1 # Internals 2 3 This section records some design and implementation details. 4 5 [TOC] 6 7 # Architecture {#Architecture} 8 9 ## SAX and DOM 10 11 The basic relationships of SAX and DOM is shown in the following UML diagram. 12 13  14 15 The core of the relationship is the `Handler` concept. From the SAX side, `Reader` parses a JSON from a stream and publish events to a `Handler`. `Writer` implements the `Handler` concept to handle the same set of events. From the DOM side, `Document` implements the `Handler` concept to build a DOM according to the events. `Value` supports a `Value::Accept(Handler&)` function, which traverses the DOM to publish events. 16 17 With this design, SAX is not dependent on DOM. Even `Reader` and `Writer` have no dependencies between them. This provides flexibility to chain event publisher and handlers. Besides, `Value` does not depends on SAX as well. So, in addition to stringify a DOM to JSON, user may also stringify it to a XML writer, or do anything else. 18 19 ## Utility Classes 20 21 Both SAX and DOM APIs depends on 3 additional concepts: `Allocator`, `Encoding` and `Stream`. Their inheritance hierarchy is shown as below. 22 23  24 25 # Value {#Value} 26 27 `Value` (actually a typedef of `GenericValue<UTF8<>>`) is the core of DOM API. This section describes the design of it. 28 29 ## Data Layout {#DataLayout} 30 31 `Value` is a [variant type](http://en.wikipedia.org/wiki/Variant_type). In RapidJSON's context, an instance of `Value` can contain 1 of 6 JSON value types. This is possible by using `union`. Each `Value` contains two members: `union Data data_` and a`unsigned flags_`. The `flags_` indiciates the JSON type, and also additional information. 32 33 The following tables show the data layout of each type. The 32-bit/64-bit columns indicates the size of the field in bytes. 34 35 | Null | |32-bit|64-bit| 36 |-------------------|----------------------------------|:----:|:----:| 37 | (unused) | |4 |8 | 38 | (unused) | |4 |4 | 39 | (unused) | |4 |4 | 40 | `unsigned flags_` | `kNullType kNullFlag` |4 |4 | 41 42 | Bool | |32-bit|64-bit| 43 |-------------------|----------------------------------------------------|:----:|:----:| 44 | (unused) | |4 |8 | 45 | (unused) | |4 |4 | 46 | (unused) | |4 |4 | 47 | `unsigned flags_` | `kBoolType` (either `kTrueFlag` or `kFalseFlag`) |4 |4 | 48 49 | String | |32-bit|64-bit| 50 |---------------------|-------------------------------------|:----:|:----:| 51 | `Ch* str` | Pointer to the string (may own) |4 |8 | 52 | `SizeType length` | Length of string |4 |4 | 53 | (unused) | |4 |4 | 54 | `unsigned flags_` | `kStringType kStringFlag ...` |4 |4 | 55 56 | Object | |32-bit|64-bit| 57 |---------------------|-------------------------------------|:----:|:----:| 58 | `Member* members` | Pointer to array of members (owned) |4 |8 | 59 | `SizeType size` | Number of members |4 |4 | 60 | `SizeType capacity` | Capacity of members |4 |4 | 61 | `unsigned flags_` | `kObjectType kObjectFlag` |4 |4 | 62 63 | Array | |32-bit|64-bit| 64 |---------------------|-------------------------------------|:----:|:----:| 65 | `Value* values` | Pointer to array of values (owned) |4 |8 | 66 | `SizeType size` | Number of values |4 |4 | 67 | `SizeType capacity` | Capacity of values |4 |4 | 68 | `unsigned flags_` | `kArrayType kArrayFlag` |4 |4 | 69 70 | Number (Int) | |32-bit|64-bit| 71 |---------------------|-------------------------------------|:----:|:----:| 72 | `int i` | 32-bit signed integer |4 |4 | 73 | (zero padding) | 0 |4 |4 | 74 | (unused) | |4 |8 | 75 | `unsigned flags_` | `kNumberType kNumberFlag kIntFlag kInt64Flag ...` |4 |4 | 76 77 | Number (UInt) | |32-bit|64-bit| 78 |---------------------|-------------------------------------|:----:|:----:| 79 | `unsigned u` | 32-bit unsigned integer |4 |4 | 80 | (zero padding) | 0 |4 |4 | 81 | (unused) | |4 |8 | 82 | `unsigned flags_` | `kNumberType kNumberFlag kUIntFlag kUInt64Flag ...` |4 |4 | 83 84 | Number (Int64) | |32-bit|64-bit| 85 |---------------------|-------------------------------------|:----:|:----:| 86 | `int64_t i64` | 64-bit signed integer |8 |8 | 87 | (unused) | |4 |8 | 88 | `unsigned flags_` | `kNumberType kNumberFlag kInt64Flag ...` |4 |4 | 89 90 | Number (Uint64) | |32-bit|64-bit| 91 |---------------------|-------------------------------------|:----:|:----:| 92 | `uint64_t i64` | 64-bit unsigned integer |8 |8 | 93 | (unused) | |4 |8 | 94 | `unsigned flags_` | `kNumberType kNumberFlag kInt64Flag ...` |4 |4 | 95 96 | Number (Double) | |32-bit|64-bit| 97 |---------------------|-------------------------------------|:----:|:----:| 98 | `uint64_t i64` | Double precision floating-point |8 |8 | 99 | (unused) | |4 |8 | 100 | `unsigned flags_` | `kNumberType kNumberFlag kDoubleFlag` |4 |4 | 101 102 Here are some notes: 103 * To reduce memory consumption for 64-bit architecture, `SizeType` is typedef as `unsigned` instead of `size_t`. 104 * Zero padding for 32-bit number may be placed after or before the actual type, according to the endianess. This makes possible for interpreting a 32-bit integer as a 64-bit integer, without any conversion. 105 * An `Int` is always an `Int64`, but the converse is not always true. 106 107 ## Flags {#Flags} 108 109 The 32-bit `flags_` contains both JSON type and other additional information. As shown in the above tables, each JSON type contains redundant `kXXXType` and `kXXXFlag`. This design is for optimizing the operation of testing bit-flags (`IsNumber()`) and obtaining a sequential number for each type (`GetType()`). 110 111 String has two optional flags. `kCopyFlag` means that the string owns a copy of the string. `kInlineStrFlag` means using [Short-String Optimization](#ShortString). 112 113 Number is a bit more complicated. For normal integer values, it can contains `kIntFlag`, `kUintFlag`, `kInt64Flag` and/or `kUint64Flag`, according to the range of the integer. For numbers with fraction, and integers larger than 64-bit range, they will be stored as `double` with `kDoubleFlag`. 114 115 ## Short-String Optimization {#ShortString} 116 117 Kosta (@Kosta-Github) provided a very neat short-string optimization. The optimization idea is given as follow. Excluding the `flags_`, a `Value` has 12 or 16 bytes (32-bit or 64-bit) for storing actual data. Instead of storing a pointer to a string, it is possible to store short strings in these space internally. For encoding with 1-byte character type (e.g. `char`), it can store maximum 11 or 15 characters string inside the `Value` type. 118 119 | ShortString (Ch=char) | |32-bit|64-bit| 120 |---------------------|-------------------------------------|:----:|:----:| 121 | `Ch str[MaxChars]` | String buffer |11 |15 | 122 | `Ch invLength` | MaxChars - Length |1 |1 | 123 | `unsigned flags_` | `kStringType kStringFlag ...` |4 |4 | 124 125 A special technique is applied. Instead of storing the length of string directly, it stores (MaxChars - length). This make it possible to store 11 characters with trailing `\0`. 126 127 This optimization can reduce memory usage for copy-string. It can also improve cache-coherence thus improve runtime performance. 128 129 # Allocator {#Allocator} 130 131 `Allocator` is a concept in RapidJSON: 132 ~~~cpp 133 concept Allocator { 134 static const bool kNeedFree; //!< Whether this allocator needs to call Free(). 135 136 // Allocate a memory block. 137 // \param size of the memory block in bytes. 138 // \returns pointer to the memory block. 139 void* Malloc(size_t size); 140 141 // Resize a memory block. 142 // \param originalPtr The pointer to current memory block. Null pointer is permitted. 143 // \param originalSize The current size in bytes. (Design issue: since some allocator may not book-keep this, explicitly pass to it can save memory.) 144 // \param newSize the new size in bytes. 145 void* Realloc(void* originalPtr, size_t originalSize, size_t newSize); 146 147 // Free a memory block. 148 // \param pointer to the memory block. Null pointer is permitted. 149 static void Free(void *ptr); 150 }; 151 ~~~ 152 153 Note that `Malloc()` and `Realloc()` are member functions but `Free()` is static member function. 154 155 ## MemoryPoolAllocator {#MemoryPoolAllocator} 156 157 `MemoryPoolAllocator` is the default allocator for DOM. It allocate but do not free memory. This is suitable for building a DOM tree. 158 159 Internally, it allocates chunks of memory from the base allocator (by default `CrtAllocator`) and stores the chunks as a singly linked list. When user requests an allocation, it allocates memory from the following order: 160 161 1. User supplied buffer if it is available. (See [User Buffer section in DOM](dom.md)) 162 2. If user supplied buffer is full, use the current memory chunk. 163 3. If the current block is full, allocate a new block of memory. 164 165 # Parsing Optimization {#ParsingOptimization} 166 167 ## Skip Whitespaces with SIMD {#SkipwhitespaceWithSIMD} 168 169 When parsing JSON from a stream, the parser need to skip 4 whitespace characters: 170 171 1. Space (`U+0020`) 172 2. Character Tabulation (`U+000B`) 173 3. Line Feed (`U+000A`) 174 4. Carriage Return (`U+000D`) 175 176 A simple implementation will be simply: 177 ~~~cpp 178 void SkipWhitespace(InputStream& s) { 179 while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t') 180 s.Take(); 181 } 182 ~~~ 183 184 However, this requires 4 comparisons and a few branching for each character. This was found to be a hot spot. 185 186 To accelerate this process, SIMD was applied to compare 16 characters with 4 white spaces for each iteration. Currently RapidJSON only supports SSE2 and SSE4.2 instructions for this. And it is only activated for UTF-8 memory streams, including string stream or *in situ* parsing. 187 188 To enable this optimization, need to define `RAPIDJSON_SSE2` or `RAPIDJSON_SSE42` before including `rapidjson.h`. Some compilers can detect the setting, as in `perftest.h`: 189 190 ~~~cpp 191 // __SSE2__ and __SSE4_2__ are recognized by gcc, clang, and the Intel compiler. 192 // We use -march=native with gmake to enable -msse2 and -msse4.2, if supported. 193 #if defined(__SSE4_2__) 194 # define RAPIDJSON_SSE42 195 #elif defined(__SSE2__) 196 # define RAPIDJSON_SSE2 197 #endif 198 ~~~ 199 200 Note that, these are compile-time settings. Running the executable on a machine without such instruction set support will make it crash. 201 202 ## Local Stream Copy {#LocalStreamCopy} 203 204 During optimization, it is found that some compilers cannot localize some member data access of streams into local variables or registers. Experimental results show that for some stream types, making a copy of the stream and used it in inner-loop can improve performance. For example, the actual (non-SIMD) implementation of `SkipWhitespace()` is implemented as: 205 206 ~~~cpp 207 template<typename InputStream> 208 void SkipWhitespace(InputStream& is) { 209 internal::StreamLocalCopy<InputStream> copy(is); 210 InputStream& s(copy.s); 211 212 while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t') 213 s.Take(); 214 } 215 ~~~ 216 217 Depending on the traits of stream, `StreamLocalCopy` will make (or not make) a copy of the stream object, use it locally and copy the states of stream back to the original stream. 218 219 ## Parsing to Double {#ParsingDouble} 220 221 Parsing string into `double` is difficult. The standard library function `strtod()` can do the job but it is slow. By default, the parsers use normal precision setting. This has has maximum 3 [ULP](http://en.wikipedia.org/wiki/Unit_in_the_last_place) error and implemented in `internal::StrtodNormalPrecision()`. 222 223 When using `kParseFullPrecisionFlag`, the parsers calls `internal::StrtodFullPrecision()` instead, and this function actually implemented 3 versions of conversion methods. 224 1. [Fast-Path](http://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/). 225 2. Custom DIY-FP implementation as in [double-conversion](https://github.com/floitsch/double-conversion). 226 3. Big Integer Method as in (Clinger, William D.How to read floating point numbers accurately. Vol. 25. No. 6. ACM, 1990). 227 228 If the first conversion methods fail, it will try the second, and so on. 229 230 # Generation Optimization {#GenerationOptimization} 231 232 ## Integer-to-String conversion {#itoa} 233 234 The naive algorithm for integer-to-string conversion involves division per each decimal digit. We have implemented various implementations and evaluated them in [itoa-benchmark](https://github.com/miloyip/itoa-benchmark). 235 236 Although SSE2 version is the fastest but the difference is minor by comparing to the first running-up `branchlut`. And `branchlut` is pure C++ implementation so we adopt `branchlut` in RapidJSON. 237 238 ## Double-to-String conversion {#dtoa} 239 240 Originally RapidJSON uses `snprintf(..., ..., "%g")` to achieve double-to-string conversion. This is not accurate as the default precision is 6. Later we also find that this is slow and there is an alternative. 241 242 Google's V8 [double-conversion](https://github.com/floitsch/double-conversion 243 ) implemented a newer, fast algorithm called Grisu3 (Loitsch, Florian. "Printing floating-point numbers quickly and accurately with integers."ACM Sigplan Notices45.6 (2010): 233-243.). 244 245 However, since it is not header-only so that we implemented a header-only version of Grisu2. This algorithm guarantees that the result is always accurate. And in most of cases it produces the shortest (optimal) string representation. 246 247 The header-only conversion function has been evaluated in [dtoa-benchmark](https://github.com/miloyip/dtoa-benchmark). 248 249 # Parser {#Parser} 250 251 ## Iterative Parser {#IterativeParser} 252 253 The iterative parser is a recursive descent LL(1) parser 254 implemented in a non-recursive manner. 255 256 ### Grammar {#IterativeParserGrammar} 257 258 The grammar used for this parser is based on strict JSON syntax: 259 ~~~~~~~~~~ 260 S -> array | object 261 array -> [ values ] 262 object -> { members } 263 values -> non-empty-values | 264 non-empty-values -> value addition-values 265 addition-values -> | , non-empty-values 266 members -> non-empty-members | 267 non-empty-members -> member addition-members 268 addition-members -> | , non-empty-members 269 member -> STRING : value 270 value -> STRING | NUMBER | NULL | BOOLEAN | object | array 271 ~~~~~~~~~~ 272 273 Note that left factoring is applied to non-terminals `values` and `members` 274 to make the grammar be LL(1). 275 276 ### Parsing Table {#IterativeParserParsingTable} 277 278 Based on the grammar, we can construct the FIRST and FOLLOW set. 279 280 The FIRST set of non-terminals is listed below: 281 282 | NON-TERMINAL | FIRST | 283 |:-----------------:|:--------------------------------:| 284 | array | [ | 285 | object | { | 286 | values | STRING NUMBER NULL BOOLEAN { [ | 287 | addition-values | COMMA | 288 | members | STRING | 289 | addition-members | COMMA | 290 | member | STRING | 291 | value | STRING NUMBER NULL BOOLEAN { [ | 292 | S | [ { | 293 | non-empty-members | STRING | 294 | non-empty-values | STRING NUMBER NULL BOOLEAN { [ | 295 296 The FOLLOW set is listed below: 297 298 | NON-TERMINAL | FOLLOW | 299 |:-----------------:|:-------:| 300 | S | $ | 301 | array | , $ } ] | 302 | object | , $ } ] | 303 | values | ] | 304 | non-empty-values | ] | 305 | addition-values | ] | 306 | members | } | 307 | non-empty-members | } | 308 | addition-members | } | 309 | member | , } | 310 | value | , } ] | 311 312 Finally the parsing table can be constructed from FIRST and FOLLOW set: 313 314 | NON-TERMINAL | [ | { | , | : | ] | } | STRING | NUMBER | NULL | BOOLEAN | 315 |:-----------------:|:---------------------:|:---------------------:|:-------------------:|:-:|:-:|:-:|:-----------------------:|:---------------------:|:---------------------:|:---------------------:| 316 | S | array | object | | | | | | | | | 317 | array | [ values ] | | | | | | | | | | 318 | object | | { members } | | | | | | | | | 319 | values | non-empty-values | non-empty-values | | | | | non-empty-values | non-empty-values | non-empty-values | non-empty-values | 320 | non-empty-values | value addition-values | value addition-values | | | | | value addition-values | value addition-values | value addition-values | value addition-values | 321 | addition-values | | | , non-empty-values | | | | | | | | 322 | members | | | | | | | non-empty-members | | | | 323 | non-empty-members | | | | | | | member addition-members | | | | 324 | addition-members | | | , non-empty-members | | | | | | | | 325 | member | | | | | | | STRING : value | | | | 326 | value | array | object | | | | | STRING | NUMBER | NULL | BOOLEAN | 327 328 There is a great [tool](http://hackingoff.com/compilers/predict-first-follow-set) for above grammar analysis. 329 330 ### Implementation {#IterativeParserImplementation} 331 332 Based on the parsing table, a direct(or conventional) implementation 333 that pushes the production body in reverse order 334 while generating a production could work. 335 336 In RapidJSON, several modifications(or adaptations to current design) are made to a direct implementation. 337 338 First, the parsing table is encoded in a state machine in RapidJSON. 339 States are constructed by the head and body of production. 340 State transitions are constructed by production rules. 341 Besides, extra states are added for productions involved with `array` and `object`. 342 In this way the generation of array values or object members would be a single state transition, 343 rather than several pop/push operations in the direct implementation. 344 This also makes the estimation of stack size more easier. 345 346 The state diagram is shown as follows: 347 348  349 350 Second, the iterative parser also keeps track of array's value count and object's member count 351 in its internal stack, which may be different from a conventional implementation. 352
