Read BON8 strings in bulk from contiguous input

- copy the valid UTF-8 of a string in one step when the input is
  contiguous (twitter.json is read in 1.68 instead of 2.52 ms,
  jeopardy.json in 196 instead of 297 ms, close to CBOR and MessagePack)
- share the new valid_utf8_prefix() with the writer's UTF-8 check, which
  now skips ASCII 8 bytes at a time
- let the fuzzer check that contiguous and stream input give the same
  value or error, and test both paths in the unit tests
- clarify that a second 0xFF after a string is an empty string

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

docs/mkdocs/docs/features/binary_formats/bon8.md

@@ -137,8 +137,8 @@ Non-negative integers are read as number_unsigned, negative integers as number_i
 !!! info
 
     Values that do not use the canonical representation, such as integers with a longer encoding than necessary,
-    arrays and objects with up to four elements that are terminated by 0xFE, unsorted object keys, or unneeded 0xFF
-    bytes after a string, are accepted.
+    arrays and objects with up to four elements that are terminated by 0xFE, unsorted object keys, or a 0xFF after a
+    string that would also end without it, are accepted. A second 0xFF is not a terminator but an empty string.
 
     Strings must be valid UTF-8, and the last string of a message must be terminated by 0xFF.
 

include/nlohmann/detail/input/binary_reader.hpp

@@ -28,6 +28,7 @@
 #include <nlohmann/detail/input/input_adapters.hpp>
 #include <nlohmann/detail/input/json_sax.hpp>
 #include <nlohmann/detail/input/lexer.hpp>
+#include <nlohmann/detail/input/string_scan.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/is_sax.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
@@ -97,6 +98,11 @@ class binary_reader
     using char_type = typename InputAdapterType::char_type;
     using char_int_type = typename char_traits<char_type>::int_type;
 
+    /// whether the input is a contiguous block of bytes that BON8 strings can
+    /// be copied from in bulk; see @ref get_bon8_string_bulk
+    static constexpr bool bon8_bulk_scan =
+        input_adapter_supports_bulk_scan<InputAdapterType>(is_detected<detect_supports_bulk_scan, InputAdapterType> {});
+
   public:
     /*!
     @brief create a binary reader
@@ -3589,6 +3595,42 @@ class binary_reader
         return bon8_error("expected a string; last byte", "key");
     }
 
+    /*!
+    @brief append the run of valid UTF-8 at the read position to a string
+
+    For contiguous input, the ASCII characters and complete well-formed UTF-8
+    sequences at the read position are appended to @a result in one step. The
+    byte that stops the run (an end-of-string marker, the first byte of the
+    next value, or an ill-formed byte) is left for @ref get_bon8_string, so
+    that strings end and errors are reported exactly as without this step.
+
+    @param[in,out] result  the string to append to
+    */
+    void get_bon8_string_bulk(string_t& result, std::true_type /*bulk*/)
+    {
+        // bytes handed back must be read through get_bon8() first
+        if (bon8_pushback_size != 0)
+        {
+            return;
+        }
+        const std::size_t remaining = ia.bulk_remaining();
+        if (remaining == 0)
+        {
+            return;
+        }
+        const auto* const data = reinterpret_cast<const unsigned char*>(ia.bulk_data());
+        const std::size_t length = valid_utf8_prefix(data, remaining);
+        if (length != 0)
+        {
+            result.append(reinterpret_cast<const typename string_t::value_type*>(data), length);
+            ia.bulk_skip(length);
+            chars_read += length;
+        }
+    }
+
+    /// input that is not contiguous: strings are read byte by byte
+    void get_bon8_string_bulk(string_t& /*result*/, std::false_type /*bulk*/) const noexcept {}
+
     /*!
     @brief read a string
 
@@ -3605,6 +3647,8 @@ class binary_reader
     {
         while (true)
         {
+            get_bon8_string_bulk(result, std::integral_constant<bool, bon8_bulk_scan> {});
+
             const auto byte = get_bon8();
 
             if (byte == char_traits<char_type>::eof())

include/nlohmann/detail/input/string_scan.hpp

@@ -201,6 +201,43 @@ inline std::size_t validate_one_utf8(const unsigned char* data, std::size_t avai
     return 0; // invalid, incomplete, or must be diagnosed by the byte path
 }
 
+// Return the length of the longest prefix of [data, data+n) that consists of
+// ASCII characters and complete well-formed UTF-8 sequences; n if all of it is
+// valid UTF-8. Unlike scalar_string_bulk_run(), quotes, escapes, and control
+// characters are ordinary characters here. ASCII is skipped 8 bytes at a time.
+inline std::size_t valid_utf8_prefix(const unsigned char* data, std::size_t n) noexcept
+{
+    constexpr std::uint64_t high = 0x8080808080808080ull;
+    std::size_t pos = 0;
+    while (pos < n)
+    {
+        if (pos + 8 <= n)
+        {
+            std::uint64_t word = 0;
+            std::memcpy(&word, data + pos, sizeof(word));
+            if ((word & high) == 0)
+            {
+                pos += 8;
+                continue;
+            }
+        }
+
+        if (data[pos] < 0x80u)
+        {
+            ++pos;
+            continue;
+        }
+
+        const std::size_t seq = validate_one_utf8(data + pos, n - pos);
+        if (seq == 0)
+        {
+            break; // ill-formed or truncated
+        }
+        pos += seq;
+    }
+    return pos;
+}
+
 // Scalar (C++11) computation of the bulk run length: the number of leading
 // bytes in [data, data+n) that are ordinary ASCII or complete well-formed UTF-8
 // sequences, stopping before the first byte that needs individual handling (the

include/nlohmann/detail/output/binary_writer.hpp

@@ -2178,7 +2178,7 @@ class binary_writer
 
                 if (N > 4)
                 {
-                    oa.write_character(to_char_type(0xFE));
+                    write_bon8_marker(0xFE, string_open);
                 }
                 break;
             }
@@ -2248,20 +2248,10 @@ class binary_writer
     {
         static_cast<void>(context); // only used when exceptions are enabled
         const auto* data = reinterpret_cast<const unsigned char*>(s.data());
-        for (std::size_t i = 0; i < s.size();)
+        const std::size_t valid = valid_utf8_prefix(data, s.size());
+        if (JSON_HEDLEY_UNLIKELY(valid != s.size()))
         {
-            if (data[i] < 0x80)
-            {
-                ++i;
-                continue;
-            }
-
-            const std::size_t length = validate_one_utf8(data + i, s.size() - i);
-            if (JSON_HEDLEY_UNLIKELY(length == 0))
-            {
-                JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", hex_byte(data[i])), &context));
-            }
-            i += length;
+            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(valid), ": 0x", hex_byte(data[valid])), &context));
         }
     }
 

single_include/nlohmann/json.hpp

@@ -8780,6 +8780,43 @@ inline std::size_t validate_one_utf8(const unsigned char* data, std::size_t avai
     return 0; // invalid, incomplete, or must be diagnosed by the byte path
 }
 
+// Return the length of the longest prefix of [data, data+n) that consists of
+// ASCII characters and complete well-formed UTF-8 sequences; n if all of it is
+// valid UTF-8. Unlike scalar_string_bulk_run(), quotes, escapes, and control
+// characters are ordinary characters here. ASCII is skipped 8 bytes at a time.
+inline std::size_t valid_utf8_prefix(const unsigned char* data, std::size_t n) noexcept
+{
+    constexpr std::uint64_t high = 0x8080808080808080ull;
+    std::size_t pos = 0;
+    while (pos < n)
+    {
+        if (pos + 8 <= n)
+        {
+            std::uint64_t word = 0;
+            std::memcpy(&word, data + pos, sizeof(word));
+            if ((word & high) == 0)
+            {
+                pos += 8;
+                continue;
+            }
+        }
+
+        if (data[pos] < 0x80u)
+        {
+            ++pos;
+            continue;
+        }
+
+        const std::size_t seq = validate_one_utf8(data + pos, n - pos);
+        if (seq == 0)
+        {
+            break; // ill-formed or truncated
+        }
+        pos += seq;
+    }
+    return pos;
+}
+
 // Scalar (C++11) computation of the bulk run length: the number of leading
 // bytes in [data, data+n) that are ordinary ASCII or complete well-formed UTF-8
 // sequences, stopping before the first byte that needs individual handling (the
@@ -12342,6 +12379,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/input/lexer.hpp>
 
+// #include <nlohmann/detail/input/string_scan.hpp>
+
 // #include <nlohmann/detail/macro_scope.hpp>
 
 // #include <nlohmann/detail/meta/is_sax.hpp>
@@ -12578,6 +12617,11 @@ class binary_reader
     using char_type = typename InputAdapterType::char_type;
     using char_int_type = typename char_traits<char_type>::int_type;
 
+    /// whether the input is a contiguous block of bytes that BON8 strings can
+    /// be copied from in bulk; see @ref get_bon8_string_bulk
+    static constexpr bool bon8_bulk_scan =
+        input_adapter_supports_bulk_scan<InputAdapterType>(is_detected<detect_supports_bulk_scan, InputAdapterType> {});
+
   public:
     /*!
     @brief create a binary reader
@@ -16070,6 +16114,42 @@ class binary_reader
         return bon8_error("expected a string; last byte", "key");
     }
 
+    /*!
+    @brief append the run of valid UTF-8 at the read position to a string
+
+    For contiguous input, the ASCII characters and complete well-formed UTF-8
+    sequences at the read position are appended to @a result in one step. The
+    byte that stops the run (an end-of-string marker, the first byte of the
+    next value, or an ill-formed byte) is left for @ref get_bon8_string, so
+    that strings end and errors are reported exactly as without this step.
+
+    @param[in,out] result  the string to append to
+    */
+    void get_bon8_string_bulk(string_t& result, std::true_type /*bulk*/)
+    {
+        // bytes handed back must be read through get_bon8() first
+        if (bon8_pushback_size != 0)
+        {
+            return;
+        }
+        const std::size_t remaining = ia.bulk_remaining();
+        if (remaining == 0)
+        {
+            return;
+        }
+        const auto* const data = reinterpret_cast<const unsigned char*>(ia.bulk_data());
+        const std::size_t length = valid_utf8_prefix(data, remaining);
+        if (length != 0)
+        {
+            result.append(reinterpret_cast<const typename string_t::value_type*>(data), length);
+            ia.bulk_skip(length);
+            chars_read += length;
+        }
+    }
+
+    /// input that is not contiguous: strings are read byte by byte
+    void get_bon8_string_bulk(string_t& /*result*/, std::false_type /*bulk*/) const noexcept {}
+
     /*!
     @brief read a string
 
@@ -16086,6 +16166,8 @@ class binary_reader
     {
         while (true)
         {
+            get_bon8_string_bulk(result, std::integral_constant<bool, bon8_bulk_scan> {});
+
             const auto byte = get_bon8();
 
             if (byte == char_traits<char_type>::eof())
@@ -21918,7 +22000,7 @@ class binary_writer
 
                 if (N > 4)
                 {
-                    oa.write_character(to_char_type(0xFE));
+                    write_bon8_marker(0xFE, string_open);
                 }
                 break;
             }
@@ -21988,20 +22070,10 @@ class binary_writer
     {
         static_cast<void>(context); // only used when exceptions are enabled
         const auto* data = reinterpret_cast<const unsigned char*>(s.data());
-        for (std::size_t i = 0; i < s.size();)
+        const std::size_t valid = valid_utf8_prefix(data, s.size());
+        if (JSON_HEDLEY_UNLIKELY(valid != s.size()))
         {
-            if (data[i] < 0x80)
-            {
-                ++i;
-                continue;
-            }
-
-            const std::size_t length = validate_one_utf8(data + i, s.size() - i);
-            if (JSON_HEDLEY_UNLIKELY(length == 0))
-            {
-                JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", hex_byte(data[i])), &context));
-            }
-            i += length;
+            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(valid), ": 0x", hex_byte(data[valid])), &context));
         }
     }
 

tests/src/fuzzer-parse_bon8.cpp

@@ -15,6 +15,10 @@ array data, it performs the following steps:
 - j2 = from_bon8(vec)
 - assert(j1 == j2)
 
+It also checks that reading the data from a stream, which reads strings byte by
+byte, gives the same value or error as reading it from contiguous memory, which
+copies strings in bulk.
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -31,9 +35,33 @@ drivers.
 
 using json = nlohmann::json;
 
+namespace
+{
+// the serialization of the value read from @a input, or the error message
+template<typename InputType>
+std::string read_bon8(InputType&& input)
+{
+    try
+    {
+        const auto vec = json::to_bon8(json::from_bon8(std::forward<InputType>(input)));
+        return {vec.begin(), vec.end()};
+    }
+    catch (const json::exception& e)
+    {
+        return e.what();
+    }
+}
+} // namespace
+
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // contiguous and stream input must be read alike
+    {
+        std::istringstream stream(std::string(reinterpret_cast<const char*>(data), size));
+        assert(read_bon8(std::vector<uint8_t>(data, data + size)) == read_bon8(stream));
+    }
+
     try
     {
         // step 1: parse input

tests/src/unit-bon8.cpp

@@ -102,6 +102,12 @@ class SaxCountdown
 
 using bytes = std::vector<std::uint8_t>;
 
+/// @return the string with the given bytes
+std::string str(const bytes& b)
+{
+    return {b.begin(), b.end()};
+}
+
 /// check that @a j is serialized to @a expected and that @a expected is read back as @a j
 void check_bon8(const json& j, const bytes& expected)
 {
@@ -112,12 +118,10 @@ void check_bon8(const json& j, const bytes& expected)
     CHECK(decoded == j);
     // integers are not read back as floats and vice versa
     CHECK(decoded.type() == j.type());
-}
 
-/// @return the string with the given bytes
-std::string str(const bytes& b)
-{
-    return {b.begin(), b.end()};
+    // a stream is read byte by byte rather than in bulk
+    std::istringstream stream(str(expected));
+    CHECK(json::from_bon8(stream) == decoded);
 }
 
 /// @return @a b followed by @a tail
@@ -320,6 +324,9 @@ TEST_CASE("BON8")
                 CHECK_THROWS_WITH_AS(_ = json::to_bon8(str({0xC2, 'a'})), "[json.exception.type_error.316] invalid UTF-8 byte at index 0: 0xC2", json::type_error&);
                 CHECK_THROWS_WITH_AS(_ = json::to_bon8(str({'a', 0xE2, 0x82})), "[json.exception.type_error.316] invalid UTF-8 byte at index 1: 0xE2", json::type_error&);
                 CHECK_THROWS_WITH_AS(_ = json::to_bon8(json::object({{str({0xFF}), 1}})), "[json.exception.type_error.316] invalid UTF-8 byte at index 0: 0xFF", json::type_error&);
+                // after a run of ASCII characters that is checked 8 bytes at a time
+                CHECK_THROWS_WITH_AS(_ = json::to_bon8(std::string(17, 'a') + str({0xC0})), "[json.exception.type_error.316] invalid UTF-8 byte at index 17: 0xC0", json::type_error&);
+                CHECK_THROWS_WITH_AS(_ = json::to_bon8(std::string(8, 'a') + "\xC3\xA4" + std::string(8, 'a') + str({0xE2, 0x82})), "[json.exception.type_error.316] invalid UTF-8 byte at index 18: 0xE2", json::type_error&);
             }
         }
 
@@ -638,6 +645,62 @@ TEST_CASE("BON8")
     }
 }
 
+TEST_CASE("BON8 strings from contiguous and stream input")
+{
+    // contiguous input copies the valid UTF-8 of a string in bulk, a stream
+    // is read byte by byte; both must end strings and report errors alike
+    const std::string ascii(20, 'a');
+    const std::vector<bytes> inputs =
+    {
+        // the string ends at 0xFF, at a marker, and at an integer
+        concat(concat(bytes(ascii.begin(), ascii.end()), {0xC3, 0xA4}), {0xFF}),
+        concat(concat({0x82}, bytes(ascii.begin(), ascii.end())), {0x91}),
+        concat(concat({0x85}, bytes(ascii.begin(), ascii.end())), {0xFE}),
+        concat(concat({0x82}, bytes(ascii.begin(), ascii.end())), {0xC2, 0x05}),
+        concat(concat({0x87}, bytes(ascii.begin(), ascii.end())), {0xE2, 0x82, 0xAC, 0xF0, 0x05, 0x00, 0x00}),
+        // invalid UTF-8 and a premature end after a run of valid characters
+        concat(bytes(ascii.begin(), ascii.end()), {0xE0, 0x80, 0x80}),
+        concat(bytes(ascii.begin(), ascii.end()), {0xE2, 0x82, 0x2F}),
+        concat(bytes(ascii.begin(), ascii.end()), {0xE2, 0x82}),
+        bytes(ascii.begin(), ascii.end()),
+        // trailing bytes after a string that ends a container
+        concat(concat({0x81}, bytes(ascii.begin(), ascii.end())), {0x91}),
+    };
+
+    for (const auto& input : inputs)
+    {
+        CAPTURE(input)
+        std::string from_vector;
+        std::string from_stream;
+        try
+        {
+            from_vector = json::from_bon8(input).dump();
+        }
+        catch (const json::parse_error& e)
+        {
+            from_vector = e.what();
+        }
+        try
+        {
+            std::istringstream stream(str(input));
+            from_stream = json::from_bon8(stream).dump();
+        }
+        catch (const json::parse_error& e)
+        {
+            from_stream = e.what();
+        }
+        CHECK(from_vector == from_stream);
+    }
+
+    json _;
+    CHECK(json::from_bon8(inputs[0]) == ascii + "\xC3\xA4");
+    CHECK(json::from_bon8(inputs[3]) == json({ascii, 45}));
+    CHECK_THROWS_WITH_AS(_ = json::from_bon8(inputs[5]), "[json.exception.parse_error.112] parse error at byte 22: syntax error while parsing BON8 string: invalid UTF-8 byte: 0x80", json::parse_error&);
+    CHECK_THROWS_WITH_AS(_ = json::from_bon8(inputs[6]), "[json.exception.parse_error.112] parse error at byte 23: syntax error while parsing BON8 string: invalid UTF-8 byte: 0x2F", json::parse_error&);
+    CHECK_THROWS_WITH_AS(_ = json::from_bon8(inputs[7]), "[json.exception.parse_error.110] parse error at byte 23: syntax error while parsing BON8 string: unexpected end of input", json::parse_error&);
+    CHECK_THROWS_WITH_AS(_ = json::from_bon8(inputs[9]), "[json.exception.parse_error.110] parse error at byte 22: syntax error while parsing BON8 value: expected end of input; last byte: 0x91", json::parse_error&);
+}
+
 // use this testcase outside [hide] to run it with Valgrind
 TEST_CASE("BON8 nesting does not consume the call stack")
 {