gecko-dev/mfbt/JSONWriter.h

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A JSON pretty-printer class. */
// A typical JSON-writing library requires you to first build up a data
// structure that represents a JSON object and then serialize it (to file, or
// somewhere else). This approach makes for a clean API, but building the data
// structure takes up memory. Sometimes that isn't desirable, such as when the
// JSON data is produced for memory reporting.
//
// The JSONWriter class instead allows JSON data to be written out
// incrementally without building up large data structures.
//
// The API is slightly uglier than you would see in a typical JSON-writing
// library, but still fairly easy to use. It's possible to generate invalid
// JSON with JSONWriter, but typically the most basic testing will identify any
// such problems.
//
// Similarly, there are no RAII facilities for automatically closing objects
// and arrays. These would be nice if you are generating all your code within
// nested functions, but in other cases you'd have to maintain an explicit
// stack of RAII objects and manually unwind it, which is no better than just
// calling "end" functions. Furthermore, the consequences of forgetting to
// close an object or array are obvious and, again, will be identified via
// basic testing, unlike other cases where RAII is typically used (e.g. smart
// pointers) and the consequences of defects are more subtle.
//
// Importantly, the class does solve the two hard problems of JSON
// pretty-printing, which are (a) correctly escaping strings, and (b) adding
// appropriate indentation and commas between items.
//
// Strings used (for property names and string property values) are |const
// char*| throughout, and can be ASCII or UTF-8.
//
// EXAMPLE
// -------
// Assume that |MyWriteFunc| is a class that implements |JSONWriteFunc|. The
// following code:
//
// JSONWriter w(MakeUnique<MyWriteFunc>());
// w.Start();
// {
// w.NullProperty("null");
// w.BoolProperty("bool", true);
// w.IntProperty("int", 1);
// w.StringProperty("string", "hello");
// w.StartArrayProperty("array");
// {
// w.DoubleElement(3.4);
// w.StartObjectElement();
// {
// w.PointerProperty("ptr", (void*)0x12345678);
// }
// w.EndObjectElement();
// }
// w.EndArrayProperty();
// }
// w.End();
//
// will produce pretty-printed output for the following JSON object:
//
// {
// "null": null,
// "bool": true,
// "int": 1,
// "string": "hello",
// "array": [
// 3.4,
// {
// "ptr": "0x12345678"
// }
// ]
// }
//
// The nesting in the example code is obviously optional, but can aid
// readability.
#ifndef mozilla_JSONWriter_h
#define mozilla_JSONWriter_h
#include "mozilla/double-conversion.h"
#include "mozilla/IntegerPrintfMacros.h"
#include "mozilla/PodOperations.h"
#include "mozilla/UniquePtr.h"
#include "mozilla/Vector.h"
#include <stdio.h>
namespace mozilla {
// A quasi-functor for JSONWriter. We don't use a true functor because that
// requires templatizing JSONWriter, and the templatization seeps to lots of
// places we don't want it to.
class JSONWriteFunc
{
public:
virtual void Write(const char* aStr) = 0;
virtual ~JSONWriteFunc() {}
};
// Ideally this would be within |EscapedString| but when compiling with GCC
// on Linux that caused link errors, whereas this formulation didn't.
namespace detail {
extern MFBT_DATA const char gTwoCharEscapes[256];
}
class JSONWriter
{
// From http://www.ietf.org/rfc/rfc4627.txt:
//
// "All Unicode characters may be placed within the quotation marks except
// for the characters that must be escaped: quotation mark, reverse
// solidus, and the control characters (U+0000 through U+001F)."
//
// This implementation uses two-char escape sequences where possible, namely:
//
// \", \\, \b, \f, \n, \r, \t
//
// All control characters not in the above list are represented with a
// six-char escape sequence, e.g. '\u000b' (a.k.a. '\v').
//
class EscapedString
{
// Only one of |mUnownedStr| and |mOwnedStr| are ever non-null. |mIsOwned|
// indicates which one is in use. They're not within a union because that
// wouldn't work with UniquePtr.
bool mIsOwned;
const char* mUnownedStr;
UniquePtr<char[]> mOwnedStr;
void SanityCheck() const
{
MOZ_ASSERT_IF( mIsOwned, mOwnedStr.get() && !mUnownedStr);
MOZ_ASSERT_IF(!mIsOwned, !mOwnedStr.get() && mUnownedStr);
}
static char hexDigitToAsciiChar(uint8_t u)
{
u = u & 0xf;
return u < 10 ? '0' + u : 'a' + (u - 10);
}
public:
explicit EscapedString(const char* aStr)
: mUnownedStr(nullptr)
, mOwnedStr(nullptr)
{
const char* p;
// First, see if we need to modify the string.
size_t nExtra = 0;
p = aStr;
while (true) {
uint8_t u = *p; // ensure it can't be interpreted as negative
if (u == 0) {
break;
}
if (detail::gTwoCharEscapes[u]) {
nExtra += 1;
} else if (u <= 0x1f) {
nExtra += 5;
}
p++;
}
if (nExtra == 0) {
// No escapes needed. Easy.
mIsOwned = false;
mUnownedStr = aStr;
return;
}
// Escapes are needed. We'll create a new string.
mIsOwned = true;
size_t len = (p - aStr) + nExtra;
mOwnedStr = MakeUnique<char[]>(len + 1);
p = aStr;
size_t i = 0;
while (true) {
uint8_t u = *p; // ensure it can't be interpreted as negative
if (u == 0) {
mOwnedStr[i] = 0;
break;
}
if (detail::gTwoCharEscapes[u]) {
mOwnedStr[i++] = '\\';
mOwnedStr[i++] = detail::gTwoCharEscapes[u];
} else if (u <= 0x1f) {
mOwnedStr[i++] = '\\';
mOwnedStr[i++] = 'u';
mOwnedStr[i++] = '0';
mOwnedStr[i++] = '0';
mOwnedStr[i++] = hexDigitToAsciiChar((u & 0x00f0) >> 4);
mOwnedStr[i++] = hexDigitToAsciiChar(u & 0x000f);
} else {
mOwnedStr[i++] = u;
}
p++;
}
}
~EscapedString()
{
SanityCheck();
}
const char* get() const
{
SanityCheck();
return mIsOwned ? mOwnedStr.get() : mUnownedStr;
}
};
const UniquePtr<JSONWriteFunc> mWriter;
Vector<bool, 8> mNeedComma; // do we need a comma at depth N?
size_t mDepth; // the current nesting depth
void Indent()
{
for (size_t i = 0; i < mDepth; i++) {
mWriter->Write(" ");
}
}
// Adds whatever is necessary (maybe a comma, and then a newline and
// whitespace) to separate an item (property or element) from what's come
// before.
void Separator()
{
if (mNeedComma[mDepth]) {
mWriter->Write(",");
}
if (mDepth > 0) {
mWriter->Write("\n");
}
Indent();
}
void PropertyNameAndColon(const char* aName)
{
EscapedString escapedName(aName);
mWriter->Write("\"");
mWriter->Write(escapedName.get());
mWriter->Write("\": ");
}
void Scalar(const char* aMaybePropertyName, const char* aStringValue)
{
Separator();
if (aMaybePropertyName) {
PropertyNameAndColon(aMaybePropertyName);
}
mWriter->Write(aStringValue);
mNeedComma[mDepth] = true;
}
void QuotedScalar(const char* aMaybePropertyName, const char* aStringValue)
{
Separator();
if (aMaybePropertyName) {
PropertyNameAndColon(aMaybePropertyName);
}
mWriter->Write("\"");
mWriter->Write(aStringValue);
mWriter->Write("\"");
mNeedComma[mDepth] = true;
}
void NewCommaEntry()
{
// If this tiny allocation OOMs we might as well just crash because we must
// be in serious memory trouble.
MOZ_RELEASE_ASSERT(mNeedComma.growByUninitialized(1));
mNeedComma[mDepth] = false;
}
void StartCollection(const char* aMaybePropertyName, const char* aStartChar)
{
Separator();
if (aMaybePropertyName) {
mWriter->Write("\"");
mWriter->Write(aMaybePropertyName);
mWriter->Write("\": ");
}
mWriter->Write(aStartChar);
mNeedComma[mDepth] = true;
mDepth++;
NewCommaEntry();
}
// Adds the whitespace and closing char necessary to end a collection.
void EndCollection(const char* aEndChar)
{
mDepth--;
mWriter->Write("\n");
Indent();
mWriter->Write(aEndChar);
}
public:
explicit JSONWriter(UniquePtr<JSONWriteFunc> aWriter)
: mWriter(Move(aWriter))
, mNeedComma()
, mDepth(0)
{
NewCommaEntry();
}
// Returns the JSONWriteFunc passed in at creation, for temporary use. The
// JSONWriter object still owns the JSONWriteFunc.
JSONWriteFunc* WriteFunc() const { return mWriter.get(); }
// For all the following functions, the "Prints:" comment indicates what the
// basic output looks like. However, it doesn't indicate the indentation and
// trailing commas, which are automatically added as required.
//
// All property names and string properties are escaped as necessary.
// Prints: {
void Start() { StartCollection(nullptr, "{"); }
// Prints: }\n
void End() { EndCollection("}\n"); }
// Prints: "<aName>": null
void NullProperty(const char* aName)
{
Scalar(aName, "null");
}
// Prints: null
void NullElement() { NullProperty(nullptr); }
// Prints: "<aName>": <aBool>
void BoolProperty(const char* aName, bool aBool)
{
Scalar(aName, aBool ? "true" : "false");
}
// Prints: <aBool>
void BoolElement(bool aBool) { BoolProperty(nullptr, aBool); }
// Prints: "<aName>": <aInt>
void IntProperty(const char* aName, int64_t aInt)
{
char buf[64];
sprintf(buf, "%" PRId64, aInt);
Scalar(aName, buf);
}
// Prints: <aInt>
void IntElement(int64_t aInt) { IntProperty(nullptr, aInt); }
// Prints: "<aName>": <aDouble>
void DoubleProperty(const char* aName, double aDouble)
{
static const size_t buflen = 64;
char buf[buflen];
const double_conversion::DoubleToStringConverter &converter =
double_conversion::DoubleToStringConverter::EcmaScriptConverter();
double_conversion::StringBuilder builder(buf, buflen);
converter.ToShortest(aDouble, &builder);
Scalar(aName, builder.Finalize());
}
// Prints: <aDouble>
void DoubleElement(double aDouble) { DoubleProperty(nullptr, aDouble); }
// Prints: "<aName>": "<aStr>"
void StringProperty(const char* aName, const char* aStr)
{
EscapedString escapedStr(aStr);
QuotedScalar(aName, escapedStr.get());
}
// Prints: "<aStr>"
void StringElement(const char* aStr) { StringProperty(nullptr, aStr); }
// Prints: "<aName>": "<aPtr>"
// The pointer is printed as a hexadecimal integer with a leading '0x'.
void PointerProperty(const char* aName, const void* aPtr)
{
char buf[32];
sprintf(buf, "0x%" PRIxPTR, uintptr_t(aPtr));
QuotedScalar(aName, buf);
}
// Prints: "<aPtr>"
// The pointer is printed as a hexadecimal integer with a leading '0x'.
void PointerElement(const void* aPtr) { PointerProperty(nullptr, aPtr); }
// Prints: "<aName>": [
void StartArrayProperty(const char* aName) { StartCollection(aName, "["); }
// Prints: [
void StartArrayElement() { StartArrayProperty(nullptr); }
// Prints: ]
void EndArray() { EndCollection("]"); }
// Prints: "<aName>": {
void StartObjectProperty(const char* aName) { StartCollection(aName, "{"); }
// Prints: {
void StartObjectElement() { StartObjectProperty(nullptr); }
// Prints: }
void EndObject() { EndCollection("}"); }
};
} // namespace mozilla
#endif /* mozilla_JSONWriter_h */