mfbt/double-conversion/utils.h

// Copyright 2010 the V8 project authors. All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * 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.
//     * Neither the name of Google Inc. nor the names of its
//       contributors may be used to endorse or promote products derived
//       from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#ifndef DOUBLE_CONVERSION_UTILS_H_
#define DOUBLE_CONVERSION_UTILS_H_

#include <stdlib.h>
#include <string.h>

#include "mozilla/Assertions.h"
#ifndef ASSERT
#define ASSERT(condition)      MOZ_ASSERT(condition)
#endif
#ifndef UNIMPLEMENTED
#define UNIMPLEMENTED() MOZ_CRASH()
#endif
#ifndef UNREACHABLE
#define UNREACHABLE()   MOZ_CRASH()
#endif

// Double operations detection based on target architecture.
// Linux uses a 80bit wide floating point stack on x86. This induces double
// rounding, which in turn leads to wrong results.
// An easy way to test if the floating-point operations are correct is to
// evaluate: 89255.0/1e22. If the floating-point stack is 64 bits wide then
// the result is equal to 89255e-22.
// The best way to test this, is to create a division-function and to compare
// the output of the division with the expected result. (Inlining must be
// disabled.)
// On Linux,x86 89255e-22 != Div_double(89255.0/1e22)
#if defined(_M_X64) || defined(__x86_64__) || \
    defined(__ARMEL__) || defined(__avr32__) || \
    defined(__hppa__) || defined(__ia64__) || \
    defined(__mips__) || \
    defined(__powerpc__) || defined(__ppc__) || defined(__ppc64__) || \
    defined(__sparc__) || defined(__sparc) || defined(__s390__) || \
    defined(__SH4__) || defined(__alpha__) || \
    defined(_MIPS_ARCH_MIPS32R2) || \
    defined(__AARCH64EL__) || defined(__aarch64__)
#define DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS 1
#elif defined(_M_IX86) || defined(__i386__) || defined(__i386)
#if defined(_WIN32)
// Windows uses a 64bit wide floating point stack.
#define DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS 1
#else
#undef DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS
#endif  // _WIN32
#else
#error Target architecture was not detected as supported by Double-Conversion.
#endif


#include <stdint.h>

// The following macro works on both 32 and 64-bit platforms.
// Usage: instead of writing 0x1234567890123456
//      write UINT64_2PART_C(0x12345678,90123456);
#define UINT64_2PART_C(a, b) (((static_cast<uint64_t>(a) << 32) + 0x##b##u))


// The expression ARRAY_SIZE(a) is a compile-time constant of type
// size_t which represents the number of elements of the given
// array. You should only use ARRAY_SIZE on statically allocated
// arrays.
#ifndef ARRAY_SIZE
#define ARRAY_SIZE(a)                                   \
  ((sizeof(a) / sizeof(*(a))) /                         \
  static_cast<size_t>(!(sizeof(a) % sizeof(*(a)))))
#endif

// A macro to disallow the evil copy constructor and operator= functions
// This should be used in the private: declarations for a class
#ifndef DISALLOW_COPY_AND_ASSIGN
#define DISALLOW_COPY_AND_ASSIGN(TypeName)      \
  TypeName(const TypeName&);                    \
  void operator=(const TypeName&)
#endif

// A macro to disallow all the implicit constructors, namely the
// default constructor, copy constructor and operator= functions.
//
// This should be used in the private: declarations for a class
// that wants to prevent anyone from instantiating it. This is
// especially useful for classes containing only static methods.
#ifndef DISALLOW_IMPLICIT_CONSTRUCTORS
#define DISALLOW_IMPLICIT_CONSTRUCTORS(TypeName) \
  TypeName();                                    \
  DISALLOW_COPY_AND_ASSIGN(TypeName)
#endif

namespace double_conversion {

static const int kCharSize = sizeof(char);

// Returns the maximum of the two parameters.
template <typename T>
static T Max(T a, T b) {
  return a < b ? b : a;
}


// Returns the minimum of the two parameters.
template <typename T>
static T Min(T a, T b) {
  return a < b ? a : b;
}


inline int StrLength(const char* string) {
  size_t length = strlen(string);
  ASSERT(length == static_cast<size_t>(static_cast<int>(length)));
  return static_cast<int>(length);
}

// This is a simplified version of V8's Vector class.
template <typename T>
class Vector {
 public:
  Vector() : start_(NULL), length_(0) {}
  Vector(T* data, int len) : start_(data), length_(len) {
    ASSERT(len == 0 || (len > 0 && data != NULL));
  }

  // Returns a vector using the same backing storage as this one,
  // spanning from and including 'from', to but not including 'to'.
  Vector<T> SubVector(int from, int to) {
    ASSERT(to <= length_);
    ASSERT(from < to);
    ASSERT(0 <= from);
    return Vector<T>(start() + from, to - from);
  }

  // Returns the length of the vector.
  int length() const { return length_; }

  // Returns whether or not the vector is empty.
  bool is_empty() const { return length_ == 0; }

  // Returns the pointer to the start of the data in the vector.
  T* start() const { return start_; }

  // Access individual vector elements - checks bounds in debug mode.
  T& operator[](int index) const {
    ASSERT(0 <= index && index < length_);
    return start_[index];
  }

  T& first() { return start_[0]; }

  T& last() { return start_[length_ - 1]; }

 private:
  T* start_;
  int length_;
};


// Helper class for building result strings in a character buffer. The
// purpose of the class is to use safe operations that checks the
// buffer bounds on all operations in debug mode.
class StringBuilder {
 public:
  StringBuilder(char* buffer, int buffer_size)
      : buffer_(buffer, buffer_size), position_(0) { }

  ~StringBuilder() { if (!is_finalized()) Finalize(); }

  int size() const { return buffer_.length(); }

  // Get the current position in the builder.
  int position() const {
    ASSERT(!is_finalized());
    return position_;
  }

  // Reset the position.
  void Reset() { position_ = 0; }

  // Add a single character to the builder. It is not allowed to add
  // 0-characters; use the Finalize() method to terminate the string
  // instead.
  void AddCharacter(char c) {
    ASSERT(c != '\0');
    ASSERT(!is_finalized() && position_ < buffer_.length());
    buffer_[position_++] = c;
  }

  // Add an entire string to the builder. Uses strlen() internally to
  // compute the length of the input string.
  void AddString(const char* s) {
    AddSubstring(s, StrLength(s));
  }

  // Add the first 'n' characters of the given string 's' to the
  // builder. The input string must have enough characters.
  void AddSubstring(const char* s, int n) {
    ASSERT(!is_finalized() && position_ + n < buffer_.length());
    ASSERT(static_cast<size_t>(n) <= strlen(s));
    memmove(&buffer_[position_], s, n * kCharSize);
    position_ += n;
  }


  // Add character padding to the builder. If count is non-positive,
  // nothing is added to the builder.
  void AddPadding(char c, int count) {
    for (int i = 0; i < count; i++) {
      AddCharacter(c);
    }
  }

  // Finalize the string by 0-terminating it and returning the buffer.
  char* Finalize() {
    ASSERT(!is_finalized() && position_ < buffer_.length());
    buffer_[position_] = '\0';
    // Make sure nobody managed to add a 0-character to the
    // buffer while building the string.
    ASSERT(strlen(buffer_.start()) == static_cast<size_t>(position_));
    position_ = -1;
    ASSERT(is_finalized());
    return buffer_.start();
  }

 private:
  Vector<char> buffer_;
  int position_;

  bool is_finalized() const { return position_ < 0; }

  DISALLOW_IMPLICIT_CONSTRUCTORS(StringBuilder);
};

// The type-based aliasing rule allows the compiler to assume that pointers of
// different types (for some definition of different) never alias each other.
// Thus the following code does not work:
//
// float f = foo();
// int fbits = *(int*)(&f);
//
// The compiler 'knows' that the int pointer can't refer to f since the types
// don't match, so the compiler may cache f in a register, leaving random data
// in fbits.  Using C++ style casts makes no difference, however a pointer to
// char data is assumed to alias any other pointer.  This is the 'memcpy
// exception'.
//
// Bit_cast uses the memcpy exception to move the bits from a variable of one
// type of a variable of another type.  Of course the end result is likely to
// be implementation dependent.  Most compilers (gcc-4.2 and MSVC 2005)
// will completely optimize BitCast away.
//
// There is an additional use for BitCast.
// Recent gccs will warn when they see casts that may result in breakage due to
// the type-based aliasing rule.  If you have checked that there is no breakage
// you can use BitCast to cast one pointer type to another.  This confuses gcc
// enough that it can no longer see that you have cast one pointer type to
// another thus avoiding the warning.
template <class Dest, class Source>
inline Dest BitCast(const Source& source) {
  static_assert(sizeof(Dest) == sizeof(Source),
                "BitCast's source and destination types must be the same size");

  Dest dest;
  memmove(&dest, &source, sizeof(dest));
  return dest;
}

template <class Dest, class Source>
inline Dest BitCast(Source* source) {
  return BitCast<Dest>(reinterpret_cast<uintptr_t>(source));
}

}  // namespace double_conversion

#endif  // DOUBLE_CONVERSION_UTILS_H_