Source/WTF/wtf/text/StringBuilder.cpp - WebKit - Git at Google

 /*
  * Copyright (C) 2010-2019 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

 #include "config.h"
 #include <wtf/text/StringBuilder.h>

 #include <wtf/dtoa.h>
 #include <wtf/MathExtras.h>

 namespace WTF {

 static constexpr unsigned maxCapacity = String::MaxLength;

 static unsigned expandedCapacity(unsigned capacity, unsigned requiredLength)
 {
     static constexpr unsigned minimumCapacity = 16;
     return std::max(requiredLength, std::max(minimumCapacity, std::min(capacity * 2, maxCapacity)));
 }

 void StringBuilder::reifyString() const
 {
     ASSERT(!hasOverflowed());

     // Check if the string already exists.
     if (!m_string.isNull()) {
         ASSERT(m_string.length() == m_length.unsafeGet<unsigned>());
         return;
     }

 #if ASSERT_ENABLED
     m_isReified = true;
 #endif

     // Check for empty.
     if (!m_length) {
         m_string = StringImpl::empty();
         return;
     }

     // Must be valid in the buffer, take a substring (unless string fills the buffer).
     ASSERT(m_buffer && m_length.unsafeGet<unsigned>() <= m_buffer->length());
     if (m_length.unsafeGet<unsigned>() == m_buffer->length())
         m_string = m_buffer.get();
     else
         m_string = StringImpl::createSubstringSharingImpl(*m_buffer, 0, m_length.unsafeGet());
 }

 void StringBuilder::resize(unsigned newSize)
 {
     if (hasOverflowed())
         return;

     // Check newSize < m_length, hence m_length > 0.
     unsigned oldLength = m_length.unsafeGet();
     ASSERT(newSize <= oldLength);
     if (newSize == oldLength)
         return;
     ASSERT(oldLength);

     m_length = newSize;
     ASSERT(!hasOverflowed());

     // If there is a buffer, we only need to duplicate it if it has more than one ref.
     if (m_buffer) {
         m_string = String(); // Clear the string to remove the reference to m_buffer if any before checking the reference count of m_buffer.
         if (!m_buffer->hasOneRef()) {
             if (m_buffer->is8Bit())
                 allocateBuffer(m_buffer->characters8(), m_buffer->length());
             else
                 allocateBuffer(m_buffer->characters16(), m_buffer->length());
         }
         ASSERT(hasOverflowed() || m_buffer->length() >= m_length.unsafeGet<unsigned>());
         return;
     }

     // Since m_length && !m_buffer, the string must be valid in m_string, and m_string.length() > 0.
     ASSERT(!m_string.isEmpty());
     ASSERT(oldLength == m_string.length());
     ASSERT(newSize < m_string.length());
     m_string = StringImpl::createSubstringSharingImpl(*m_string.impl(), 0, newSize);
 }

 // Allocate a new 8 bit buffer, copying in currentCharacters (these may come from either m_string
 // or m_buffer, neither will be reassigned until the copy has completed).
 void StringBuilder::allocateBuffer(const LChar* currentCharacters, unsigned requiredLength)
 {
     ASSERT(!hasOverflowed());
     ASSERT(m_is8Bit);
     // Copy the existing data into a new buffer, set result to point to the end of the existing data.
     auto buffer = StringImpl::tryCreateUninitialized(requiredLength, m_bufferCharacters8);
     if (UNLIKELY(!buffer))
         return didOverflow();
     std::memcpy(m_bufferCharacters8, currentCharacters, m_length.unsafeGet());

     // Update the builder state.
     m_buffer = WTFMove(buffer);
     m_string = String();
     ASSERT(m_buffer->length() == requiredLength);
 }

 // Allocate a new 16 bit buffer, copying in currentCharacters (these may come from either m_string
 // or m_buffer,  neither will be reassigned until the copy has completed).
 void StringBuilder::allocateBuffer(const UChar* currentCharacters, unsigned requiredLength)
 {
     ASSERT(!hasOverflowed());
     ASSERT(!m_is8Bit);
     // Copy the existing data into a new buffer, set result to point to the end of the existing data.
     auto buffer = StringImpl::tryCreateUninitialized(requiredLength, m_bufferCharacters16);
     if (UNLIKELY(!buffer))
         return didOverflow();
     std::memcpy(m_bufferCharacters16, currentCharacters, static_cast<size_t>(m_length.unsafeGet()) * sizeof(UChar)); // This can't overflow.

     // Update the builder state.
     m_buffer = WTFMove(buffer);
     m_string = String();
     ASSERT(m_buffer->length() == requiredLength);
 }

 // Allocate a new 16 bit buffer, copying in currentCharacters (which is 8 bit and may come
 // from either m_string or m_buffer, neither will be reassigned until the copy has completed).
 void StringBuilder::allocateBufferUpConvert(const LChar* currentCharacters, unsigned requiredLength)
 {
     ASSERT(!hasOverflowed());
     ASSERT(m_is8Bit);
     unsigned length = m_length.unsafeGet();
     ASSERT(requiredLength <= maxCapacity && requiredLength >= length);
     // Copy the existing data into a new buffer, set result to point to the end of the existing data.
     auto buffer = StringImpl::tryCreateUninitialized(requiredLength, m_bufferCharacters16);
     if (UNLIKELY(!buffer))
         return didOverflow(); // Treat a failure to allcoate as an overflow.
     for (unsigned i = 0; i < length; ++i)
         m_bufferCharacters16[i] = currentCharacters[i];

     m_is8Bit = false;

     // Update the builder state.
     m_buffer = WTFMove(buffer);
     m_string = String();
     ASSERT(m_buffer->length() == requiredLength);
 }

 template<>
 void StringBuilder::reallocateBuffer<LChar>(unsigned requiredLength)
 {
     // If the buffer has only one ref (by this StringBuilder), reallocate it,
     // otherwise fall back to "allocate and copy" method.
     m_string = String();

     ASSERT(m_is8Bit);
     ASSERT(m_buffer->is8Bit());

     if (m_buffer->hasOneRef()) {
         auto expectedStringImpl = StringImpl::tryReallocate(m_buffer.releaseNonNull(), requiredLength, m_bufferCharacters8);
         if (UNLIKELY(!expectedStringImpl))
             return didOverflow();
         m_buffer = WTFMove(expectedStringImpl.value());
     } else
         allocateBuffer(m_buffer->characters8(), requiredLength);
     ASSERT(hasOverflowed() || m_buffer->length() == requiredLength);
 }

 template<>
 void StringBuilder::reallocateBuffer<UChar>(unsigned requiredLength)
 {
     // If the buffer has only one ref (by this StringBuilder), reallocate it,
     // otherwise fall back to "allocate and copy" method.
     m_string = String();

     if (m_buffer->is8Bit())
         allocateBufferUpConvert(m_buffer->characters8(), requiredLength);
     else if (m_buffer->hasOneRef()) {
         auto expectedStringImpl = StringImpl::tryReallocate(m_buffer.releaseNonNull(), requiredLength, m_bufferCharacters16);
         if (UNLIKELY(!expectedStringImpl))
             return didOverflow();
         m_buffer = WTFMove(expectedStringImpl.value());
     } else
         allocateBuffer(m_buffer->characters16(), requiredLength);
     ASSERT(hasOverflowed() || m_buffer->length() == requiredLength);
 }

 void StringBuilder::reserveCapacity(unsigned newCapacity)
 {
     if (hasOverflowed())
         return;
     ASSERT(newCapacity <= String::MaxLength);
     if (m_buffer) {
         // If there is already a buffer, then grow if necessary.
         if (newCapacity > m_buffer->length()) {
             if (m_buffer->is8Bit())
                 reallocateBuffer<LChar>(newCapacity);
             else
                 reallocateBuffer<UChar>(newCapacity);
         }
     } else {
         // Grow the string, if necessary.
         unsigned length = m_length.unsafeGet();
         if (newCapacity > length) {
             if (!length) {
                 LChar* nullPlaceholder = nullptr;
                 allocateBuffer(nullPlaceholder, newCapacity);
             } else if (m_string.is8Bit())
                 allocateBuffer(m_string.characters8(), newCapacity);
             else
                 allocateBuffer(m_string.characters16(), newCapacity);
         }
     }
     ASSERT(hasOverflowed() || !newCapacity || m_buffer->length() >= newCapacity);
 }

 // Make 'additionalLength' additional capacity be available in m_buffer, update m_string & m_length,
 // return a pointer to the newly allocated storage.
 // Returns nullptr if the size of the new builder would have overflowed
 template<typename CharacterType> ALWAYS_INLINE CharacterType* StringBuilder::extendBufferForAppending(unsigned additionalLength)
 {
     ASSERT(additionalLength);

     // Calculate the new size of the builder after appending.
     CheckedInt32 requiredLength = m_length + additionalLength;
     if (requiredLength.hasOverflowed()) {
         didOverflow();
         return nullptr;
     }

     return extendBufferForAppendingWithoutOverflowCheck<CharacterType>(requiredLength);
 }

 template<typename CharacterType> ALWAYS_INLINE CharacterType* StringBuilder::extendBufferForAppendingWithoutOverflowCheck(CheckedInt32 requiredLength)
 {
     ASSERT(!requiredLength.hasOverflowed());

     if (m_buffer && (requiredLength.unsafeGet<unsigned>() <= m_buffer->length())) {
         // If the buffer is valid it must be at least as long as the current builder contents!
         ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
         unsigned currentLength = m_length.unsafeGet();
         m_string = String();
         m_length = requiredLength;
         return getBufferCharacters<CharacterType>() + currentLength;
     }

     return extendBufferForAppendingSlowCase<CharacterType>(requiredLength.unsafeGet());
 }

 LChar* StringBuilder::extendBufferForAppending8(CheckedInt32 requiredLength)
 {
     if (UNLIKELY(requiredLength.hasOverflowed())) {
         didOverflow();
         return nullptr;
     }
     return extendBufferForAppendingWithoutOverflowCheck<LChar>(requiredLength);
 }

 UChar* StringBuilder::extendBufferForAppending16(CheckedInt32 requiredLength)
 {
     if (UNLIKELY(requiredLength.hasOverflowed())) {
         didOverflow();
         return nullptr;
     }
     if (m_is8Bit) {
         const LChar* characters;
         if (m_buffer) {
             ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
             characters = m_buffer->characters8();
         } else {
             ASSERT(m_string.length() == m_length.unsafeGet<unsigned>());
             characters = m_string.isNull() ? nullptr : m_string.characters8();
         }
         allocateBufferUpConvert(characters, expandedCapacity(capacity(), requiredLength.unsafeGet()));
         if (UNLIKELY(hasOverflowed()))
             return nullptr;
         unsigned oldLength = m_length.unsafeGet();
         m_length = requiredLength.unsafeGet();
         return m_bufferCharacters16 + oldLength;
     }
     return extendBufferForAppendingWithoutOverflowCheck<UChar>(requiredLength);
 }

 // Make 'requiredLength' capacity be available in m_buffer, update m_string & m_length,
 // return a pointer to the newly allocated storage.
 template<typename CharacterType> CharacterType* StringBuilder::extendBufferForAppendingSlowCase(unsigned requiredLength)
 {
     ASSERT(!hasOverflowed());
     ASSERT(requiredLength);

     if (m_buffer) {
         // If the buffer is valid it must be at least as long as the current builder contents!
         ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());

         reallocateBuffer<CharacterType>(expandedCapacity(capacity(), requiredLength));
     } else {
         ASSERT(m_string.length() == m_length.unsafeGet<unsigned>());
         allocateBuffer(m_length ? m_string.characters<CharacterType>() : nullptr, expandedCapacity(capacity(), requiredLength));
     }
     if (UNLIKELY(hasOverflowed()))
         return nullptr;

     CharacterType* result = getBufferCharacters<CharacterType>() + m_length.unsafeGet();
     m_length = requiredLength;
     ASSERT(!hasOverflowed());
     ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
     return result;
 }

 void StringBuilder::appendCharacters(const UChar* characters, unsigned length)
 {
     if (!length || hasOverflowed())
         return;

     ASSERT(characters);

     if (m_is8Bit && length == 1 && isLatin1(characters[0])) {
         append(static_cast<LChar>(characters[0]));
         return;
     }

     // FIXME: Should we optimize memory by keeping the string 8-bit when all the characters are Latin-1?

     UChar* destination = extendBufferForAppending16(m_length + length);
     if (UNLIKELY(!destination))
         return;
     std::memcpy(destination, characters, static_cast<size_t>(length) * sizeof(UChar));
     ASSERT(!hasOverflowed());
     ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
 }

 void StringBuilder::appendCharacters(const LChar* characters, unsigned length)
 {
     if (!length || hasOverflowed())
         return;

     ASSERT(characters);

     if (m_is8Bit) {
         LChar* destination = extendBufferForAppending<LChar>(length);
         if (!destination) {
             ASSERT(hasOverflowed());
             return;
         }
         if (length > 8)
             std::memcpy(destination, characters, length);
         else {
             // FIXME: How strong is our evidence that this is faster than memcpy? What platforms is this true for?
             const LChar* end = characters + length;
             while (characters < end)
                 *destination++ = *characters++;
         }
     } else {
         UChar* destination = extendBufferForAppending<UChar>(length);
         if (!destination) {
             ASSERT(hasOverflowed());
             return;
         }
         const LChar* end = characters + length;
         while (characters < end)
             *destination++ = *characters++;
     }
 }

 #if USE(CF)

 void StringBuilder::append(CFStringRef string)
 {
     // Fast path: avoid constructing a temporary String when possible.
     if (auto* characters = CFStringGetCStringPtr(string, kCFStringEncodingISOLatin1)) {
         appendCharacters(reinterpret_cast<const LChar*>(characters), CFStringGetLength(string));
         return;
     }
     append(String(string));
 }

 #endif

 void StringBuilder::appendNumber(int number)
 {
     numberToStringSigned<StringBuilder>(number, this);
 }

 void StringBuilder::appendNumber(unsigned number)
 {
     numberToStringUnsigned<StringBuilder>(number, this);
 }

 void StringBuilder::appendNumber(long number)
 {
     numberToStringSigned<StringBuilder>(number, this);
 }

 void StringBuilder::appendNumber(unsigned long number)
 {
     numberToStringUnsigned<StringBuilder>(number, this);
 }

 void StringBuilder::appendNumber(long long number)
 {
     numberToStringSigned<StringBuilder>(number, this);
 }

 void StringBuilder::appendNumber(unsigned long long number)
 {
     numberToStringUnsigned<StringBuilder>(number, this);
 }

 void StringBuilder::appendNumber(float number)
 {
     NumberToStringBuffer buffer;
     append(numberToString(number, buffer));
 }

 void StringBuilder::appendNumber(double number)
 {
     NumberToStringBuffer buffer;
     append(numberToString(number, buffer));
 }

 bool StringBuilder::canShrink() const
 {
     if (hasOverflowed())
         return false;
     // Only shrink the buffer if it's less than 80% full.
     // FIXME: We should tune this heuristic based some actual test case measurements.
     unsigned length = m_length.unsafeGet();
     return m_buffer && m_buffer->length() > (length + (length >> 2));
 }

 void StringBuilder::shrinkToFit()
 {
     if (canShrink()) {
         if (m_is8Bit)
             reallocateBuffer<LChar>(m_length.unsafeGet());
         else
             reallocateBuffer<UChar>(m_length.unsafeGet());
         ASSERT(!hasOverflowed());
         m_string = WTFMove(m_buffer);
     }
 }

 } // namespace WTF
	/*
	* Copyright (C) 2010-2019 Apple Inc. All rights reserved.
	* Copyright (C) 2012 Google Inc. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
	* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
	* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
	* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
	* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
	* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
	* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#include "config.h"
	#include <wtf/text/StringBuilder.h>

	#include <wtf/dtoa.h>
	#include <wtf/MathExtras.h>

	namespace WTF {

	static constexpr unsigned maxCapacity = String::MaxLength;

	static unsigned expandedCapacity(unsigned capacity, unsigned requiredLength)
	{
	static constexpr unsigned minimumCapacity = 16;
	return std::max(requiredLength, std::max(minimumCapacity, std::min(capacity * 2, maxCapacity)));
	}

	void StringBuilder::reifyString() const
	{
	ASSERT(!hasOverflowed());

	// Check if the string already exists.
	if (!m_string.isNull()) {
	ASSERT(m_string.length() == m_length.unsafeGet<unsigned>());
	return;
	}

	#if ASSERT_ENABLED
	m_isReified = true;
	#endif

	// Check for empty.
	if (!m_length) {
	m_string = StringImpl::empty();
	return;
	}

	// Must be valid in the buffer, take a substring (unless string fills the buffer).
	ASSERT(m_buffer && m_length.unsafeGet<unsigned>() <= m_buffer->length());
	if (m_length.unsafeGet<unsigned>() == m_buffer->length())
	m_string = m_buffer.get();
	else
	m_string = StringImpl::createSubstringSharingImpl(*m_buffer, 0, m_length.unsafeGet());
	}

	void StringBuilder::resize(unsigned newSize)
	{
	if (hasOverflowed())
	return;

	// Check newSize < m_length, hence m_length > 0.
	unsigned oldLength = m_length.unsafeGet();
	ASSERT(newSize <= oldLength);
	if (newSize == oldLength)
	return;
	ASSERT(oldLength);

	m_length = newSize;
	ASSERT(!hasOverflowed());

	// If there is a buffer, we only need to duplicate it if it has more than one ref.
	if (m_buffer) {
	m_string = String(); // Clear the string to remove the reference to m_buffer if any before checking the reference count of m_buffer.
	if (!m_buffer->hasOneRef()) {
	if (m_buffer->is8Bit())
	allocateBuffer(m_buffer->characters8(), m_buffer->length());
	else
	allocateBuffer(m_buffer->characters16(), m_buffer->length());
	}
	ASSERT(hasOverflowed() \|\| m_buffer->length() >= m_length.unsafeGet<unsigned>());
	return;
	}

	// Since m_length && !m_buffer, the string must be valid in m_string, and m_string.length() > 0.
	ASSERT(!m_string.isEmpty());
	ASSERT(oldLength == m_string.length());
	ASSERT(newSize < m_string.length());
	m_string = StringImpl::createSubstringSharingImpl(*m_string.impl(), 0, newSize);
	}

	// Allocate a new 8 bit buffer, copying in currentCharacters (these may come from either m_string
	// or m_buffer, neither will be reassigned until the copy has completed).
	void StringBuilder::allocateBuffer(const LChar* currentCharacters, unsigned requiredLength)
	{
	ASSERT(!hasOverflowed());
	ASSERT(m_is8Bit);
	// Copy the existing data into a new buffer, set result to point to the end of the existing data.
	auto buffer = StringImpl::tryCreateUninitialized(requiredLength, m_bufferCharacters8);
	if (UNLIKELY(!buffer))
	return didOverflow();
	std::memcpy(m_bufferCharacters8, currentCharacters, m_length.unsafeGet());

	// Update the builder state.
	m_buffer = WTFMove(buffer);
	m_string = String();
	ASSERT(m_buffer->length() == requiredLength);
	}

	// Allocate a new 16 bit buffer, copying in currentCharacters (these may come from either m_string
	// or m_buffer, neither will be reassigned until the copy has completed).
	void StringBuilder::allocateBuffer(const UChar* currentCharacters, unsigned requiredLength)
	{
	ASSERT(!hasOverflowed());
	ASSERT(!m_is8Bit);
	// Copy the existing data into a new buffer, set result to point to the end of the existing data.
	auto buffer = StringImpl::tryCreateUninitialized(requiredLength, m_bufferCharacters16);
	if (UNLIKELY(!buffer))
	return didOverflow();
	std::memcpy(m_bufferCharacters16, currentCharacters, static_cast<size_t>(m_length.unsafeGet()) * sizeof(UChar)); // This can't overflow.

	// Update the builder state.
	m_buffer = WTFMove(buffer);
	m_string = String();
	ASSERT(m_buffer->length() == requiredLength);
	}

	// Allocate a new 16 bit buffer, copying in currentCharacters (which is 8 bit and may come
	// from either m_string or m_buffer, neither will be reassigned until the copy has completed).
	void StringBuilder::allocateBufferUpConvert(const LChar* currentCharacters, unsigned requiredLength)
	{
	ASSERT(!hasOverflowed());
	ASSERT(m_is8Bit);
	unsigned length = m_length.unsafeGet();
	ASSERT(requiredLength <= maxCapacity && requiredLength >= length);
	// Copy the existing data into a new buffer, set result to point to the end of the existing data.
	auto buffer = StringImpl::tryCreateUninitialized(requiredLength, m_bufferCharacters16);
	if (UNLIKELY(!buffer))
	return didOverflow(); // Treat a failure to allcoate as an overflow.
	for (unsigned i = 0; i < length; ++i)
	m_bufferCharacters16[i] = currentCharacters[i];

	m_is8Bit = false;

	// Update the builder state.
	m_buffer = WTFMove(buffer);
	m_string = String();
	ASSERT(m_buffer->length() == requiredLength);
	}

	template<>
	void StringBuilder::reallocateBuffer<LChar>(unsigned requiredLength)
	{
	// If the buffer has only one ref (by this StringBuilder), reallocate it,
	// otherwise fall back to "allocate and copy" method.
	m_string = String();

	ASSERT(m_is8Bit);
	ASSERT(m_buffer->is8Bit());

	if (m_buffer->hasOneRef()) {
	auto expectedStringImpl = StringImpl::tryReallocate(m_buffer.releaseNonNull(), requiredLength, m_bufferCharacters8);
	if (UNLIKELY(!expectedStringImpl))
	return didOverflow();
	m_buffer = WTFMove(expectedStringImpl.value());
	} else
	allocateBuffer(m_buffer->characters8(), requiredLength);
	ASSERT(hasOverflowed() \|\| m_buffer->length() == requiredLength);
	}

	template<>
	void StringBuilder::reallocateBuffer<UChar>(unsigned requiredLength)
	{
	// If the buffer has only one ref (by this StringBuilder), reallocate it,
	// otherwise fall back to "allocate and copy" method.
	m_string = String();

	if (m_buffer->is8Bit())
	allocateBufferUpConvert(m_buffer->characters8(), requiredLength);
	else if (m_buffer->hasOneRef()) {
	auto expectedStringImpl = StringImpl::tryReallocate(m_buffer.releaseNonNull(), requiredLength, m_bufferCharacters16);
	if (UNLIKELY(!expectedStringImpl))
	return didOverflow();
	m_buffer = WTFMove(expectedStringImpl.value());
	} else
	allocateBuffer(m_buffer->characters16(), requiredLength);
	ASSERT(hasOverflowed() \|\| m_buffer->length() == requiredLength);
	}

	void StringBuilder::reserveCapacity(unsigned newCapacity)
	{
	if (hasOverflowed())
	return;
	ASSERT(newCapacity <= String::MaxLength);
	if (m_buffer) {
	// If there is already a buffer, then grow if necessary.
	if (newCapacity > m_buffer->length()) {
	if (m_buffer->is8Bit())
	reallocateBuffer<LChar>(newCapacity);
	else
	reallocateBuffer<UChar>(newCapacity);
	}
	} else {
	// Grow the string, if necessary.
	unsigned length = m_length.unsafeGet();
	if (newCapacity > length) {
	if (!length) {
	LChar* nullPlaceholder = nullptr;
	allocateBuffer(nullPlaceholder, newCapacity);
	} else if (m_string.is8Bit())
	allocateBuffer(m_string.characters8(), newCapacity);
	else
	allocateBuffer(m_string.characters16(), newCapacity);
	}
	}
	ASSERT(hasOverflowed() \|\| !newCapacity \|\| m_buffer->length() >= newCapacity);
	}

	// Make 'additionalLength' additional capacity be available in m_buffer, update m_string & m_length,
	// return a pointer to the newly allocated storage.
	// Returns nullptr if the size of the new builder would have overflowed
	template<typename CharacterType> ALWAYS_INLINE CharacterType* StringBuilder::extendBufferForAppending(unsigned additionalLength)
	{
	ASSERT(additionalLength);

	// Calculate the new size of the builder after appending.
	CheckedInt32 requiredLength = m_length + additionalLength;
	if (requiredLength.hasOverflowed()) {
	didOverflow();
	return nullptr;
	}

	return extendBufferForAppendingWithoutOverflowCheck<CharacterType>(requiredLength);
	}

	template<typename CharacterType> ALWAYS_INLINE CharacterType* StringBuilder::extendBufferForAppendingWithoutOverflowCheck(CheckedInt32 requiredLength)
	{
	ASSERT(!requiredLength.hasOverflowed());

	if (m_buffer && (requiredLength.unsafeGet<unsigned>() <= m_buffer->length())) {
	// If the buffer is valid it must be at least as long as the current builder contents!
	ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
	unsigned currentLength = m_length.unsafeGet();
	m_string = String();
	m_length = requiredLength;
	return getBufferCharacters<CharacterType>() + currentLength;
	}

	return extendBufferForAppendingSlowCase<CharacterType>(requiredLength.unsafeGet());
	}

	LChar* StringBuilder::extendBufferForAppending8(CheckedInt32 requiredLength)
	{
	if (UNLIKELY(requiredLength.hasOverflowed())) {
	didOverflow();
	return nullptr;
	}
	return extendBufferForAppendingWithoutOverflowCheck<LChar>(requiredLength);
	}

	UChar* StringBuilder::extendBufferForAppending16(CheckedInt32 requiredLength)
	{
	if (UNLIKELY(requiredLength.hasOverflowed())) {
	didOverflow();
	return nullptr;
	}
	if (m_is8Bit) {
	const LChar* characters;
	if (m_buffer) {
	ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
	characters = m_buffer->characters8();
	} else {
	ASSERT(m_string.length() == m_length.unsafeGet<unsigned>());
	characters = m_string.isNull() ? nullptr : m_string.characters8();
	}
	allocateBufferUpConvert(characters, expandedCapacity(capacity(), requiredLength.unsafeGet()));
	if (UNLIKELY(hasOverflowed()))
	return nullptr;
	unsigned oldLength = m_length.unsafeGet();
	m_length = requiredLength.unsafeGet();
	return m_bufferCharacters16 + oldLength;
	}
	return extendBufferForAppendingWithoutOverflowCheck<UChar>(requiredLength);
	}

	// Make 'requiredLength' capacity be available in m_buffer, update m_string & m_length,
	// return a pointer to the newly allocated storage.
	template<typename CharacterType> CharacterType* StringBuilder::extendBufferForAppendingSlowCase(unsigned requiredLength)
	{
	ASSERT(!hasOverflowed());
	ASSERT(requiredLength);

	if (m_buffer) {
	// If the buffer is valid it must be at least as long as the current builder contents!
	ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());

	reallocateBuffer<CharacterType>(expandedCapacity(capacity(), requiredLength));
	} else {
	ASSERT(m_string.length() == m_length.unsafeGet<unsigned>());
	allocateBuffer(m_length ? m_string.characters<CharacterType>() : nullptr, expandedCapacity(capacity(), requiredLength));
	}
	if (UNLIKELY(hasOverflowed()))
	return nullptr;

	CharacterType* result = getBufferCharacters<CharacterType>() + m_length.unsafeGet();
	m_length = requiredLength;
	ASSERT(!hasOverflowed());
	ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
	return result;
	}

	void StringBuilder::appendCharacters(const UChar* characters, unsigned length)
	{
	if (!length \|\| hasOverflowed())
	return;

	ASSERT(characters);

	if (m_is8Bit && length == 1 && isLatin1(characters[0])) {
	append(static_cast<LChar>(characters[0]));
	return;
	}

	// FIXME: Should we optimize memory by keeping the string 8-bit when all the characters are Latin-1?

	UChar* destination = extendBufferForAppending16(m_length + length);
	if (UNLIKELY(!destination))
	return;
	std::memcpy(destination, characters, static_cast<size_t>(length) * sizeof(UChar));
	ASSERT(!hasOverflowed());
	ASSERT(m_buffer->length() >= m_length.unsafeGet<unsigned>());
	}

	void StringBuilder::appendCharacters(const LChar* characters, unsigned length)
	{
	if (!length \|\| hasOverflowed())
	return;

	ASSERT(characters);

	if (m_is8Bit) {
	LChar* destination = extendBufferForAppending<LChar>(length);
	if (!destination) {
	ASSERT(hasOverflowed());
	return;
	}
	if (length > 8)
	std::memcpy(destination, characters, length);
	else {
	// FIXME: How strong is our evidence that this is faster than memcpy? What platforms is this true for?
	const LChar* end = characters + length;
	while (characters < end)
	destination++ = characters++;
	}
	} else {
	UChar* destination = extendBufferForAppending<UChar>(length);
	if (!destination) {
	ASSERT(hasOverflowed());
	return;
	}
	const LChar* end = characters + length;
	while (characters < end)
	destination++ = characters++;
	}
	}

	#if USE(CF)

	void StringBuilder::append(CFStringRef string)
	{
	// Fast path: avoid constructing a temporary String when possible.
	if (auto* characters = CFStringGetCStringPtr(string, kCFStringEncodingISOLatin1)) {
	appendCharacters(reinterpret_cast<const LChar*>(characters), CFStringGetLength(string));
	return;
	}
	append(String(string));
	}

	#endif

	void StringBuilder::appendNumber(int number)
	{
	numberToStringSigned<StringBuilder>(number, this);
	}

	void StringBuilder::appendNumber(unsigned number)
	{
	numberToStringUnsigned<StringBuilder>(number, this);
	}

	void StringBuilder::appendNumber(long number)
	{
	numberToStringSigned<StringBuilder>(number, this);
	}

	void StringBuilder::appendNumber(unsigned long number)
	{
	numberToStringUnsigned<StringBuilder>(number, this);
	}

	void StringBuilder::appendNumber(long long number)
	{
	numberToStringSigned<StringBuilder>(number, this);
	}

	void StringBuilder::appendNumber(unsigned long long number)
	{
	numberToStringUnsigned<StringBuilder>(number, this);
	}

	void StringBuilder::appendNumber(float number)
	{
	NumberToStringBuffer buffer;
	append(numberToString(number, buffer));
	}

	void StringBuilder::appendNumber(double number)
	{
	NumberToStringBuffer buffer;
	append(numberToString(number, buffer));
	}

	bool StringBuilder::canShrink() const
	{
	if (hasOverflowed())
	return false;
	// Only shrink the buffer if it's less than 80% full.
	// FIXME: We should tune this heuristic based some actual test case measurements.
	unsigned length = m_length.unsafeGet();
	return m_buffer && m_buffer->length() > (length + (length >> 2));
	}

	void StringBuilder::shrinkToFit()
	{
	if (canShrink()) {
	if (m_is8Bit)
	reallocateBuffer<LChar>(m_length.unsafeGet());
	else
	reallocateBuffer<UChar>(m_length.unsafeGet());
	ASSERT(!hasOverflowed());
	m_string = WTFMove(m_buffer);
	}
	}

	} // namespace WTF