conversions.cc 21.7 KB
Newer Older
1
// Copyright 2006-2008 the V8 project authors. All rights reserved.
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57
// 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
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER 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 <stdarg.h>

#include "v8.h"

#include "conversions-inl.h"
#include "factory.h"
#include "scanner.h"

namespace v8 { namespace internal {

int HexValue(uc32 c) {
  if ('0' <= c && c <= '9')
    return c - '0';
  if ('a' <= c && c <= 'f')
    return c - 'a' + 10;
  if ('A' <= c && c <= 'F')
    return c - 'A' + 10;
  return -1;
}


// Provide a common interface to getting a character at a certain
// index from a char* or a String object.
static inline int GetChar(const char* str, int index) {
  ASSERT(index >= 0 && index < static_cast<int>(strlen(str)));
  return str[index];
}


static inline int GetChar(String* str, int index) {
58 59
  StringShape shape(str);
  return str->Get(shape, index);
60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78
}


static inline int GetLength(const char* str) {
  return strlen(str);
}


static inline int GetLength(String* str) {
  return str->length();
}


static inline const char* GetCString(const char* str, int index) {
  return str + index;
}


static inline const char* GetCString(String* str, int index) {
79 80 81 82 83 84 85
  StringShape shape(str);
  int length = str->length(shape);
  char* result = NewArray<char>(length + 1);
  for (int i = index; i < length; i++) {
    uc16 c = str->Get(shape, i);
    if (c <= 127) {
      result[i - index] = static_cast<char>(c);
86 87 88 89
    } else {
      result[i - index] = 127;  // Force number parsing to fail.
    }
  }
90
  result[length - index] = '\0';
91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110
  return result;
}


static inline void ReleaseCString(const char* original, const char* str) {
}


static inline void ReleaseCString(String* original, const char* str) {
  DeleteArray(const_cast<char *>(str));
}


static inline bool IsSpace(const char* str, int index) {
  ASSERT(index >= 0 && index < static_cast<int>(strlen(str)));
  return Scanner::kIsWhiteSpace.get(str[index]);
}


static inline bool IsSpace(String* str, int index) {
111 112
  StringShape shape(str);
  return Scanner::kIsWhiteSpace.get(str->Get(shape, index));
113 114 115 116 117 118 119 120 121 122 123 124
}


static inline bool SubStringEquals(const char* str,
                                   int index,
                                   const char* other) {
  return strncmp(str + index, other, strlen(other)) != 0;
}


static inline bool SubStringEquals(String* str, int index, const char* other) {
  HandleScope scope;
125
  int str_length = str->length();
126 127 128 129
  int other_length = strlen(other);
  int end = index + other_length < str_length ?
            index + other_length :
            str_length;
130
  Handle<String> slice =
131
      Factory::NewStringSlice(Handle<String>(str), index, end);
132
  return slice->IsEqualTo(Vector<const char>(other, other_length));
133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279
}


// Check if a string should be parsed as an octal number.  The string
// can be either a char* or a String*.
template<class S>
static bool ShouldParseOctal(S* s, int i) {
  int index = i;
  int len = GetLength(s);
  if (index < len && GetChar(s, index) != '0') return false;

  // If the first real character (following '0') is not an octal
  // digit, bail out early. This also takes care of numbers of the
  // forms 0.xxx and 0exxx by not allowing the first 0 to be
  // interpreted as an octal.
  index++;
  if (index < len) {
    int d = GetChar(s, index) - '0';
    if (d < 0 || d > 7) return false;
  } else {
    return false;
  }

  // Traverse all digits (including the first). If there is an octal
  // prefix which is not a part of a longer decimal prefix, we return
  // true. Otherwise, false is returned.
  while (index < len) {
    int d = GetChar(s, index++) - '0';
    if (d == 8 || d == 9) return false;
    if (d <  0 || d >  7) return true;
  }
  return true;
}


extern "C" double gay_strtod(const char* s00, const char** se);


// Parse an int from a string starting a given index and in a given
// radix.  The string can be either a char* or a String*.
template <class S>
static int InternalStringToInt(S* s, int i, int radix, double* value) {
  int len = GetLength(s);

  // Setup limits for computing the value.
  ASSERT(2 <= radix && radix <= 36);
  int lim_0 = '0' + (radix < 10 ? radix : 10);
  int lim_a = 'a' + (radix - 10);
  int lim_A = 'A' + (radix - 10);

  // NOTE: The code for computing the value may seem a bit complex at
  // first glance. It is structured to use 32-bit multiply-and-add
  // loops as long as possible to avoid loosing precision.

  double v = 0.0;
  int j;
  for (j = i; j < len;) {
    // Parse the longest part of the string starting at index j
    // possible while keeping the multiplier, and thus the part
    // itself, within 32 bits.
    uint32_t part = 0, multiplier = 1;
    int k;
    for (k = j; k < len; k++) {
      int c = GetChar(s, k);
      if (c >= '0' && c < lim_0) {
        c = c - '0';
      } else if (c >= 'a' && c < lim_a) {
        c = c - 'a' + 10;
      } else if (c >= 'A' && c < lim_A) {
        c = c - 'A' + 10;
      } else {
        break;
      }

      // Update the value of the part as long as the multiplier fits
      // in 32 bits. When we can't guarantee that the next iteration
      // will not overflow the multiplier, we stop parsing the part
      // by leaving the loop.
      static const uint32_t kMaximumMultiplier = 0xffffffffU / 36;
      uint32_t m = multiplier * radix;
      if (m > kMaximumMultiplier) break;
      part = part * radix + c;
      multiplier = m;
      ASSERT(multiplier > part);
    }

    // Compute the number of part digits. If no digits were parsed;
    // we're done parsing the entire string.
    int digits = k - j;
    if (digits == 0) break;

    // Update the value and skip the part in the string.
    ASSERT(multiplier ==
           pow(static_cast<double>(radix), static_cast<double>(digits)));
    v = v * multiplier + part;
    j = k;
  }

  // If the resulting value is larger than 2^53 the value does not fit
  // in the mantissa of the double and there is a loss of precision.
  // When the value is larger than 2^53 the rounding depends on the
  // code generation.  If the code generator spills the double value
  // it uses 64 bits and if it does not it uses 80 bits.
  //
  // If there is a potential for overflow we resort to strtod for
  // radix 10 numbers to get higher precision.  For numbers in another
  // radix we live with the loss of precision.
  static const double kPreciseConversionLimit = 9007199254740992.0;
  if (radix == 10 && v > kPreciseConversionLimit) {
    const char* cstr = GetCString(s, i);
    const char* end;
    v = gay_strtod(cstr, &end);
    ReleaseCString(s, cstr);
  }

  *value = v;
  return j;
}


int StringToInt(String* str, int index, int radix, double* value) {
  return InternalStringToInt(str, index, radix, value);
}


int StringToInt(const char* str, int index, int radix, double* value) {
  return InternalStringToInt(const_cast<char*>(str), index, radix, value);
}


static const double JUNK_STRING_VALUE = OS::nan_value();


// Convert a string to a double value.  The string can be either a
// char* or a String*.
template<class S>
static double InternalStringToDouble(S* str,
                                     int flags,
                                     double empty_string_val) {
  double result = 0.0;
  int index = 0;

  int len = GetLength(str);

  // Skip leading spaces.
  while ((index < len) && IsSpace(str, index)) index++;

280 281 282 283 284 285 286 287 288 289 290 291 292
  // Is the string empty?
  if (index >= len) return empty_string_val;

  // Get the first character.
  uint16_t first = GetChar(str, index);

  // Numbers can only start with '-', '+', '.', 'I' (Infinity), or a digit.
  if (first != '-' && first != '+' && first != '.' && first != 'I' &&
      (first > '9' || first < '0')) {
    return JUNK_STRING_VALUE;
  }

  // Compute sign of result based on first character.
293
  int sign = 1;
294
  if (first == '-') {
295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710
    sign = -1;
    index++;
    // String only containing a '-' are junk chars.
    if (index == len) return JUNK_STRING_VALUE;
  }

  // do we have a hex number?
  // (since the string is 0-terminated, it's ok to look one char beyond the end)
  if ((flags & ALLOW_HEX) != 0 &&
      (index + 1) < len &&
      GetChar(str, index) == '0' &&
      (GetChar(str, index + 1) == 'x' || GetChar(str, index + 1) == 'X')) {
    index += 2;
    index = StringToInt(str, index, 16, &result);
  } else if ((flags & ALLOW_OCTALS) != 0 && ShouldParseOctal(str, index)) {
    // NOTE: We optimistically try to parse the number as an octal (if
    // we're allowed to), even though this is not as dictated by
    // ECMA-262. The reason for doing this is compatibility with IE and
    // Firefox.
    index = StringToInt(str, index, 8, &result);
  } else {
    const char* cstr = GetCString(str, index);
    const char* end;
    // Optimistically parse the number and then, if that fails,
    // check if it might have been {+,-,}Infinity.
    result = gay_strtod(cstr, &end);
    ReleaseCString(str, cstr);
    if (result != 0.0 || end != cstr) {
      // It appears that strtod worked
      index += end - cstr;
    } else {
      // Check for {+,-,}Infinity
      bool is_negative = (GetChar(str, index) == '-');
      if (GetChar(str, index) == '+' || GetChar(str, index) == '-')
        index++;
      if (!SubStringEquals(str, index, "Infinity"))
        return JUNK_STRING_VALUE;
      result = is_negative ? -INFINITY : INFINITY;
      index += 8;
    }
  }

  if ((flags & ALLOW_TRAILING_JUNK) == 0) {
    // skip trailing spaces
    while ((index < len) && IsSpace(str, index)) index++;
    // string ending with junk?
    if (index < len) return JUNK_STRING_VALUE;
  }

  return sign * result;
}


double StringToDouble(String* str, int flags, double empty_string_val) {
  return InternalStringToDouble(str, flags, empty_string_val);
}


double StringToDouble(const char* str, int flags, double empty_string_val) {
  return InternalStringToDouble(str, flags, empty_string_val);
}


extern "C" char* dtoa(double d, int mode, int ndigits,
                      int* decpt, int* sign, char** rve);

extern "C" void freedtoa(char* s);

const char* DoubleToCString(double v, Vector<char> buffer) {
  StringBuilder builder(buffer.start(), buffer.length());

  switch (fpclassify(v)) {
    case FP_NAN:
      builder.AddString("NaN");
      break;

    case FP_INFINITE:
      if (v < 0.0) {
        builder.AddString("-Infinity");
      } else {
        builder.AddString("Infinity");
      }
      break;

    case FP_ZERO:
      builder.AddCharacter('0');
      break;

    default: {
      int decimal_point;
      int sign;

      char* decimal_rep = dtoa(v, 0, 0, &decimal_point, &sign, NULL);
      int length = strlen(decimal_rep);

      if (sign) builder.AddCharacter('-');

      if (length <= decimal_point && decimal_point <= 21) {
        // ECMA-262 section 9.8.1 step 6.
        builder.AddString(decimal_rep);
        builder.AddPadding('0', decimal_point - length);

      } else if (0 < decimal_point && decimal_point <= 21) {
        // ECMA-262 section 9.8.1 step 7.
        builder.AddSubstring(decimal_rep, decimal_point);
        builder.AddCharacter('.');
        builder.AddString(decimal_rep + decimal_point);

      } else if (decimal_point <= 0 && decimal_point > -6) {
        // ECMA-262 section 9.8.1 step 8.
        builder.AddString("0.");
        builder.AddPadding('0', -decimal_point);
        builder.AddString(decimal_rep);

      } else {
        // ECMA-262 section 9.8.1 step 9 and 10 combined.
        builder.AddCharacter(decimal_rep[0]);
        if (length != 1) {
          builder.AddCharacter('.');
          builder.AddString(decimal_rep + 1);
        }
        builder.AddCharacter('e');
        builder.AddCharacter((decimal_point >= 0) ? '+' : '-');
        int exponent = decimal_point - 1;
        if (exponent < 0) exponent = -exponent;
        builder.AddFormatted("%d", exponent);
      }

      freedtoa(decimal_rep);
    }
  }
  return builder.Finalize();
}


const char* IntToCString(int n, Vector<char> buffer) {
  bool negative = false;
  if (n < 0) {
    // We must not negate the most negative int.
    if (n == kMinInt) return DoubleToCString(n, buffer);
    negative = true;
    n = -n;
  }
  // Build the string backwards from the least significant digit.
  int i = buffer.length();
  buffer[--i] = '\0';
  do {
    buffer[--i] = '0' + (n % 10);
    n /= 10;
  } while (n);
  if (negative) buffer[--i] = '-';
  return buffer.start() + i;
}


char* DoubleToFixedCString(double value, int f) {
  ASSERT(f >= 0);

  bool negative = false;
  double abs_value = value;
  if (value < 0) {
    abs_value = -value;
    negative = true;
  }

  if (abs_value >= 1e21) {
    char arr[100];
    Vector<char> buffer(arr, ARRAY_SIZE(arr));
    return StrDup(DoubleToCString(value, buffer));
  }

  // Find a sufficiently precise decimal representation of n.
  int decimal_point;
  int sign;
  char* decimal_rep = dtoa(abs_value, 3, f, &decimal_point, &sign, NULL);
  int decimal_rep_length = strlen(decimal_rep);

  // Create a representation that is padded with zeros if needed.
  int zero_prefix_length = 0;
  int zero_postfix_length = 0;

  if (decimal_point <= 0) {
    zero_prefix_length = -decimal_point + 1;
    decimal_point = 1;
  }

  if (zero_prefix_length + decimal_rep_length < decimal_point + f) {
    zero_postfix_length = decimal_point + f - decimal_rep_length -
                          zero_prefix_length;
  }

  unsigned rep_length =
      zero_prefix_length + decimal_rep_length + zero_postfix_length;
  StringBuilder rep_builder(rep_length + 1);
  rep_builder.AddPadding('0', zero_prefix_length);
  rep_builder.AddString(decimal_rep);
  rep_builder.AddPadding('0', zero_postfix_length);
  char* rep = rep_builder.Finalize();
  freedtoa(decimal_rep);

  // Create the result string by appending a minus and putting in a
  // decimal point if needed.
  unsigned result_size = decimal_point + f + 2;
  StringBuilder builder(result_size + 1);
  if (negative) builder.AddCharacter('-');
  builder.AddSubstring(rep, decimal_point);
  if (f > 0) {
    builder.AddCharacter('.');
    builder.AddSubstring(rep + decimal_point, f);
  }
  DeleteArray(rep);
  return builder.Finalize();
}


static char* CreateExponentialRepresentation(char* decimal_rep,
                                             int exponent,
                                             bool negative,
                                             int significant_digits) {
  bool negative_exponent = false;
  if (exponent < 0) {
    negative_exponent = true;
    exponent = -exponent;
  }

  // Leave room in the result for appending a minus, for a period, the
  // letter 'e', a minus or a plus depending on the exponent, and a
  // three digit exponent.
  unsigned result_size = significant_digits + 7;
  StringBuilder builder(result_size + 1);

  if (negative) builder.AddCharacter('-');
  builder.AddCharacter(decimal_rep[0]);
  if (significant_digits != 1) {
    builder.AddCharacter('.');
    builder.AddString(decimal_rep + 1);
    builder.AddPadding('0', significant_digits - strlen(decimal_rep));
  }

  builder.AddCharacter('e');
  builder.AddCharacter(negative_exponent ? '-' : '+');
  builder.AddFormatted("%d", exponent);
  return builder.Finalize();
}



char* DoubleToExponentialCString(double value, int f) {
  // f might be -1 to signal that f was undefined in JavaScript.
  ASSERT(f >= -1 && f <= 20);

  bool negative = false;
  if (value < 0) {
    value = -value;
    negative = true;
  }

  // Find a sufficiently precise decimal representation of n.
  int decimal_point;
  int sign;
  char* decimal_rep = NULL;
  if (f == -1) {
    decimal_rep = dtoa(value, 0, 0, &decimal_point, &sign, NULL);
    f = strlen(decimal_rep) - 1;
  } else {
    decimal_rep = dtoa(value, 2, f + 1, &decimal_point, &sign, NULL);
  }
  int decimal_rep_length = strlen(decimal_rep);
  ASSERT(decimal_rep_length > 0);
  ASSERT(decimal_rep_length <= f + 1);
  USE(decimal_rep_length);

  int exponent = decimal_point - 1;
  char* result =
      CreateExponentialRepresentation(decimal_rep, exponent, negative, f+1);

  freedtoa(decimal_rep);

  return result;
}


char* DoubleToPrecisionCString(double value, int p) {
  ASSERT(p >= 1 && p <= 21);

  bool negative = false;
  if (value < 0) {
    value = -value;
    negative = true;
  }

  // Find a sufficiently precise decimal representation of n.
  int decimal_point;
  int sign;
  char* decimal_rep = dtoa(value, 2, p, &decimal_point, &sign, NULL);
  int decimal_rep_length = strlen(decimal_rep);
  ASSERT(decimal_rep_length <= p);

  int exponent = decimal_point - 1;

  char* result = NULL;

  if (exponent < -6 || exponent >= p) {
    result =
        CreateExponentialRepresentation(decimal_rep, exponent, negative, p);
  } else {
    // Use fixed notation.
    //
    // Leave room in the result for appending a minus, a period and in
    // the case where decimal_point is not positive for a zero in
    // front of the period.
    unsigned result_size = (decimal_point <= 0)
        ? -decimal_point + p + 3
        : p + 2;
    StringBuilder builder(result_size + 1);
    if (negative) builder.AddCharacter('-');
    if (decimal_point <= 0) {
      builder.AddString("0.");
      builder.AddPadding('0', -decimal_point);
      builder.AddString(decimal_rep);
      builder.AddPadding('0', p - decimal_rep_length);
    } else {
      const int m = Min(decimal_rep_length, decimal_point);
      builder.AddSubstring(decimal_rep, m);
      builder.AddPadding('0', decimal_point - decimal_rep_length);
      if (decimal_point < p) {
        builder.AddCharacter('.');
        const int extra = negative ? 2 : 1;
        if (decimal_rep_length > decimal_point) {
          const int len = strlen(decimal_rep + decimal_point);
          const int n = Min(len, p - (builder.position() - extra));
          builder.AddSubstring(decimal_rep + decimal_point, n);
        }
        builder.AddPadding('0', extra + (p - builder.position()));
      }
    }
    result = builder.Finalize();
  }

  freedtoa(decimal_rep);
  return result;
}


char* DoubleToRadixCString(double value, int radix) {
  ASSERT(radix >= 2 && radix <= 36);

  // Character array used for conversion.
  static const char chars[] = "0123456789abcdefghijklmnopqrstuvwxyz";

  // Buffer for the integer part of the result. 1024 chars is enough
  // for max integer value in radix 2.  We need room for a sign too.
  static const int kBufferSize = 1100;
  char integer_buffer[kBufferSize];
  integer_buffer[kBufferSize - 1] = '\0';

  // Buffer for the decimal part of the result.  We only generate up
  // to kBufferSize - 1 chars for the decimal part.
  char decimal_buffer[kBufferSize];
  decimal_buffer[kBufferSize - 1] = '\0';

  // Make sure the value is positive.
  bool is_negative = value < 0.0;
  if (is_negative) value = -value;

  // Get the integer part and the decimal part.
  double integer_part = floor(value);
  double decimal_part = value - integer_part;

  // Convert the integer part starting from the back.  Always generate
  // at least one digit.
  int integer_pos = kBufferSize - 2;
  do {
    integer_buffer[integer_pos--] =
        chars[static_cast<int>(fmod(integer_part, radix))];
    integer_part /= radix;
  } while (integer_part >= 1.0);
  // Sanity check.
  ASSERT(integer_pos > 0);
  // Add sign if needed.
  if (is_negative) integer_buffer[integer_pos--] = '-';

  // Convert the decimal part.  Repeatedly multiply by the radix to
  // generate the next char.  Never generate more than kBufferSize - 1
  // chars.
  //
  // TODO(1093998): We will often generate a full decimal_buffer of
  // chars because hitting zero will often not happen.  The right
  // solution would be to continue until the string representation can
  // be read back and yield the original value.  To implement this
  // efficiently, we probably have to modify dtoa.
  int decimal_pos = 0;
  while ((decimal_part > 0.0) && (decimal_pos < kBufferSize - 1)) {
    decimal_part *= radix;
    decimal_buffer[decimal_pos++] =
        chars[static_cast<int>(floor(decimal_part))];
    decimal_part -= floor(decimal_part);
  }
  decimal_buffer[decimal_pos] = '\0';

  // Compute the result size.
  int integer_part_size = kBufferSize - 2 - integer_pos;
  // Make room for zero termination.
  unsigned result_size = integer_part_size + decimal_pos;
  // If the number has a decimal part, leave room for the period.
  if (decimal_pos > 0) result_size++;
  // Allocate result and fill in the parts.
  StringBuilder builder(result_size + 1);
  builder.AddSubstring(integer_buffer + integer_pos + 1, integer_part_size);
  if (decimal_pos > 0) builder.AddCharacter('.');
  builder.AddSubstring(decimal_buffer, decimal_pos);
  return builder.Finalize();
}


} }  // namespace v8::internal