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@@ -4,24 +4,234 @@ This section records some design and implementation details.
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[TOC]
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+# Architecture {#Architecture}
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+
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+## SAX and DOM
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+
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+The basic relationships of SAX and DOM is shown in the following UML diagram.
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+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.
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+
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+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.
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+
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+## Utility Classes
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+
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+Both SAX and DOM APIs depends on 3 additional concepts: `Allocator`, `Encoding` and `Stream`. Their inheritance hierarchy is shown as below.
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+
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+
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# Value {#Value}
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+`Value` (actually a typedef of `GenericValue<UTF8<>>`) is the core of DOM API. This section describes the design of it.
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+
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## Data Layout {#DataLayout}
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+`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 possble 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.
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+
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+The following tables show the data layout of each type. The 32-bit/64-bit columns indicates the size of the field in bytes.
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+
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+| Null | |32-bit|64-bit|
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+|-------------------|----------------------------------|:----:|:----:|
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+| (unused) | |4 |8 |
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+| (unused) | |4 |4 |
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+| (unused) | |4 |4 |
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+| `unsigned flags_` | `kNullType | kNullFlag` |4 |4 |
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+
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+| Bool | |32-bit|64-bit|
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+|-------------------|----------------------------------------------------|:----:|:----:|
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+| (unused) | |4 |8 |
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+| (unused) | |4 |4 |
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+| (unused) | |4 |4 |
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+| `unsigned flags_` | `kBoolType | `(either `kTrueFlag` or `kFalseFlag`) |4 |4 |
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+
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+| String | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `Ch* str` | Pointer to the string (may own) |4 |8 |
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+| `SizeType length` | Length of string |4 |4 |
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+| (unused) | |4 |4 |
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+| `unsigned flags_` | `kStringType | kStringFlag | ...` |4 |4 |
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+
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+| Object | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `Member* members` | Pointer to array of members (owned) |4 |8 |
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+| `SizeType size` | Number of members |4 |4 |
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+| `SizeType capacity` | Capacity of members |4 |4 |
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+| `unsigned flags_` | `kObjectType | kObjectFlag` |4 |4 |
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+
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+| Array | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `Value* values` | Pointer to array of values (owned) |4 |8 |
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+| `SizeType size` | Number of values |4 |4 |
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+| `SizeType capacity` | Capacity of values |4 |4 |
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+| `unsigned flags_` | `kArrayType | kArrayFlag` |4 |4 |
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+
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+| Number (Int) | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `int i` | 32-bit signed integer |4 |4 |
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+| (zero padding) | 0 |4 |4 |
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+| (unused) | |4 |8 |
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+| `unsigned flags_` | `kNumberType | kNumberFlag | kIntFlag | kInt64Flag | ...` |4 |4 |
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+
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+| Number (UInt) | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `unsigned u` | 32-bit unsigned integer |4 |4 |
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+| (zero padding) | 0 |4 |4 |
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+| (unused) | |4 |8 |
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+| `unsigned flags_` | `kNumberType | kNumberFlag | kUIntFlag | kUInt64Flag | ...` |4 |4 |
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+
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+| Number (Int64) | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `int64_t i64` | 64-bit signed integer |8 |8 |
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+| (unused) | |4 |8 |
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+| `unsigned flags_` | `kNumberType | kNumberFlag | kInt64Flag | ...` |4 |4 |
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+
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+| Number (Uint64) | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `uint64_t i64` | 64-bit unsigned integer |8 |8 |
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+| (unused) | |4 |8 |
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+| `unsigned flags_` | `kNumberType | kNumberFlag | kInt64Flag | ...` |4 |4 |
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+
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+| Number (Double) | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `uint64_t i64` | Double precision floating-point |8 |8 |
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+| (unused) | |4 |8 |
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+| `unsigned flags_` | `kNumberType | kNumberFlag | kDoubleFlag` |4 |4 |
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+
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+Here are some notes:
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+* To reduce memory consumption for 64-bit architecture, `SizeType` is typedef as `unsigned` instead of `size_t`.
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+* 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.
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+* An `Int` is always an `Int64`, but the converse is not always true.
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+
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## Flags {#Flags}
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+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 sequental number for each type (`GetType()`).
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+
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+String has two optional flags. `kCopyFlag` means that the string owns a copy of the string. `kInlineStrFlag` means using [Short-String Optimizatoin](#ShortString).
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+
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+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`.
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+
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+## Short-String Optimization {#ShortString}
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+
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+ 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 maxium 11 or 15 characters string inside the `Value` type.
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+
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+| ShortString (Ch=char) | |32-bit|64-bit|
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+|---------------------|-------------------------------------|:----:|:----:|
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+| `Ch str[MaxChars]` | String buffer |11 |15 |
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+| `Ch invLength` | MaxChars - Length |1 |1 |
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+| `unsigned flags_` | `kStringType | kStringFlag | ...` |4 |4 |
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+
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+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`.
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+
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+This optimization can reduce memory usage for copy-string. It can also improve cache-coherence thus improve runtime performance.
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+
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# Allocator {#Allocator}
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+`Allocator` is a concept in RapidJSON:
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+~~~cpp
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+concept Allocator {
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+ static const bool kNeedFree; //!< Whether this allocator needs to call Free().
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+
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+ // Allocate a memory block.
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+ // \param size of the memory block in bytes.
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+ // \returns pointer to the memory block.
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+ void* Malloc(size_t size);
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+
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+ // Resize a memory block.
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+ // \param originalPtr The pointer to current memory block. Null pointer is permitted.
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+ // \param originalSize The current size in bytes. (Design issue: since some allocator may not book-keep this, explicitly pass to it can save memory.)
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+ // \param newSize the new size in bytes.
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+ void* Realloc(void* originalPtr, size_t originalSize, size_t newSize);
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+
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+ // Free a memory block.
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+ // \param pointer to the memory block. Null pointer is permitted.
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+ static void Free(void *ptr);
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+};
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+~~~
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+
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+Note that `Malloc()` and `Realloc()` are member functions but `Free()` is static member function.
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+
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## MemoryPoolAllocator {#MemoryPoolAllocator}
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+`MemoryPoolAllocator` is the default allocator for DOM. It allocate but do not free memory. This is suitable for building a DOM tree.
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+
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+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:
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+
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+1. User supplied buffer if it is available. (See [User Buffer section in DOM](dom.md))
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+2. If user supplied buffer is full, use the current memory chunk.
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+3. If the current block is full, allocate a new block of memory.
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+
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# Parsing Optimization {#ParsingOptimization}
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-## Skip Whitespace with SIMD {#SkipwhitespaceWithSIMD}
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+## Skip Whitespaces with SIMD {#SkipwhitespaceWithSIMD}
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+
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+When parsing JSON from a stream, the parser need to skip 4 whitespace characters:
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+
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+1. Space (`U+0020`)
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+2. Character Tabulation (`U+000B`)
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+3. Line Feed (`U+000A`)
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+4. Carriage Return (`U+000D`)
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+
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+A simple implementation will be simply:
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+~~~cpp
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+void SkipWhitespace(InputStream& s) {
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+ while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t')
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+ s.Take();
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+}
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+~~~
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-## Pow10() {#Pow10}
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+However, this requires 4 comparisons and a few branching for each character. This was found to be a hot spot.
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+
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+To accelerate this process, SIMD was applied to compare 16 characters with 4 white spaces for each iteration. Currently RapidJSON only supports SSE2 and SSE4.1 instructions for this. And it is only activated for UTF-8 memory streams, including string stream or *in situ* parsing.
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## Local Stream Copy {#LocalStreamCopy}
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+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:
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+
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+~~~cpp
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+template<typename InputStream>
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+void SkipWhitespace(InputStream& is) {
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+ internal::StreamLocalCopy<InputStream> copy(is);
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+ InputStream& s(copy.s);
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+
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+ while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t')
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+ s.Take();
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+}
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+~~~
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+
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+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.
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+
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+## Parsing to Double {#ParsingDouble}
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+
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+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()`.
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+
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+When using `kParseFullPrecisionFlag`, the parsers calls `internal::StrtodFullPrecision()` instead, and this function actually implemented 3 versions of conversion methods.
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+1. [Fast-Path](http://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/).
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+2. Custom DIY-FP implementation as in [double-conversion](https://github.com/floitsch/double-conversion).
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+3. Big Integer Method as in (Clinger, William D. How to read floating point numbers accurately. Vol. 25. No. 6. ACM, 1990).
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+
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+If the first conversion methods fail, it will try the second, and so on.
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+
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+# Generation Optimizatoin {#GenerationOptimization}
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+
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+## Integer-to-String conversion {#itoa}
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+
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+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).
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+
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+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.
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+
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+## Double-to-String conversion {#dtoa}
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+
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+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.
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+
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+Google's V8 [double-conversion](https://github.com/floitsch/double-conversion
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+) implemented a newer, fast algorithm called Grisu3 (Loitsch, Florian. "Printing floating-point numbers quickly and accurately with integers." ACM Sigplan Notices 45.6 (2010): 233-243.).
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+
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+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.
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+
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+The header-only conversion function has been evaluated in [dtoa-benchmark](https://github.com/miloyip/dtoa-benchmark).
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+
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# Parser {#Parser}
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## Iterative Parser {#IterativeParser}
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miloyip
