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diff --git a/src/s3select/rapidjson/doc/sax.md b/src/s3select/rapidjson/doc/sax.md new file mode 100644 index 000000000..d42d04388 --- /dev/null +++ b/src/s3select/rapidjson/doc/sax.md @@ -0,0 +1,509 @@ +# SAX + +The term "SAX" originated from [Simple API for XML](http://en.wikipedia.org/wiki/Simple_API_for_XML). We borrowed this term for JSON parsing and generation. + +In RapidJSON, `Reader` (typedef of `GenericReader<...>`) is the SAX-style parser for JSON, and `Writer` (typedef of `GenericWriter<...>`) is the SAX-style generator for JSON. + +[TOC] + +# Reader {#Reader} + +`Reader` parses a JSON from a stream. While it reads characters from the stream, it analyzes the characters according to the syntax of JSON, and publishes events to a handler. + +For example, here is a JSON. + +~~~~~~~~~~js +{ + "hello": "world", + "t": true , + "f": false, + "n": null, + "i": 123, + "pi": 3.1416, + "a": [1, 2, 3, 4] +} +~~~~~~~~~~ + +When a `Reader` parses this JSON, it publishes the following events to the handler sequentially: + +~~~~~~~~~~ +StartObject() +Key("hello", 5, true) +String("world", 5, true) +Key("t", 1, true) +Bool(true) +Key("f", 1, true) +Bool(false) +Key("n", 1, true) +Null() +Key("i") +Uint(123) +Key("pi") +Double(3.1416) +Key("a") +StartArray() +Uint(1) +Uint(2) +Uint(3) +Uint(4) +EndArray(4) +EndObject(7) +~~~~~~~~~~ + +These events can be easily matched with the JSON, but some event parameters need further explanation. Let's see the `simplereader` example which produces exactly the same output as above: + +~~~~~~~~~~cpp +#include "rapidjson/reader.h" +#include <iostream> + +using namespace rapidjson; +using namespace std; + +struct MyHandler : public BaseReaderHandler<UTF8<>, MyHandler> { + bool Null() { cout << "Null()" << endl; return true; } + bool Bool(bool b) { cout << "Bool(" << boolalpha << b << ")" << endl; return true; } + bool Int(int i) { cout << "Int(" << i << ")" << endl; return true; } + bool Uint(unsigned u) { cout << "Uint(" << u << ")" << endl; return true; } + bool Int64(int64_t i) { cout << "Int64(" << i << ")" << endl; return true; } + bool Uint64(uint64_t u) { cout << "Uint64(" << u << ")" << endl; return true; } + bool Double(double d) { cout << "Double(" << d << ")" << endl; return true; } + bool String(const char* str, SizeType length, bool copy) { + cout << "String(" << str << ", " << length << ", " << boolalpha << copy << ")" << endl; + return true; + } + bool StartObject() { cout << "StartObject()" << endl; return true; } + bool Key(const char* str, SizeType length, bool copy) { + cout << "Key(" << str << ", " << length << ", " << boolalpha << copy << ")" << endl; + return true; + } + bool EndObject(SizeType memberCount) { cout << "EndObject(" << memberCount << ")" << endl; return true; } + bool StartArray() { cout << "StartArray()" << endl; return true; } + bool EndArray(SizeType elementCount) { cout << "EndArray(" << elementCount << ")" << endl; return true; } +}; + +void main() { + const char json[] = " { \"hello\" : \"world\", \"t\" : true , \"f\" : false, \"n\": null, \"i\":123, \"pi\": 3.1416, \"a\":[1, 2, 3, 4] } "; + + MyHandler handler; + Reader reader; + StringStream ss(json); + reader.Parse(ss, handler); +} +~~~~~~~~~~ + +Note that RapidJSON uses templates to statically bind the `Reader` type and the handler type, instead of using classes with virtual functions. This paradigm can improve performance by inlining functions. + +## Handler {#Handler} + +As shown in the previous example, the user needs to implement a handler which consumes the events (via function calls) from the `Reader`. The handler must contain the following member functions. + +~~~~~~~~~~cpp +class Handler { + bool Null(); + bool Bool(bool b); + bool Int(int i); + bool Uint(unsigned i); + bool Int64(int64_t i); + bool Uint64(uint64_t i); + bool Double(double d); + bool RawNumber(const Ch* str, SizeType length, bool copy); + bool String(const Ch* str, SizeType length, bool copy); + bool StartObject(); + bool Key(const Ch* str, SizeType length, bool copy); + bool EndObject(SizeType memberCount); + bool StartArray(); + bool EndArray(SizeType elementCount); +}; +~~~~~~~~~~ + +`Null()` is called when the `Reader` encounters a JSON null value. + +`Bool(bool)` is called when the `Reader` encounters a JSON true or false value. + +When the `Reader` encounters a JSON number, it chooses a suitable C++ type mapping. And then it calls *one* function out of `Int(int)`, `Uint(unsigned)`, `Int64(int64_t)`, `Uint64(uint64_t)` and `Double(double)`. If `kParseNumbersAsStrings` is enabled, `Reader` will always calls `RawNumber()` instead. + +`String(const char* str, SizeType length, bool copy)` is called when the `Reader` encounters a string. The first parameter is pointer to the string. The second parameter is the length of the string (excluding the null terminator). Note that RapidJSON supports null character `\0` inside a string. If such situation happens, `strlen(str) < length`. The last `copy` indicates whether the handler needs to make a copy of the string. For normal parsing, `copy = true`. Only when *insitu* parsing is used, `copy = false`. And be aware that the character type depends on the target encoding, which will be explained later. + +When the `Reader` encounters the beginning of an object, it calls `StartObject()`. An object in JSON is a set of name-value pairs. If the object contains members it first calls `Key()` for the name of member, and then calls functions depending on the type of the value. These calls of name-value pairs repeat until calling `EndObject(SizeType memberCount)`. Note that the `memberCount` parameter is just an aid for the handler; users who do not need this parameter may ignore it. + +Arrays are similar to objects, but simpler. At the beginning of an array, the `Reader` calls `BeginArray()`. If there is elements, it calls functions according to the types of element. Similarly, in the last call `EndArray(SizeType elementCount)`, the parameter `elementCount` is just an aid for the handler. + +Every handler function returns a `bool`. Normally it should return `true`. If the handler encounters an error, it can return `false` to notify the event publisher to stop further processing. + +For example, when we parse a JSON with `Reader` and the handler detects that the JSON does not conform to the required schema, the handler can return `false` and let the `Reader` stop further parsing. This will place the `Reader` in an error state, with error code `kParseErrorTermination`. + +## GenericReader {#GenericReader} + +As mentioned before, `Reader` is a typedef of a template class `GenericReader`: + +~~~~~~~~~~cpp +namespace rapidjson { + +template <typename SourceEncoding, typename TargetEncoding, typename Allocator = MemoryPoolAllocator<> > +class GenericReader { + // ... +}; + +typedef GenericReader<UTF8<>, UTF8<> > Reader; + +} // namespace rapidjson +~~~~~~~~~~ + +The `Reader` uses UTF-8 as both source and target encoding. The source encoding means the encoding in the JSON stream. The target encoding means the encoding of the `str` parameter in `String()` calls. For example, to parse a UTF-8 stream and output UTF-16 string events, you can define a reader by: + +~~~~~~~~~~cpp +GenericReader<UTF8<>, UTF16<> > reader; +~~~~~~~~~~ + +Note that, the default character type of `UTF16` is `wchar_t`. So this `reader` needs to call `String(const wchar_t*, SizeType, bool)` of the handler. + +The third template parameter `Allocator` is the allocator type for internal data structure (actually a stack). + +## Parsing {#SaxParsing} + +The main function of `Reader` is used to parse JSON. + +~~~~~~~~~~cpp +template <unsigned parseFlags, typename InputStream, typename Handler> +bool Parse(InputStream& is, Handler& handler); + +// with parseFlags = kDefaultParseFlags +template <typename InputStream, typename Handler> +bool Parse(InputStream& is, Handler& handler); +~~~~~~~~~~ + +If an error occurs during parsing, it will return `false`. User can also call `bool HasParseError()`, `ParseErrorCode GetParseErrorCode()` and `size_t GetErrorOffset()` to obtain the error states. In fact, `Document` uses these `Reader` functions to obtain parse errors. Please refer to [DOM](doc/dom.md) for details about parse errors. + +## Token-by-Token Parsing {#TokenByTokenParsing} + +Some users may wish to parse a JSON input stream a single token at a time, instead of immediately parsing an entire document without stopping. To parse JSON this way, instead of calling `Parse`, you can use the `IterativeParse` set of functions: + +~~~~~~~~~~cpp + void IterativeParseInit(); + + template <unsigned parseFlags, typename InputStream, typename Handler> + bool IterativeParseNext(InputStream& is, Handler& handler); + + bool IterativeParseComplete(); +~~~~~~~~~~ + +Here is an example of iteratively parsing JSON, token by token: + +~~~~~~~~~~cpp + reader.IterativeParseInit(); + while (!reader.IterativeParseComplete()) { + reader.IterativeParseNext<kParseDefaultFlags>(is, handler); + // Your handler has been called once. + } +~~~~~~~~~~ + +# Writer {#Writer} + +`Reader` converts (parses) JSON into events. `Writer` does exactly the opposite. It converts events into JSON. + +`Writer` is very easy to use. If your application only need to converts some data into JSON, it may be a good choice to use `Writer` directly, instead of building a `Document` and then stringifying it with a `Writer`. + +In `simplewriter` example, we do exactly the reverse of `simplereader`. + +~~~~~~~~~~cpp +#include "rapidjson/writer.h" +#include "rapidjson/stringbuffer.h" +#include <iostream> + +using namespace rapidjson; +using namespace std; + +void main() { + StringBuffer s; + Writer<StringBuffer> writer(s); + + writer.StartObject(); + writer.Key("hello"); + writer.String("world"); + writer.Key("t"); + writer.Bool(true); + writer.Key("f"); + writer.Bool(false); + writer.Key("n"); + writer.Null(); + writer.Key("i"); + writer.Uint(123); + writer.Key("pi"); + writer.Double(3.1416); + writer.Key("a"); + writer.StartArray(); + for (unsigned i = 0; i < 4; i++) + writer.Uint(i); + writer.EndArray(); + writer.EndObject(); + + cout << s.GetString() << endl; +} +~~~~~~~~~~ + +~~~~~~~~~~ +{"hello":"world","t":true,"f":false,"n":null,"i":123,"pi":3.1416,"a":[0,1,2,3]} +~~~~~~~~~~ + +There are two `String()` and `Key()` overloads. One is the same as defined in handler concept with 3 parameters. It can handle string with null characters. Another one is the simpler version used in the above example. + +Note that, the example code does not pass any parameters in `EndArray()` and `EndObject()`. An `SizeType` can be passed but it will be simply ignored by `Writer`. + +You may doubt that, why not just using `sprintf()` or `std::stringstream` to build a JSON? + +There are various reasons: +1. `Writer` must output a well-formed JSON. If there is incorrect event sequence (e.g. `Int()` just after `StartObject()`), it generates assertion fail in debug mode. +2. `Writer::String()` can handle string escaping (e.g. converting code point `U+000A` to `\n`) and Unicode transcoding. +3. `Writer` handles number output consistently. +4. `Writer` implements the event handler concept. It can be used to handle events from `Reader`, `Document` or other event publisher. +5. `Writer` can be optimized for different platforms. + +Anyway, using `Writer` API is even simpler than generating a JSON by ad hoc methods. + +## Template {#WriterTemplate} + +`Writer` has a minor design difference to `Reader`. `Writer` is a template class, not a typedef. There is no `GenericWriter`. The following is the declaration. + +~~~~~~~~~~cpp +namespace rapidjson { + +template<typename OutputStream, typename SourceEncoding = UTF8<>, typename TargetEncoding = UTF8<>, typename Allocator = CrtAllocator<>, unsigned writeFlags = kWriteDefaultFlags> +class Writer { +public: + Writer(OutputStream& os, Allocator* allocator = 0, size_t levelDepth = kDefaultLevelDepth) +// ... +}; + +} // namespace rapidjson +~~~~~~~~~~ + +The `OutputStream` template parameter is the type of output stream. It cannot be deduced and must be specified by user. + +The `SourceEncoding` template parameter specifies the encoding to be used in `String(const Ch*, ...)`. + +The `TargetEncoding` template parameter specifies the encoding in the output stream. + +The `Allocator` is the type of allocator, which is used for allocating internal data structure (a stack). + +The `writeFlags` are combination of the following bit-flags: + +Parse flags | Meaning +------------------------------|----------------------------------- +`kWriteNoFlags` | No flag is set. +`kWriteDefaultFlags` | Default write flags. It is equal to macro `RAPIDJSON_WRITE_DEFAULT_FLAGS`, which is defined as `kWriteNoFlags`. +`kWriteValidateEncodingFlag` | Validate encoding of JSON strings. +`kWriteNanAndInfFlag` | Allow writing of `Infinity`, `-Infinity` and `NaN`. + +Besides, the constructor of `Writer` has a `levelDepth` parameter. This parameter affects the initial memory allocated for storing information per hierarchy level. + +## PrettyWriter {#PrettyWriter} + +While the output of `Writer` is the most condensed JSON without white-spaces, suitable for network transfer or storage, it is not easily readable by human. + +Therefore, RapidJSON provides a `PrettyWriter`, which adds indentation and line feeds in the output. + +The usage of `PrettyWriter` is exactly the same as `Writer`, expect that `PrettyWriter` provides a `SetIndent(Ch indentChar, unsigned indentCharCount)` function. The default is 4 spaces. + +## Completeness and Reset {#CompletenessReset} + +A `Writer` can only output a single JSON, which can be any JSON type at the root. Once the singular event for root (e.g. `String()`), or the last matching `EndObject()` or `EndArray()` event, is handled, the output JSON is well-formed and complete. User can detect this state by calling `Writer::IsComplete()`. + +When a JSON is complete, the `Writer` cannot accept any new events. Otherwise the output will be invalid (i.e. having more than one root). To reuse the `Writer` object, user can call `Writer::Reset(OutputStream& os)` to reset all internal states of the `Writer` with a new output stream. + +# Techniques {#SaxTechniques} + +## Parsing JSON to Custom Data Structure {#CustomDataStructure} + +`Document`'s parsing capability is completely based on `Reader`. Actually `Document` is a handler which receives events from a reader to build a DOM during parsing. + +User may uses `Reader` to build other data structures directly. This eliminates building of DOM, thus reducing memory and improving performance. + +In the following `messagereader` example, `ParseMessages()` parses a JSON which should be an object with key-string pairs. + +~~~~~~~~~~cpp +#include "rapidjson/reader.h" +#include "rapidjson/error/en.h" +#include <iostream> +#include <string> +#include <map> + +using namespace std; +using namespace rapidjson; + +typedef map<string, string> MessageMap; + +struct MessageHandler + : public BaseReaderHandler<UTF8<>, MessageHandler> { + MessageHandler() : state_(kExpectObjectStart) { + } + + bool StartObject() { + switch (state_) { + case kExpectObjectStart: + state_ = kExpectNameOrObjectEnd; + return true; + default: + return false; + } + } + + bool String(const char* str, SizeType length, bool) { + switch (state_) { + case kExpectNameOrObjectEnd: + name_ = string(str, length); + state_ = kExpectValue; + return true; + case kExpectValue: + messages_.insert(MessageMap::value_type(name_, string(str, length))); + state_ = kExpectNameOrObjectEnd; + return true; + default: + return false; + } + } + + bool EndObject(SizeType) { return state_ == kExpectNameOrObjectEnd; } + + bool Default() { return false; } // All other events are invalid. + + MessageMap messages_; + enum State { + kExpectObjectStart, + kExpectNameOrObjectEnd, + kExpectValue, + }state_; + std::string name_; +}; + +void ParseMessages(const char* json, MessageMap& messages) { + Reader reader; + MessageHandler handler; + StringStream ss(json); + if (reader.Parse(ss, handler)) + messages.swap(handler.messages_); // Only change it if success. + else { + ParseErrorCode e = reader.GetParseErrorCode(); + size_t o = reader.GetErrorOffset(); + cout << "Error: " << GetParseError_En(e) << endl;; + cout << " at offset " << o << " near '" << string(json).substr(o, 10) << "...'" << endl; + } +} + +int main() { + MessageMap messages; + + const char* json1 = "{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\" }"; + cout << json1 << endl; + ParseMessages(json1, messages); + + for (MessageMap::const_iterator itr = messages.begin(); itr != messages.end(); ++itr) + cout << itr->first << ": " << itr->second << endl; + + cout << endl << "Parse a JSON with invalid schema." << endl; + const char* json2 = "{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\", \"foo\" : {} }"; + cout << json2 << endl; + ParseMessages(json2, messages); + + return 0; +} +~~~~~~~~~~ + +~~~~~~~~~~ +{ "greeting" : "Hello!", "farewell" : "bye-bye!" } +farewell: bye-bye! +greeting: Hello! + +Parse a JSON with invalid schema. +{ "greeting" : "Hello!", "farewell" : "bye-bye!", "foo" : {} } +Error: Terminate parsing due to Handler error. + at offset 59 near '} }...' +~~~~~~~~~~ + +The first JSON (`json1`) was successfully parsed into `MessageMap`. Since `MessageMap` is a `std::map`, the printing order are sorted by the key. This order is different from the JSON's order. + +In the second JSON (`json2`), `foo`'s value is an empty object. As it is an object, `MessageHandler::StartObject()` will be called. However, at that moment `state_ = kExpectValue`, so that function returns `false` and cause the parsing process be terminated. The error code is `kParseErrorTermination`. + +## Filtering of JSON {#Filtering} + +As mentioned earlier, `Writer` can handle the events published by `Reader`. `condense` example simply set a `Writer` as handler of a `Reader`, so it can remove all white-spaces in JSON. `pretty` example uses the same relationship, but replacing `Writer` by `PrettyWriter`. So `pretty` can be used to reformat a JSON with indentation and line feed. + +Actually, we can add intermediate layer(s) to filter the contents of JSON via these SAX-style API. For example, `capitalize` example capitalize all strings in a JSON. + +~~~~~~~~~~cpp +#include "rapidjson/reader.h" +#include "rapidjson/writer.h" +#include "rapidjson/filereadstream.h" +#include "rapidjson/filewritestream.h" +#include "rapidjson/error/en.h" +#include <vector> +#include <cctype> + +using namespace rapidjson; + +template<typename OutputHandler> +struct CapitalizeFilter { + CapitalizeFilter(OutputHandler& out) : out_(out), buffer_() { + } + + bool Null() { return out_.Null(); } + bool Bool(bool b) { return out_.Bool(b); } + bool Int(int i) { return out_.Int(i); } + bool Uint(unsigned u) { return out_.Uint(u); } + bool Int64(int64_t i) { return out_.Int64(i); } + bool Uint64(uint64_t u) { return out_.Uint64(u); } + bool Double(double d) { return out_.Double(d); } + bool RawNumber(const char* str, SizeType length, bool copy) { return out_.RawNumber(str, length, copy); } + bool String(const char* str, SizeType length, bool) { + buffer_.clear(); + for (SizeType i = 0; i < length; i++) + buffer_.push_back(std::toupper(str[i])); + return out_.String(&buffer_.front(), length, true); // true = output handler need to copy the string + } + bool StartObject() { return out_.StartObject(); } + bool Key(const char* str, SizeType length, bool copy) { return String(str, length, copy); } + bool EndObject(SizeType memberCount) { return out_.EndObject(memberCount); } + bool StartArray() { return out_.StartArray(); } + bool EndArray(SizeType elementCount) { return out_.EndArray(elementCount); } + + OutputHandler& out_; + std::vector<char> buffer_; +}; + +int main(int, char*[]) { + // Prepare JSON reader and input stream. + Reader reader; + char readBuffer[65536]; + FileReadStream is(stdin, readBuffer, sizeof(readBuffer)); + + // Prepare JSON writer and output stream. + char writeBuffer[65536]; + FileWriteStream os(stdout, writeBuffer, sizeof(writeBuffer)); + Writer<FileWriteStream> writer(os); + + // JSON reader parse from the input stream and let writer generate the output. + CapitalizeFilter<Writer<FileWriteStream> > filter(writer); + if (!reader.Parse(is, filter)) { + fprintf(stderr, "\nError(%u): %s\n", (unsigned)reader.GetErrorOffset(), GetParseError_En(reader.GetParseErrorCode())); + return 1; + } + + return 0; +} +~~~~~~~~~~ + +Note that, it is incorrect to simply capitalize the JSON as a string. For example: +~~~~~~~~~~ +["Hello\nWorld"] +~~~~~~~~~~ + +Simply capitalizing the whole JSON would contain incorrect escape character: +~~~~~~~~~~ +["HELLO\NWORLD"] +~~~~~~~~~~ + +The correct result by `capitalize`: +~~~~~~~~~~ +["HELLO\nWORLD"] +~~~~~~~~~~ + +More complicated filters can be developed. However, since SAX-style API can only provide information about a single event at a time, user may need to book-keeping the contextual information (e.g. the path from root value, storage of other related values). Some processing may be easier to be implemented in DOM than SAX. |