Optimize binary get_number implementation by reading multiple bytes at once (#4391)

* multibyte binary reader

* wide_string_input_adapter fallback to get_character

Update input_adapters.hpp

* Update json.hpp

* Add from msgpack test

* Test for broken msgpack with stream, address some warnings

* Reading binary number from wchar as an error, address warnings

* Not casting float to int, it violates strict aliasing rule

include/nlohmann/detail/input/input_adapters.hpp

@@ -68,6 +68,13 @@ class file_input_adapter
         return std::fgetc(m_file);
     }
 
+    // returns the number of characters successfully read
+    template<class T>
+    std::size_t get_elements(T* dest, std::size_t count = 1)
+    {
+        return fread(dest, 1, sizeof(T) * count, m_file);
+    }
+
   private:
     /// the file pointer to read from
     std::FILE* m_file;
@@ -127,6 +134,17 @@ class input_stream_adapter
         return res;
     }
 
+    template<class T>
+    std::size_t get_elements(T* dest, std::size_t count = 1)
+    {
+        auto res = static_cast<std::size_t>(sb->sgetn(reinterpret_cast<char*>(dest), static_cast<std::streamsize>(count * sizeof(T))));
+        if (JSON_HEDLEY_UNLIKELY(res < count * sizeof(T)))
+        {
+            is->clear(is->rdstate() | std::ios::eofbit);
+        }
+        return res;
+    }
+
   private:
     /// the associated input stream
     std::istream* is = nullptr;
@@ -158,6 +176,26 @@ class iterator_input_adapter
         return char_traits<char_type>::eof();
     }
 
+    // for general iterators, we cannot really do something better than falling back to processing the range one-by-one
+    template<class T>
+    std::size_t get_elements(T* dest, std::size_t count = 1)
+    {
+        auto* ptr = reinterpret_cast<char*>(dest);
+        for (std::size_t read_index = 0; read_index < count * sizeof(T); ++read_index)
+        {
+            if (JSON_HEDLEY_LIKELY(current != end))
+            {
+                ptr[read_index] = static_cast<char>(*current);
+                std::advance(current, 1);
+            }
+            else
+            {
+                return read_index;
+            }
+        }
+        return count * sizeof(T);
+    }
+
   private:
     IteratorType current;
     IteratorType end;
@@ -321,6 +359,13 @@ class wide_string_input_adapter
         return utf8_bytes[utf8_bytes_index++];
     }
 
+    // parsing binary with wchar doesn't make sense, but since the parsing mode can be runtime, we need something here
+    template<class T>
+    std::size_t get_elements(T* /*dest*/, std::size_t /*count*/ = 1)
+    {
+        JSON_THROW(parse_error::create(112, 1, "wide string type cannot be interpreted as binary data", nullptr));
+    }
+
   private:
     BaseInputAdapter base_adapter;