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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.
+//
+// By default, every property is placed on its own line. However, it is
+// possible to request that objects and arrays be placed entirely on a single
+// line, which can reduce output size significantly in some cases.
+//
+// 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.StartArrayProperty("array");
+// {
+// w.StringElement("string");
+// w.StartObjectElement();
+// {
+// w.DoubleProperty("double", 3.4);
+// w.StartArrayProperty("single-line array", w.SingleLineStyle);
+// {
+// w.IntElement(1);
+// w.StartObjectElement(); // SingleLineStyle is inherited from
+// w.EndObjectElement(); // above for this collection
+// }
+// w.EndArray();
+// }
+// w.EndObjectElement();
+// }
+// w.EndArrayProperty();
+// }
+// w.End();
+//
+// will produce pretty-printed output for the following JSON object:
+//
+// {
+// "null": null,
+// "bool": true,
+// "int": 1,
+// "array": [
+// "string",
+// {
+// "double": 3.4,
+// "single-line array": [1, {}]
+// }
+// ]
+// }
+//
+// 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/Sprintf.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];
+} // namespace detail
+
+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;
+ }
+ };
+
+public:
+ // Collections (objects and arrays) are printed in a multi-line style by
+ // default. This can be changed to a single-line style if SingleLineStyle is
+ // specified. If a collection is printed in single-line style, every nested
+ // collection within it is also printed in single-line style, even if
+ // multi-line style is requested.
+ enum CollectionStyle {
+ MultiLineStyle, // the default
+ SingleLineStyle
+ };
+
+protected:
+ const UniquePtr<JSONWriteFunc> mWriter;
+ Vector<bool, 8> mNeedComma; // do we need a comma at depth N?
+ Vector<bool, 8> mNeedNewlines; // do we need newlines 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 && mNeedNewlines[mDepth]) {
+ mWriter->Write("\n");
+ Indent();
+ } else if (mNeedComma[mDepth]) {
+ mWriter->Write(" ");
+ }
+ }
+
+ 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 NewVectorEntries()
+ {
+ // If these tiny allocations OOM we might as well just crash because we
+ // must be in serious memory trouble.
+ MOZ_RELEASE_ASSERT(mNeedComma.resizeUninitialized(mDepth + 1));
+ MOZ_RELEASE_ASSERT(mNeedNewlines.resizeUninitialized(mDepth + 1));
+ mNeedComma[mDepth] = false;
+ mNeedNewlines[mDepth] = true;
+ }
+
+ void StartCollection(const char* aMaybePropertyName, const char* aStartChar,
+ CollectionStyle aStyle = MultiLineStyle)
+ {
+ Separator();
+ if (aMaybePropertyName) {
+ mWriter->Write("\"");
+ mWriter->Write(aMaybePropertyName);
+ mWriter->Write("\": ");
+ }
+ mWriter->Write(aStartChar);
+ mNeedComma[mDepth] = true;
+ mDepth++;
+ NewVectorEntries();
+ mNeedNewlines[mDepth] =
+ mNeedNewlines[mDepth - 1] && aStyle == MultiLineStyle;
+ }
+
+ // Adds the whitespace and closing char necessary to end a collection.
+ void EndCollection(const char* aEndChar)
+ {
+ if (mNeedNewlines[mDepth]) {
+ mWriter->Write("\n");
+ mDepth--;
+ Indent();
+ } else {
+ mDepth--;
+ }
+ mWriter->Write(aEndChar);
+ }
+
+public:
+ explicit JSONWriter(UniquePtr<JSONWriteFunc> aWriter)
+ : mWriter(Move(aWriter))
+ , mNeedComma()
+ , mNeedNewlines()
+ , mDepth(0)
+ {
+ NewVectorEntries();
+ }
+
+ // 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 whitespace and
+ // trailing commas, which are automatically added as required.
+ //
+ // All property names and string properties are escaped as necessary.
+
+ // Prints: {
+ void Start(CollectionStyle aStyle = MultiLineStyle)
+ {
+ StartCollection(nullptr, "{", aStyle);
+ }
+
+ // Prints: }
+ 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];
+ SprintfLiteral(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>": [
+ void StartArrayProperty(const char* aName,
+ CollectionStyle aStyle = MultiLineStyle)
+ {
+ StartCollection(aName, "[", aStyle);
+ }
+
+ // Prints: [
+ void StartArrayElement(CollectionStyle aStyle = MultiLineStyle)
+ {
+ StartArrayProperty(nullptr, aStyle);
+ }
+
+ // Prints: ]
+ void EndArray() { EndCollection("]"); }
+
+ // Prints: "<aName>": {
+ void StartObjectProperty(const char* aName,
+ CollectionStyle aStyle = MultiLineStyle)
+ {
+ StartCollection(aName, "{", aStyle);
+ }
+
+ // Prints: {
+ void StartObjectElement(CollectionStyle aStyle = MultiLineStyle)
+ {
+ StartObjectProperty(nullptr, aStyle);
+ }
+
+ // Prints: }
+ void EndObject() { EndCollection("}"); }
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_JSONWriter_h */
+