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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * 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/. */

#ifndef vm_String_h
#define vm_String_h

#include "mozilla/MemoryReporting.h"
#include "mozilla/PodOperations.h"

#include "jsapi.h"
#include "jsfriendapi.h"
#include "jsstr.h"

#include "gc/Barrier.h"
#include "gc/Heap.h"
#include "gc/Marking.h"
#include "gc/Rooting.h"
#include "js/CharacterEncoding.h"
#include "js/RootingAPI.h"

class JSDependentString;
class JSExtensibleString;
class JSExternalString;
class JSInlineString;
class JSRope;

namespace js {

class StaticStrings;
class PropertyName;

/* The buffer length required to contain any unsigned 32-bit integer. */
static const size_t UINT32_CHAR_BUFFER_LENGTH = sizeof("4294967295") - 1;

} /* namespace js */

/*
 * JavaScript strings
 *
 * Conceptually, a JS string is just an array of chars and a length. This array
 * of chars may or may not be null-terminated and, if it is, the null character
 * is not included in the length.
 *
 * To improve performance of common operations, the following optimizations are
 * made which affect the engine's representation of strings:
 *
 *  - The plain vanilla representation is a "flat" string which consists of a
 *    string header in the GC heap and a malloc'd null terminated char array.
 *
 *  - To avoid copying a substring of an existing "base" string , a "dependent"
 *    string (JSDependentString) can be created which points into the base
 *    string's char array.
 *
 *  - To avoid O(n^2) char buffer copying, a "rope" node (JSRope) can be created
 *    to represent a delayed string concatenation. Concatenation (called
 *    flattening) is performed if and when a linear char array is requested. In
 *    general, ropes form a binary dag whose internal nodes are JSRope string
 *    headers with no associated char array and whose leaf nodes are either flat
 *    or dependent strings.
 *
 *  - To avoid copying the left-hand side when flattening, the left-hand side's
 *    buffer may be grown to make space for a copy of the right-hand side (see
 *    comment in JSString::flatten). This optimization requires that there are
 *    no external pointers into the char array. We conservatively maintain this
 *    property via a flat string's "extensible" property.
 *
 *  - To avoid allocating small char arrays, short strings can be stored inline
 *    in the string header (JSInlineString). To increase the max size of such
 *    inline strings, extra-large string headers can be used (JSShortString).
 *
 *  - To avoid comparing O(n) string equality comparison, strings can be
 *    canonicalized to "atoms" (JSAtom) such that there is a single atom with a
 *    given (length,chars).
 *
 *  - To avoid copying all strings created through the JSAPI, an "external"
 *    string (JSExternalString) can be created whose chars are managed by the
 *    JSAPI client.
 *
 * Although all strings share the same basic memory layout, we can conceptually
 * arrange them into a hierarchy of operations/invariants and represent this
 * hierarchy in C++ with classes:
 *
 * C++ type                     operations+fields / invariants+properties
 * ==========================   =========================================
 * JSString (abstract)          getCharsZ, getChars, length / -
 *  | \
 *  | JSRope                    leftChild, rightChild / -
 *  |
 * JSLinearString (abstract)    chars / might be null-terminated
 *  | \
 *  | JSDependentString         base / -
 *  |
 * JSFlatString                 - / null terminated
 *  |  |
 *  |  +-- JSExternalString     - / char array memory managed by embedding
 *  |  |
 *  |  +-- JSExtensibleString   capacity / no external pointers into char array
 *  |  |
 *  |  +-- JSUndependedString   original dependent base / -
 *  |  |
 *  |  +-- JSInlineString       - / chars stored in header
 *  |         \
 *  |         JSShortString     - / header is fat
 *  |
 * JSAtom                       - / string equality === pointer equality
 *  |
 * js::PropertyName             - / chars don't contain an index (uint32_t)
 *
 * Classes marked with (abstract) above are not literally C++ Abstract Base
 * Classes (since there are no virtual functions, pure or not, in this
 * hierarchy), but have the same meaning: there are no strings with this type as
 * its most-derived type.
 *
 * Atoms can additionally be permanent, i.e. unable to be collected, and can
 * be combined with other string types to create additional most-derived types
 * that satisfy the invariants of more than one of the abovementioned
 * most-derived types:
 *  - InlineAtom = JSInlineString + JSAtom (atom with inline chars)
 *  - ShortAtom  = JSShortString  + JSAtom (atom with (more) inline chars)
 *
 * Derived string types can be queried from ancestor types via isX() and
 * retrieved with asX() debug-only-checked casts.
 *
 * The ensureX() operations mutate 'this' in place to effectively the type to be
 * at least X (e.g., ensureLinear will change a JSRope to be a JSFlatString).
 */

class JSString : public js::gc::BarrieredCell<JSString>
{
  protected:
    static const size_t NUM_INLINE_CHARS = 2 * sizeof(void*) / sizeof(jschar);

    /* Fields only apply to string types commented on the right. */
    struct Data
    {
        size_t                     lengthAndFlags;      /* JSString */
        union {
            const jschar*          chars;              /* JSLinearString */
            JSString*              left;               /* JSRope */
        } u1;
        union {
            jschar                 inlineStorage[NUM_INLINE_CHARS]; /* JS(Inline|Short)String */
            struct {
                union {
                    JSLinearString* base;               /* JS(Dependent|Undepended)String */
                    JSString*      right;              /* JSRope */
                    size_t         capacity;            /* JSFlatString (extensible) */
                    const JSStringFinalizer* externalFinalizer;/* JSExternalString */
                } u2;
                union {
                    JSString*      parent;             /* JSRope (temporary) */
                    size_t         reserved;            /* may use for bug 615290 */
                } u3;
            } s;
        };
    } d;

  public:
    /* Flags exposed only for jits */

    /*
     * The low LENGTH_SHIFT bits of lengthAndFlags are used to encode the type
     * of the string. The remaining bits store the string length (which must be
     * less or equal than MAX_LENGTH).
     *
     * Instead of using a dense index to represent the most-derived type, string
     * types are encoded to allow single-op tests for hot queries (isRope,
     * isDependent, isFlat, isAtom) which, in view of subtyping, would require
     * slower (isX() || isY() || isZ()).
     *
     * The string type encoding can be summarized as follows. The "instance
     * encoding" entry for a type specifies the flag bits used to create a
     * string instance of that type. Abstract types have no instances and thus
     * have no such entry. The "subtype predicate" entry for a type specifies
     * the predicate used to query whether a JSString instance is subtype
     * (reflexively) of that type.
     *
     *   Rope          0000       0000
     *   Linear        -         !0000
     *   HasBase       -          xxx1
     *   Dependent     0001       0001
     *   Flat          -          isLinear && !isDependent
     *   Undepended    0011       0011
     *   Extensible    0010       0010
     *   Inline        0100       isFlat && !isExtensible && (u1.chars == inlineStorage)
     *   Short         0100       header in FINALIZE_SHORT_STRING arena
     *   External      0100       header in FINALIZE_EXTERNAL_STRING arena
     *   Atom          -          1xxx
     *   PermanentAtom 1100       1100
     *   InlineAtom    -          isAtom && is Inline
     *   ShortAtom     -          isAtom && is Short
     *
     *  "HasBase" here refers to the two string types that have a 'base' field:
     *  JSDependentString and JSUndependedString.
     *  A JSUndependedString is a JSDependentString which has been 'fixed' (by ensureFixed)
     *  to be null-terminated.  In such cases, the string must keep marking its base since
     *  there may be any number of *other* JSDependentStrings transitively depending on it.
     *
     */

    static const size_t LENGTH_SHIFT          = 4;
    static const size_t FLAGS_MASK            = JS_BITMASK(LENGTH_SHIFT);

    static const size_t ROPE_FLAGS            = 0;
    static const size_t DEPENDENT_FLAGS       = JS_BIT(0);
    static const size_t UNDEPENDED_FLAGS      = JS_BIT(0) | JS_BIT(1);
    static const size_t EXTENSIBLE_FLAGS      = JS_BIT(1);
    static const size_t FIXED_FLAGS           = JS_BIT(2);

    static const size_t INT32_MASK            = JS_BITMASK(3);
    static const size_t INT32_FLAGS           = JS_BIT(1) | JS_BIT(2);

    static const size_t HAS_BASE_BIT          = JS_BIT(0);
    static const size_t PERMANENT_BIT         = JS_BIT(2);
    static const size_t ATOM_BIT              = JS_BIT(3);

    static const size_t PERMANENT_ATOM_FLAGS  = JS_BIT(2) | JS_BIT(3);

    static const size_t MAX_LENGTH            = JS_BIT(32 - LENGTH_SHIFT) - 1;

    size_t buildLengthAndFlags(size_t length, size_t flags) {
        JS_ASSERT(length <= MAX_LENGTH);
        JS_ASSERT(flags <= FLAGS_MASK);
        return (length << LENGTH_SHIFT) | flags;
    }

    /*
     * Helper function to validate that a string of a given length is
     * representable by a JSString. An allocation overflow is reported if false
     * is returned.
     */
    static inline bool validateLength(js::ThreadSafeContext* maybecx, size_t length);

    static void staticAsserts() {
        JS_STATIC_ASSERT(JS_BITS_PER_WORD >= 32);
        JS_STATIC_ASSERT(((JSString::MAX_LENGTH << JSString::LENGTH_SHIFT) >>
                           JSString::LENGTH_SHIFT) == JSString::MAX_LENGTH);
        JS_STATIC_ASSERT(sizeof(JSString) ==
                         offsetof(JSString, d.inlineStorage) + NUM_INLINE_CHARS * sizeof(jschar));
        JS_STATIC_ASSERT(offsetof(JSString, d.u1.chars) ==
                         offsetof(js::shadow::Atom, chars));
    }

    /* Avoid lame compile errors in JSRope::flatten */
    friend class JSRope;

  public:
    /* All strings have length. */

    MOZ_ALWAYS_INLINE
    size_t length() const {
        return d.lengthAndFlags >> LENGTH_SHIFT;
    }

    MOZ_ALWAYS_INLINE
    bool empty() const {
        return d.lengthAndFlags <= FLAGS_MASK;
    }

    /*
     * All strings have a fallible operation to get an array of chars.
     * getCharsZ additionally ensures the array is null terminated.
     */

    inline const jschar* getChars(js::ExclusiveContext* cx);
    inline const jschar* getCharsZ(js::ExclusiveContext* cx);
    inline bool getChar(js::ExclusiveContext* cx, size_t index, jschar* code);

    /*
     * A string has "pure" chars if it can return a pointer to its chars
     * infallibly without mutating anything so they are safe to be from off the
     * main thread. If a string does not have pure chars, the caller can call
     * copyNonPureChars to allocate a copy of the chars which is also a
     * non-mutating threadsafe operation. Beware, this is an O(n) operation
     * (involving a DAG traversal for ropes).
     */
    bool hasPureChars() const { return isLinear(); }
    bool hasPureCharsZ() const { return isFlat(); }
    inline const jschar* pureChars() const;
    inline const jschar* pureCharsZ() const;
    inline bool copyNonPureChars(js::ThreadSafeContext* cx,
                                 js::ScopedJSFreePtr<jschar>& out) const;
    inline bool copyNonPureCharsZ(js::ThreadSafeContext* cx,
                                  js::ScopedJSFreePtr<jschar>& out) const;

    /* Fallible conversions to more-derived string types. */

    inline JSLinearString* ensureLinear(js::ExclusiveContext* cx);
    inline JSFlatString* ensureFlat(js::ExclusiveContext* cx);

    static bool ensureLinear(js::ExclusiveContext* cx, JSString* str) {
        return str->ensureLinear(cx) != nullptr;
    }

    /* Type query and debug-checked casts */

    MOZ_ALWAYS_INLINE
    bool isRope() const {
        return (d.lengthAndFlags & FLAGS_MASK) == ROPE_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSRope& asRope() const {
        JS_ASSERT(isRope());
        return *(JSRope*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isLinear() const {
        return !isRope();
    }

    MOZ_ALWAYS_INLINE
    JSLinearString& asLinear() const {
        JS_ASSERT(JSString::isLinear());
        return *(JSLinearString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isDependent() const {
        return (d.lengthAndFlags & FLAGS_MASK) == DEPENDENT_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSDependentString& asDependent() const {
        JS_ASSERT(isDependent());
        return *(JSDependentString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isFlat() const {
        return isLinear() && !isDependent();
    }

    MOZ_ALWAYS_INLINE
    JSFlatString& asFlat() const {
        JS_ASSERT(isFlat());
        return *(JSFlatString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isExtensible() const {
        return (d.lengthAndFlags & FLAGS_MASK) == EXTENSIBLE_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSExtensibleString& asExtensible() const {
        JS_ASSERT(isExtensible());
        return *(JSExtensibleString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isInline() const {
        return isFlat() && !isExtensible() && (d.u1.chars == d.inlineStorage);
    }

    MOZ_ALWAYS_INLINE
    JSInlineString& asInline() const {
        JS_ASSERT(isInline());
        return *(JSInlineString*)this;
    }

    bool isShort() const;

    /* For hot code, prefer other type queries. */
    bool isExternal() const;

    MOZ_ALWAYS_INLINE
    JSExternalString& asExternal() const {
        JS_ASSERT(isExternal());
        return *(JSExternalString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isUndepended() const {
        return (d.lengthAndFlags & FLAGS_MASK) == UNDEPENDED_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    bool isAtom() const {
        return (d.lengthAndFlags & ATOM_BIT);
    }

    MOZ_ALWAYS_INLINE
    bool isPermanentAtom() const {
        return (d.lengthAndFlags & FLAGS_MASK) == PERMANENT_ATOM_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSAtom& asAtom() const {
        JS_ASSERT(isAtom());
        return *(JSAtom*)this;
    }

    /* Only called by the GC for dependent or undepended strings. */

    inline bool hasBase() const {
        JS_STATIC_ASSERT((DEPENDENT_FLAGS | JS_BIT(1)) == UNDEPENDED_FLAGS);
        return (d.lengthAndFlags & HAS_BASE_BIT);
    }

    inline JSLinearString* base() const;

    inline void markBase(JSTracer* trc);

    /* Only called by the GC for strings with the FINALIZE_STRING kind. */

    inline void finalize(js::FreeOp* fop);

    /* Gets the number of bytes that the chars take on the heap. */

    size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf);

    /* Offsets for direct field from jit code. */

    static size_t offsetOfLengthAndFlags() {
        return offsetof(JSString, d.lengthAndFlags);
    }

    static size_t offsetOfChars() {
        return offsetof(JSString, d.u1.chars);
    }

    js::gc::AllocKind getAllocKind() const { return tenuredGetAllocKind(); }

    static inline js::ThingRootKind rootKind() { return js::THING_ROOT_STRING; }

#ifdef DEBUG
    void dump();
    static void dumpChars(const jschar* s, size_t len);
    bool equals(const char* s);
#endif

    static MOZ_ALWAYS_INLINE void readBarrier(JSString* thing) {
#ifdef JSGC_INCREMENTAL
        if (thing->isPermanentAtom())
            return;

        js::gc::BarrieredCell<JSString>::readBarrier(thing);
#endif
    }

    static MOZ_ALWAYS_INLINE void writeBarrierPre(JSString* thing) {
#ifdef JSGC_INCREMENTAL
        if (isNullLike(thing) || thing->isPermanentAtom())
            return;

        js::gc::BarrieredCell<JSString>::writeBarrierPre(thing);
#endif
    }

  private:
    JSString() MOZ_DELETE;
    JSString(const JSString& other) MOZ_DELETE;
    void operator=(const JSString& other) MOZ_DELETE;
};

class JSRope : public JSString
{
    bool copyNonPureCharsInternal(js::ThreadSafeContext* cx,
                                  js::ScopedJSFreePtr<jschar>& out,
                                  bool nullTerminate) const;
    bool copyNonPureChars(js::ThreadSafeContext* cx, js::ScopedJSFreePtr<jschar>& out) const;
    bool copyNonPureCharsZ(js::ThreadSafeContext* cx, js::ScopedJSFreePtr<jschar>& out) const;

    enum UsingBarrier { WithIncrementalBarrier, NoBarrier };
    template<UsingBarrier b>
    JSFlatString* flattenInternal(js::ExclusiveContext* cx);

    friend class JSString;
    JSFlatString* flatten(js::ExclusiveContext* cx);

    void init(js::ThreadSafeContext* cx, JSString* left, JSString* right, size_t length);

  public:
    template <js::AllowGC allowGC>
    static inline JSRope* new_(js::ThreadSafeContext* cx,
                               typename js::MaybeRooted<JSString*, allowGC>::HandleType left,
                               typename js::MaybeRooted<JSString*, allowGC>::HandleType right,
                               size_t length);

    inline JSString* leftChild() const {
        JS_ASSERT(isRope());
        return d.u1.left;
    }

    inline JSString* rightChild() const {
        JS_ASSERT(isRope());
        return d.s.u2.right;
    }

    inline void markChildren(JSTracer* trc);

    inline static size_t offsetOfLeft() {
        return offsetof(JSRope, d.u1.left);
    }
    inline static size_t offsetOfRight() {
        return offsetof(JSRope, d.s.u2.right);
    }
};

JS_STATIC_ASSERT(sizeof(JSRope) == sizeof(JSString));

class JSLinearString : public JSString
{
    friend class JSString;

    /* Vacuous and therefore unimplemented. */
    JSLinearString* ensureLinear(JSContext* cx) MOZ_DELETE;
    bool isLinear() const MOZ_DELETE;
    JSLinearString& asLinear() const MOZ_DELETE;

  public:
    MOZ_ALWAYS_INLINE
    const jschar* chars() const {
        JS_ASSERT(JSString::isLinear());
        return d.u1.chars;
    }

    JS::TwoByteChars range() const {
        JS_ASSERT(JSString::isLinear());
        return JS::TwoByteChars(d.u1.chars, length());
    }
};

JS_STATIC_ASSERT(sizeof(JSLinearString) == sizeof(JSString));

class JSDependentString : public JSLinearString
{
    bool copyNonPureCharsZ(js::ThreadSafeContext* cx, js::ScopedJSFreePtr<jschar>& out) const;

    friend class JSString;
    JSFlatString* undepend(js::ExclusiveContext* cx);

    void init(js::ThreadSafeContext* cx, JSLinearString* base, const jschar* chars,
              size_t length);

    /* Vacuous and therefore unimplemented. */
    bool isDependent() const MOZ_DELETE;
    JSDependentString& asDependent() const MOZ_DELETE;

  public:
    static inline JSLinearString* new_(js::ExclusiveContext* cx, JSLinearString* base,
                                       const jschar* chars, size_t length);
};

JS_STATIC_ASSERT(sizeof(JSDependentString) == sizeof(JSString));

class JSFlatString : public JSLinearString
{
    /* Vacuous and therefore unimplemented. */
    JSFlatString* ensureFlat(JSContext* cx) MOZ_DELETE;
    bool isFlat() const MOZ_DELETE;
    JSFlatString& asFlat() const MOZ_DELETE;

    bool isIndexSlow(uint32_t* indexp) const;

    void init(const jschar* chars, size_t length);

  public:
    template <js::AllowGC allowGC>
    static inline JSFlatString* new_(js::ThreadSafeContext* cx,
                                     const jschar* chars, size_t length);

    MOZ_ALWAYS_INLINE
    const jschar* charsZ() const {
        JS_ASSERT(JSString::isFlat());
        return chars();
    }

    /*
     * Returns true if this string's characters store an unsigned 32-bit
     * integer value, initializing *indexp to that value if so.  (Thus if
     * calling isIndex returns true, js::IndexToString(cx, *indexp) will be a
     * string equal to this string.)
     */
    inline bool isIndex(uint32_t* indexp) const {
        const jschar* s = chars();
        return JS7_ISDEC(*s) && isIndexSlow(indexp);
    }

    /*
     * Returns a property name represented by this string, or null on failure.
     * You must verify that this is not an index per isIndex before calling
     * this method.
     */
    inline js::PropertyName* toPropertyName(JSContext* cx);

    /*
     * Once a JSFlatString sub-class has been added to the atom state, this
     * operation changes the string to the JSAtom type, in place.
     */
    MOZ_ALWAYS_INLINE JSAtom* morphAtomizedStringIntoAtom() {
        d.lengthAndFlags = buildLengthAndFlags(length(), ATOM_BIT);
        return &asAtom();
    }
    MOZ_ALWAYS_INLINE JSAtom* morphAtomizedStringIntoPermanentAtom() {
        d.lengthAndFlags = buildLengthAndFlags(length(), PERMANENT_ATOM_FLAGS);
        return &asAtom();
    }

    inline void finalize(js::FreeOp* fop);
};

JS_STATIC_ASSERT(sizeof(JSFlatString) == sizeof(JSString));

class JSExtensibleString : public JSFlatString
{
    /* Vacuous and therefore unimplemented. */
    bool isExtensible() const MOZ_DELETE;
    JSExtensibleString& asExtensible() const MOZ_DELETE;

  public:
    MOZ_ALWAYS_INLINE
    size_t capacity() const {
        JS_ASSERT(JSString::isExtensible());
        return d.s.u2.capacity;
    }
};

JS_STATIC_ASSERT(sizeof(JSExtensibleString) == sizeof(JSString));

class JSInlineString : public JSFlatString
{
    static const size_t MAX_INLINE_LENGTH = NUM_INLINE_CHARS - 1;

  public:
    template <js::AllowGC allowGC>
    static inline JSInlineString* new_(js::ThreadSafeContext* cx);

    inline jschar* init(size_t length);

    inline void resetLength(size_t length);

    static bool lengthFits(size_t length) {
        return length <= MAX_INLINE_LENGTH;
    }

    static size_t offsetOfInlineStorage() {
        return offsetof(JSInlineString, d.inlineStorage);
    }
};

JS_STATIC_ASSERT(sizeof(JSInlineString) == sizeof(JSString));

class JSShortString : public JSInlineString
{
    /* This can be any value that is a multiple of CellSize. */
    static const size_t INLINE_EXTENSION_CHARS = sizeof(JSString::Data) / sizeof(jschar);

    static void staticAsserts() {
        JS_STATIC_ASSERT(INLINE_EXTENSION_CHARS % js::gc::CellSize == 0);
        JS_STATIC_ASSERT(MAX_SHORT_LENGTH + 1 ==
                         (sizeof(JSShortString) -
                          offsetof(JSShortString, d.inlineStorage)) / sizeof(jschar));
    }

  protected: /* to fool clang into not warning this is unused */
    jschar inlineStorageExtension[INLINE_EXTENSION_CHARS];

  public:
    template <js::AllowGC allowGC>
    static inline JSShortString* new_(js::ThreadSafeContext* cx);

    static const size_t MAX_SHORT_LENGTH = JSString::NUM_INLINE_CHARS +
                                           INLINE_EXTENSION_CHARS
                                           -1 /* null terminator */;

    static bool lengthFits(size_t length) {
        return length <= MAX_SHORT_LENGTH;
    }

    /* Only called by the GC for strings with the FINALIZE_SHORT_STRING kind. */

    MOZ_ALWAYS_INLINE void finalize(js::FreeOp* fop);
};

JS_STATIC_ASSERT(sizeof(JSShortString) == 2 * sizeof(JSString));

class JSExternalString : public JSFlatString
{
    void init(const jschar* chars, size_t length, const JSStringFinalizer* fin);

    /* Vacuous and therefore unimplemented. */
    bool isExternal() const MOZ_DELETE;
    JSExternalString& asExternal() const MOZ_DELETE;

  public:
    static inline JSExternalString* new_(JSContext* cx, const jschar* chars, size_t length,
                                         const JSStringFinalizer* fin);

    const JSStringFinalizer* externalFinalizer() const {
        JS_ASSERT(JSString::isExternal());
        return d.s.u2.externalFinalizer;
    }

    /* Only called by the GC for strings with the FINALIZE_EXTERNAL_STRING kind. */

    inline void finalize(js::FreeOp* fop);
};

JS_STATIC_ASSERT(sizeof(JSExternalString) == sizeof(JSString));

class JSUndependedString : public JSFlatString
{
    /*
     * JSUndependedString is not explicitly used and is only present for
     * consistency. See JSDependentString::undepend for how a JSDependentString
     * gets morphed into a JSUndependedString.
     */
};

JS_STATIC_ASSERT(sizeof(JSUndependedString) == sizeof(JSString));

class JSAtom : public JSFlatString
{
    /* Vacuous and therefore unimplemented. */
    bool isAtom() const MOZ_DELETE;
    JSAtom& asAtom() const MOZ_DELETE;

  public:
    /* Returns the PropertyName for this.  isIndex() must be false. */
    inline js::PropertyName* asPropertyName();

    inline void finalize(js::FreeOp* fop);

    MOZ_ALWAYS_INLINE
    bool isPermanent() const {
        return (d.lengthAndFlags & PERMANENT_BIT);
    }

    // Transform this atom into a permanent atom. This is only done during
    // initialization of the runtime.
    MOZ_ALWAYS_INLINE void morphIntoPermanentAtom() {
        d.lengthAndFlags = buildLengthAndFlags(length(), PERMANENT_ATOM_FLAGS);
    }

#ifdef DEBUG
    void dump();
#endif
};

JS_STATIC_ASSERT(sizeof(JSAtom) == sizeof(JSString));

namespace js {

/*
 * Thread safe RAII wrapper for inspecting the contents of JSStrings. The
 * thread safe operations such as |getCharsNonDestructive| require allocation
 * of a char array. This allocation is not always required, such as when the
 * string is already linear. This wrapper makes dealing with this detail more
 * convenient by encapsulating the allocation logic.
 *
 * As the name suggests, this class is scoped. Return values from chars() and
 * range() may not be valid after the inspector goes out of scope.
 */

class ScopedThreadSafeStringInspector
{
  private:
    JSString* str_;
    ScopedJSFreePtr<jschar> scopedChars_;
    const jschar* chars_;

  public:
    ScopedThreadSafeStringInspector(JSString* str)
      : str_(str),
        chars_(nullptr)
    { }

    bool ensureChars(ThreadSafeContext* cx);

    const jschar* chars() {
        JS_ASSERT(chars_);
        return chars_;
    }

    JS::TwoByteChars range() {
        JS_ASSERT(chars_);
        return JS::TwoByteChars(chars_, str_->length());
    }
};

class StaticStrings
{
  private:
    /* Bigger chars cannot be in a length-2 string. */
    static const size_t SMALL_CHAR_LIMIT    = 128U;
    static const size_t NUM_SMALL_CHARS     = 64U;

    JSAtom* length2StaticTable[NUM_SMALL_CHARS * NUM_SMALL_CHARS];

  public:
    /* We keep these public for the JITs. */
    static const size_t UNIT_STATIC_LIMIT   = 256U;
    JSAtom* unitStaticTable[UNIT_STATIC_LIMIT];

    static const size_t INT_STATIC_LIMIT    = 256U;
    JSAtom* intStaticTable[INT_STATIC_LIMIT];

    StaticStrings() {
        mozilla::PodZero(this);
    }

    bool init(JSContext* cx);
    void trace(JSTracer* trc);

    static bool hasUint(uint32_t u) { return u < INT_STATIC_LIMIT; }

    JSAtom* getUint(uint32_t u) {
        JS_ASSERT(hasUint(u));
        return intStaticTable[u];
    }

    static bool hasInt(int32_t i) {
        return uint32_t(i) < INT_STATIC_LIMIT;
    }

    JSAtom* getInt(int32_t i) {
        JS_ASSERT(hasInt(i));
        return getUint(uint32_t(i));
    }

    static bool hasUnit(jschar c) { return c < UNIT_STATIC_LIMIT; }

    JSAtom* getUnit(jschar c) {
        JS_ASSERT(hasUnit(c));
        return unitStaticTable[c];
    }

    /* May not return atom, returns null on (reported) failure. */
    inline JSLinearString* getUnitStringForElement(JSContext* cx, JSString* str, size_t index);

    static bool isStatic(JSAtom* atom);

    /* Return null if no static atom exists for the given (chars, length). */
    JSAtom* lookup(const jschar* chars, size_t length) {
        switch (length) {
          case 1:
            if (chars[0] < UNIT_STATIC_LIMIT)
                return getUnit(chars[0]);
            return nullptr;
          case 2:
            if (fitsInSmallChar(chars[0]) && fitsInSmallChar(chars[1]))
                return getLength2(chars[0], chars[1]);
            return nullptr;
          case 3:
            /*
             * Here we know that JSString::intStringTable covers only 256 (or at least
             * not 1000 or more) chars. We rely on order here to resolve the unit vs.
             * int string/length-2 string atom identity issue by giving priority to unit
             * strings for "0" through "9" and length-2 strings for "10" through "99".
             */
            JS_STATIC_ASSERT(INT_STATIC_LIMIT <= 999);
            if ('1' <= chars[0] && chars[0] <= '9' &&
                '0' <= chars[1] && chars[1] <= '9' &&
                '0' <= chars[2] && chars[2] <= '9') {
                int i = (chars[0] - '0') * 100 +
                          (chars[1] - '0') * 10 +
                          (chars[2] - '0');

                if (unsigned(i) < INT_STATIC_LIMIT)
                    return getInt(i);
            }
            return nullptr;
        }

        return nullptr;
    }

  private:
    typedef uint8_t SmallChar;
    static const SmallChar INVALID_SMALL_CHAR = -1;

    static bool fitsInSmallChar(jschar c) {
        return c < SMALL_CHAR_LIMIT && toSmallChar[c] != INVALID_SMALL_CHAR;
    }

    static const SmallChar toSmallChar[];

    JSAtom* getLength2(jschar c1, jschar c2);
    JSAtom* getLength2(uint32_t u) {
        JS_ASSERT(u < 100);
        return getLength2('0' + u / 10, '0' + u % 10);
    }
};

/*
 * Represents an atomized string which does not contain an index (that is, an
 * unsigned 32-bit value).  Thus for any PropertyName propname,
 * ToString(ToUint32(propname)) never equals propname.
 *
 * To more concretely illustrate the utility of PropertyName, consider that it
 * is used to partition, in a type-safe manner, the ways to refer to a
 * property, as follows:
 *
 *   - uint32_t indexes,
 *   - PropertyName strings which don't encode uint32_t indexes, and
 *   - jsspecial special properties (non-ES5 properties like object-valued
 *     jsids, JSID_EMPTY, JSID_VOID, and maybe in the future Harmony-proposed
 *     private names).
 */
class PropertyName : public JSAtom
{};

JS_STATIC_ASSERT(sizeof(PropertyName) == sizeof(JSString));

static MOZ_ALWAYS_INLINE jsid
NameToId(PropertyName* name)
{
    return NON_INTEGER_ATOM_TO_JSID(name);
}

typedef HeapPtr<JSAtom> HeapPtrAtom;

class AutoNameVector : public AutoVectorRooter<PropertyName*>
{
    typedef AutoVectorRooter<PropertyName*> BaseType;
  public:
    explicit AutoNameVector(JSContext* cx
                            MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
        : AutoVectorRooter<PropertyName*>(cx, NAMEVECTOR)
    {
        MOZ_GUARD_OBJECT_NOTIFIER_INIT;
    }

    HandlePropertyName operator[](size_t i) const {
        return HandlePropertyName::fromMarkedLocation(&begin()[i]);
    }

    MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
};

} /* namespace js */

/* Avoid requiring vm/String-inl.h just to call getChars. */

MOZ_ALWAYS_INLINE const jschar*
JSString::getChars(js::ExclusiveContext* cx)
{
    if (JSLinearString* str = ensureLinear(cx))
        return str->chars();
    return nullptr;
}

MOZ_ALWAYS_INLINE bool
JSString::getChar(js::ExclusiveContext* cx, size_t index, jschar* code)
{
    JS_ASSERT(index < length());

    /*
     * Optimization for one level deep ropes.
     * This is common for the following pattern:
     *
     * while() {
     *   text = text.substr(0, x) + "bla" + text.substr(x)
     *   test.charCodeAt(x + 1)
     * }
     */
    const jschar* chars;
    if (isRope()) {
        JSRope* rope = &asRope();
        if (uint32_t(index) < rope->leftChild()->length()) {
            chars = rope->leftChild()->getChars(cx);
        } else {
            chars = rope->rightChild()->getChars(cx);
            index -= rope->leftChild()->length();
        }
    } else {
        chars = getChars(cx);
    }

    if (!chars)
        return false;

    *code = chars[index];
    return true;
}

MOZ_ALWAYS_INLINE const jschar*
JSString::getCharsZ(js::ExclusiveContext* cx)
{
    if (JSFlatString* str = ensureFlat(cx))
        return str->chars();
    return nullptr;
}

MOZ_ALWAYS_INLINE const jschar*
JSString::pureChars() const
{
    JS_ASSERT(hasPureChars());
    return asLinear().chars();
}

MOZ_ALWAYS_INLINE const jschar*
JSString::pureCharsZ() const
{
    JS_ASSERT(hasPureCharsZ());
    return asFlat().charsZ();
}

MOZ_ALWAYS_INLINE bool
JSString::copyNonPureChars(js::ThreadSafeContext* cx, js::ScopedJSFreePtr<jschar>& out) const
{
    JS_ASSERT(!hasPureChars());
    return asRope().copyNonPureChars(cx, out);
}

MOZ_ALWAYS_INLINE bool
JSString::copyNonPureCharsZ(js::ThreadSafeContext* cx, js::ScopedJSFreePtr<jschar>& out) const
{
    JS_ASSERT(!hasPureChars());
    if (isDependent())
        return asDependent().copyNonPureCharsZ(cx, out);
    return asRope().copyNonPureCharsZ(cx, out);
}

MOZ_ALWAYS_INLINE JSLinearString*
JSString::ensureLinear(js::ExclusiveContext* cx)
{
    return isLinear()
           ? &asLinear()
           : asRope().flatten(cx);
}

MOZ_ALWAYS_INLINE JSFlatString*
JSString::ensureFlat(js::ExclusiveContext* cx)
{
    return isFlat()
           ? &asFlat()
           : isDependent()
             ? asDependent().undepend(cx)
             : asRope().flatten(cx);
}

inline JSLinearString*
JSString::base() const
{
    JS_ASSERT(hasBase());
    JS_ASSERT(!d.s.u2.base->isInline());
    return d.s.u2.base;
}

inline js::PropertyName*
JSAtom::asPropertyName()
{
#ifdef DEBUG
    uint32_t dummy;
    JS_ASSERT(!isIndex(&dummy));
#endif
    return static_cast<js::PropertyName*>(this);
}

#endif /* vm_String_h */