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+# Schema
+
+## Status: experimental, shall be included in v1.1
+
+JSON Schema is a draft standard for describing format of JSON. The schema itself is also a JSON. By validating a JSON with JSON Schema, your code can safely access the JSON without manually checking types, or whether a key exists, etc.
+
+RapidJSON implemented a JSON Schema validator for [JSON Schema Draft v4](http://json-schema.org/documentation.html). If you do not familiar with JSON Schema, you may refer to [Understanding JSON Schema](http://spacetelescope.github.io/understanding-json-schema/).
+
+## Basic Usage
+
+First of all, you need to parse a JSON Schema into `Document`, and then compile the `Document` into `SchemaDocument`.
+
+Secondly, construct a `SchemaValidator` with the `SchedmaDocument`. It is similar to a `Writer` in the sense of handling SAX events. So, you can use `document.Accept(validator)` to validate a document, and then check the validity.
+
+~~~cpp
+#include "rapidjson/schema.h"
+
+// ...
+
+Document sd;
+if (!sd.Parse(schemaJson)) {
+    // the schema is not a valid JSON.
+    // ...       
+}
+SchemaDocument schema(sd); // Compile a Document to SchemaDocument
+// sd is no longer needed here.
+
+Document d;
+if (!d.Parse(inputJson)) {
+    // the input is not a valid JSON.
+    // ...       
+}
+
+SchemaValidator validator(schema);
+if (!d.Accept(validator)) {
+    // Input JSON is invalid according to the schema
+    // Output diagnostic information
+    StringBuffer sb;
+    validator.GetInvalidSchemaPointer().StringifyUriFragment(sb);
+    printf("Invalid schema: %s\n", sb.GetString());
+    printf("Invalid keyword: %s\n", validator.GetInvalidSchemaKeyword());
+    sb.Clear();
+    validator.GetInvalidDocumentPointer().StringifyUriFragment(sb);
+    printf("Invalid document: %s\n", sb.GetString());
+}
+~~~
+
+Some notes:
+
+* One `SchemaDocment` can be referenced by multiple `SchemaValidator`s. It will not be modified by `SchemaValidator`s.
+* A `SchemaValidator` may be reused to validate multiple documents. To run it for other documents, call `validator.Reset()` first.
+
+## Validation during parsing/serialization
+
+Differ to most JSON Schema validator implementations, RapidJSON provides a SAX-based schema validator. Therefore, you can parse a JSON from a stream while validating it on the fly. If the validator encounters a JSON value that invalidates the supplied schema, the parsing will be terminated immediately. This design is especially useful for parsing large JSON files.
+
+### DOM parsing
+
+For using DOM in parsing, `Document` needs some preparation and finailizaing tasks, in addition to recieving SAX events, thus it needs some work to route the reader, validator and the document. `SchemaValidatingReader` is a helper class that doing such work.
+
+~~~cpp
+#include "rapidjson/filereadstream.h"
+
+// ...
+SchemaDocument schema(sd); // Compile a Document to SchemaDocument
+
+// Use reader to parse the JSON
+FILE* fp = fopen("big.json", "r");
+FileReadStream is(fp, buffer, sizeof(buffer));
+
+// Parse JSON from reader, validate the SAX events, and store in d.
+Document d;
+SchemaValidatingReader<kParseDefaultFlags, FileReadStream, UTF8<> > reader(is, schema);
+d.Populate(reader);
+
+if (!reader.GetParseResult()) {
+    // Not a valid JSON
+    // When reader.GetParseResult().Code() == kParseErrorTermination,
+    // it may be terminated by:
+    // (1) the validator found that the JSON is invalid according to schema; or
+    // (2) the input stream has I/O error.
+
+    // Check the validation result
+    if (!reader.IsValid()) {
+        // Input JSON is invalid according to the schema
+        // Output diagnostic information
+        StringBuffer sb;
+        reader.GetInvalidSchemaPointer().StringifyUriFragment(sb);
+        printf("Invalid schema: %s\n", sb.GetString());
+        printf("Invalid keyword: %s\n", reader.GetInvalidSchemaKeyword());
+        sb.Clear();
+        reader.GetInvalidDocumentPointer().StringifyUriFragment(sb);
+        printf("Invalid document: %s\n", sb.GetString());
+    }
+}
+~~~
+
+### SAX parsing
+
+For using SAX in parsing, it is much simpler. If it only need to validate the JSON without further processing, it is simply:
+
+~~~
+SchemaValidator validator(schema);
+Reader reader;
+if (!reader.Parse(stream, validator)) {
+    if (!validator.IsValid()) {
+        // ...    
+    }
+}
+~~~
+
+This is exactly the method used in [schemavalidator](example/schemavalidator/schemavalidator.cpp) example. The distinct advantage is low memory usage, no matter how big the JSON was (the memory usage depends on the complexity of the schema).
+
+If you need to handle the SAX events further, then you need to use the template class `GenericSchemaValidator` to set the output handler of the validator:
+
+~~~
+MyHandler handler;
+GenericSchemaValidator<SchemaDocument, MyHandler> validator(schema, handler);
+Reader reader;
+if (!reader.Parse(ss, validator)) {
+    if (!validator.IsValid()) {
+        // ...    
+    }
+}
+~~~
+
+### Serialization
+
+It is also possible to do validation during serializing. This can ensure the result JSON is valid according to the JSON schema.
+
+~~~
+StringBuffer sb;
+Writer<StringBuffer> writer(sb);
+GenericSchemaValidator<SchemaDocument, Writer<StringBuffer> > validator(s, writer);
+if (!d.Accept(validator)) {
+    // Some problem during Accept(), it may be validation or encoding issues.
+    if (!validator.IsValid()) {
+        // ...
+    }
+}
+~~~
+
+Of course, if your appication only needs SAX-style serialization, it can simply send SAX events to `SchemaValidator` instead of `Writer`.
+
+## Remote Schema
+
+JSON Schema supports [`$ref` keyword](http://spacetelescope.github.io/understanding-json-schema/structuring.html), which is a [JSON pointer](pointer.md) referencing to a local or remote schema. Local pointer is prefixed with `#`, while remote pointer is an relative or absolute URI. For example:
+
+~~~js
+{ "$ref": "definitions.json#/address" }
+~~~
+
+As `SchemaValidator` does not know how to resolve such URI, it needs a user-provided `IRemoteSchemaDocumentProvider` instance to do so.
+
+~~~
+class MyRemoteSchemaDocumentProvider : public IRemoteSchemaDocumentProvider {
+public:
+    virtual const SchemaDocument* GetRemoteDocument(const char* uri, SizeTyp length) {
+        // Resolve the uri and returns a pointer to that schema.
+    }
+};
+
+// ...
+
+MyRemoteSchemaDocumentProvider provider;
+SchemaValidator validator(schema, &provider);
+~~~
+
+## Conformance
+
+RapidJSON passed 262 out of 263 tests in [JSON Schema Test Suite](https://github.com/json-schema/JSON-Schema-Test-Suite) (Json Schema draft 4).
+
+The failed test is "changed scope ref invalid" of "change resolution scope" in `refRemote.json`. It is due to that `id` schema keyword and URI combining function are not implemented.
+
+Besides, the `format` schema keyword for string values is ignored, since it is not required by the specification.
+
+### Regular Expression
+
+The schema keyword `pattern` and `patternProperties` uses regular expression to match the required pattern.
+
+RapidJSON implemented a simple DFA regular expression engine, which is used by default. It supports the following syntax.
+
+|Syntax|Description|
+|------|-----------|
+|`ab`    | Concatenation
+|`a|b`   | Alternation
+|`a?`    | Zero or one
+|`a*`    | Zero or more
+|`a+`    | One or more
+|`a{3}`  | Exactly 3 times
+|`a{3,}` | At least 3 times
+|`a{3,5}`| 3 to 5 times
+|`(ab)`  | Grouping
+|`^a`    | At the beginning
+|`a$`    | At the end
+|`.`     | Any character
+|`[abc]` | Character classes
+|`[a-c]` | Character class range
+|`[a-z0-9_]` | Character class combination
+|`[^abc]` | Negated character classes
+|`[^a-c]` | Negated character class range
+|`[\b]`   | Backspace (U+0008)
+|``\|`, `\\`, ...  | Escape characters
+|`\f` | Form feed (U+000C)
+|`\n` | Line feed (U+000A)
+|`\r` | Carriage return (U+000D)
+|`\t` | Tab (U+0009)
+|`\v` | Vertical tab (U+000B)
+
+For C++11 compiler, it is also possible to use the `std::regex` by defining `RAPIDJSON_SCHEMA_USE_INTERNALREGEX=0` and `RAPIDJSON_SCHEMA_USE_STDREGEX=1`. If your schemas do not need `pattern` and `patternProperties`, you can set both macros to zero to disable this feature, which will reduce some code size.
+
+## Performance
+
+Most C++ JSON libraries have not yet supporting JSON Schema. So we tried to evluate the performance of RapidJSON's JSON Schema validator according to [json-schema-benchmark](https://github.com/ebdrup/json-schema-benchmark), which tests 11 JavaScript libraries running on Node.js.
+
+That benchmark runs validations on [JSON Schema Test Suite](https://github.com/json-schema/JSON-Schema-Test-Suite), in which some test suites and tests are excluded.
+
+On a Mac Book Pro (2.8 GHz Intel Core i7), the following results are gathered.
+
+|Validator|Relative speed|Number of test runs per second|
+|---------|:------------:|:----------------------------:|
+|RapidJSON|36521%|7220217|
+|[`ajv`](https://github.com/epoberezkin/ajv)|100%|19770 (± 1.31%)|
+|[`is-my-json-valid`](https://github.com/mafintosh/is-my-json-valid)|70%|13835 (± 2.84%)|
+|[`jsen`](https://github.com/bugventure/jsen)|57.7%|11411 (± 1.27%)|
+|[`schemasaurus`](https://github.com/AlexeyGrishin/schemasaurus)|26%|5145 (± 1.62%)|
+|[`themis`](https://github.com/playlyfe/themis)|19.9%|3935 (± 2.69%)|
+|[`z-schema`](https://github.com/zaggino/z-schema)|7%|1388 (± 0.84%)|
+|[`jsck`](https://github.com/pandastrike/jsck#readme)|3.1%|606 (± 2.84%)|
+|[`jsonschema`](https://github.com/tdegrunt/jsonschema#readme)|0.9%|185 (± 1.01%)|
+|[`skeemas`](https://github.com/Prestaul/skeemas#readme)|0.8%|154 (± 0.79%)|
+|tv4|0.5%|93 (± 0.94%)|
+|[`jayschema`](https://github.com/natesilva/jayschema)|0.1%|21 (± 1.14%)|
+
+That is, RapidJSON is about ~365 times faster than the fastest JavaScript library (ajv). And ~344 thousand times faster than the slowest one.