Add lookahead parser example

example/CMakeLists.txt

@@ -10,6 +10,7 @@ set(EXAMPLES
     filterkey
     filterkeydom
     jsonx
+    lookaheadparser
     messagereader
     parsebyparts
     pretty

example/lookaheadparser/lookaheadparser.cpp

+#include "rapidjson/reader.h"
+#include "rapidjson/document.h"
+#include <iostream>
+
+// This example demonstrates JSON token-by-token parsing with an API that is
+// more direct; you don't need to design your logic around a handler object and
+// callbacks. Instead, you retrieve values from the JSON stream by calling
+// GetInt(), GetDouble(), GetString() and GetBool(), traverse into structures
+// by calling EnterObject() and EnterArray(), and skip over unwanted data by
+// calling SkipValue(). When you know your JSON's structure, this can be quite
+// convenient.
+//
+// If you aren't sure of what's next in the JSON data, you can use PeekType() and
+// PeekValue() to look ahead to the next object before reading it.
+//
+// If you call the wrong retrieval method--e.g. GetInt when the next JSON token is
+// not an int, EnterObject or EnterArray when there isn't actually an object or array
+// to read--the stream parsing will end immediately and no more data will be delivered.
+//
+// After calling EnterObject, you retrieve keys via NextObjectKey() and values via
+// the normal getters. When NextObjectKey() returns null, you have exited the
+// object, or you can call ExitObject() to skip to the end of the object
+// immediately. If you fetch the entire object (i.e. NextObjectKey() returned  null),
+// you should not call ExitObject().
+//
+// After calling EnterArray(), you must alternate between calling NextArrayValue()
+// to see if the array has more data, and then retrieving values via the normal
+// getters. You can call ExitArray() to skip to the end of the array immediately.
+// If you fetch the entire array (i.e. NextArrayValue() returned null),
+// you should not call ExitArray().
+//
+// This parser uses in-situ strings, so the JSON buffer will be altered during the
+// parse.
+
+using namespace rapidjson;
+
+
+class LookaheadParserHandler {
+public:
+    bool Null() { st_ = kHasValue; v_.SetNull(); return true; }
+    bool Bool(bool b) { st_ = kHasValue; v_.SetBool(b); return true; }
+    bool Int(int i) { st_ = kHasValue; v_.SetInt(i); return true; }
+    bool Uint(unsigned u) { st_ = kHasValue; v_.SetUint(u); return true; }
+    bool Int64(int64_t i) { st_ = kHasValue; v_.SetInt64(i); return true; }
+    bool Uint64(uint64_t u) { st_ = kHasValue; v_.SetUint64(u); return true; }
+    bool Double(double d) { st_ = kHasValue; v_.SetDouble(d); return true; }
+    bool RawNumber(const char*, SizeType, bool) { return false; }
+    bool String(const char* str, SizeType length, bool) { st_ = kHasValue; v_.SetString(str, length); return true; }
+    bool StartObject() { st_ = kEnteringObject; return true; }
+    bool Key(const char* str, SizeType length, bool) { st_ = kHasKey; v_.SetString(str, length); return true; }
+    bool EndObject(SizeType) { st_ = kExitingObject; return true; }
+    bool StartArray() { st_ = kEnteringArray; return true; }
+    bool EndArray(SizeType) { st_ = kExitingArray; return true; }
+
+protected:
+    LookaheadParserHandler(char* str);
+    void ParseNext();
+
+protected:
+    enum LookaheadParsingState {
+        kError,
+        kHasValue,
+        kHasKey,
+        kEnteringObject,
+        kExitingObject,
+        kEnteringArray,
+        kExitingArray
+    };
+    
+    Value v_;
+    LookaheadParsingState st_;
+    Reader r_;
+    InsituStringStream ss_;
+    
+    static const int parseFlags = kParseDefaultFlags | kParseInsituFlag;
+};
+
+LookaheadParserHandler::LookaheadParserHandler(char* str) : ss_(str) {
+    r_.IterativeParseInit();
+    ParseNext();
+}
+
+void LookaheadParserHandler::ParseNext() {
+    if (r_.HasParseError()) {
+        st_ = kError;
+        return;
+    }
+    
+    r_.IterativeParseNext<parseFlags>(ss_, *this);
+}
+
+class LookaheadParser : protected LookaheadParserHandler {
+public:
+    LookaheadParser(char* str) : LookaheadParserHandler(str) {}
+    
+    void EnterObject();
+    void EnterArray();
+    void ExitObject();
+    void ExitArray();
+    const char* NextObjectKey();
+    bool NextArrayValue();
+    int GetInt();
+    double GetDouble();
+    const char* GetString();
+    bool GetBool();
+    void GetNull();
+
+    void SkipValue();
+    Value* PeekValue();
+    int PeekType(); // returns a rapidjson::Type, or -1 for no value (at end of object/array)
+    
+    bool IsValid() { return st_ != kError; }
+};
+
+void LookaheadParser::EnterObject() {
+    if (st_ != kEnteringObject) {
+        st_  = kError;
+        return;
+    }
+    
+    ParseNext();
+}
+
+void LookaheadParser::EnterArray() {
+    if (st_ != kEnteringArray) {
+        st_  = kError;
+        return;
+    }
+    
+    ParseNext();
+}
+
+void LookaheadParser::ExitObject() {
+    while (NextObjectKey()) {
+        SkipValue();
+    }
+}
+
+void LookaheadParser::ExitArray() {
+    while (NextArrayValue()) {
+        SkipValue();
+    }
+}
+
+const char* LookaheadParser::NextObjectKey() {
+    if (st_ == kExitingObject) {
+        ParseNext();
+        return 0;
+    }
+
+    if (st_ != kHasKey || !v_.IsString()) {
+        st_ = kError;
+        return 0;
+    }
+    
+    const char* result = v_.GetString();
+    ParseNext();
+    return result;
+}
+
+bool LookaheadParser::NextArrayValue() {
+    if (st_ == kExitingArray) {
+        ParseNext();
+        return false;
+    }
+    
+    return true;
+}
+
+int LookaheadParser::GetInt() {
+    if (st_ != kHasValue || !v_.IsInt()) {
+        st_ = kError;
+        return 0;
+    }
+
+    int result = v_.GetInt();
+    ParseNext();
+    return result;
+}
+
+double LookaheadParser::GetDouble() {
+    if (st_ != kHasValue || !v_.IsNumber()) {
+        st_  = kError;
+        return 0.;
+    }
+    
+    double result = v_.GetDouble();
+    ParseNext();
+    return result;
+}
+
+bool LookaheadParser::GetBool() {
+    if (st_ != kHasValue || !v_.IsBool()) {
+        st_  = kError;
+        return false;
+    }
+    
+    bool result = v_.GetBool();
+    ParseNext();
+    return result;
+}
+
+void LookaheadParser::GetNull() {
+    if (st_ != kHasValue || !v_.IsNull()) {
+        st_  = kError;
+        return;
+    }
+
+    ParseNext();
+}
+
+const char* LookaheadParser::GetString() {
+    if (st_ != kHasValue || !v_.IsString()) {
+        st_  = kError;
+        return 0;
+    }
+    
+    const char* result = v_.GetString();
+    ParseNext();
+    return result;
+}
+
+void LookaheadParser::SkipValue() {
+    int depth = 0;
+    do {
+        switch (st_) {
+            case kEnteringArray:
+            case kEnteringObject:
+                ++depth;
+                break;
+
+            case kExitingArray:
+            case kExitingObject:
+                --depth;
+                break;
+            
+            case kError:
+                return;
+                
+            case kHasKey:
+            case kHasValue:
+                break;
+        }
+        ParseNext();
+    }
+    while (depth > 0);
+}
+
+Value* LookaheadParser::PeekValue() {
+    if (st_ == kHasValue || st_ == kHasKey) {
+        return &v_;
+    }
+    
+    return 0;
+}
+
+int LookaheadParser::PeekType() {
+    switch (st_) {
+        case kHasValue:
+        case kHasKey:
+            return v_.GetType();
+        
+        case kEnteringArray:
+            return kArrayType;
+
+        case kEnteringObject:
+            return kObjectType;
+
+        case kExitingArray:
+        case kExitingObject:
+        case kError:
+            return -1;
+    }
+}
+
+//-------------------------------------------------------------------------
+
+int main() {
+    using namespace std;
+
+    char json[] = " { \"hello\" : \"world\", \"t\" : true , \"f\" : false, \"n\": null,"
+        "\"i\":123, \"pi\": 3.1416, \"a\":[-1, 2, 3, 4, \"array\", []], \"skipArrays\":[1, 2, [[[3]]]], "
+        "\"skipObject\":{ \"i\":0, \"t\":true, \"n\":null, \"d\":123.45 }, "
+        "\"skipNested\":[[[[{\"\":0}, {\"\":[-9.87]}]]], [], []], "
+        "\"skipString\":\"zzz\", \"reachedEnd\":null, \"t\":true }";
+
+    LookaheadParser r(json);
+    
+    RAPIDJSON_ASSERT(r.PeekType() == kObjectType);
+
+    r.EnterObject();
+    while (const char* key = r.NextObjectKey()) {
+        if (0 == strcmp(key, "hello")) {
+            RAPIDJSON_ASSERT(r.PeekType() == kStringType);
+            cout << key << ":" << r.GetString() << endl;
+        }
+        else if (0 == strcmp(key, "t") || 0 == strcmp(key, "f")) {
+            RAPIDJSON_ASSERT(r.PeekType() == kTrueType || r.PeekType() == kFalseType);
+            cout << key << ":" << r.GetBool() << endl;
+            continue;
+        }
+        else if (0 == strcmp(key, "n")) {
+            RAPIDJSON_ASSERT(r.PeekType() == kNullType);
+            r.GetNull();
+            cout << key << endl;
+            continue;
+        }
+        else if (0 == strcmp(key, "pi")) {
+            RAPIDJSON_ASSERT(r.PeekType() == kNumberType);
+            cout << key << ":" << r.GetDouble() << endl;
+            continue;
+        }
+        else if (0 == strcmp(key, "a")) {
+            RAPIDJSON_ASSERT(r.PeekType() == kArrayType);
+            
+            r.EnterArray();
+            
+            cout << key << ":[ ";
+            while (r.NextArrayValue()) {
+                if (r.PeekType() == kNumberType) {
+                    cout << r.GetDouble() << " ";
+                }
+                else if (r.PeekType() == kStringType) {
+                    cout << r.GetString() << " ";
+                }
+                else {
+                    r.ExitArray();
+                    break;
+                }
+            }
+            
+            cout << "]" << endl;
+        }
+        else {
+            cout << key << ":skipped" << endl;
+            r.SkipValue();
+        }
+    }
+    
+    return 0;
+}