o);
+                CHECK(o_back.dump() == o.dump());
+            }
+
             SECTION("ndarray that would not be read back as an annotated object stays as object")
             {
                 // the reader only restores an annotated object from an ND-array