Added precision mode to fast-dtoa.

src/fast-dtoa.h

Index: src/fast-dtoa.h
===================================================================
--- src/fast-dtoa.h	(revision 4612)
+++ src/fast-dtoa.h	(working copy)
@@ -31,27 +31,52 @@
 namespace v8 {
 namespace internal {
 
+enum FastDtoaMode {
+  // Computes the shortest representation of the given input. The returned
+  // result will be the most accurate number of this length. Longer
+  // representations might be more accurate.
+  FAST_DTOA_SHORTEST,
+  // Computes a representation where the precision (number of digits) is
+  // given as input. The precision is independent of the decimal point.
+  FAST_DTOA_PRECISION
+};
+
 // FastDtoa will produce at most kFastDtoaMaximalLength digits. This does not
 // include the terminating '\0' character.
 static const int kFastDtoaMaximalLength = 17;
 
 // Provides a decimal representation of v.
-// v must be a strictly positive finite double.
+// The result should be interpreted as buffer * 10^(point - length).
+//
+// Precondition:
+//   * v must be a strictly positive finite double.
+//
 // Returns true if it succeeds, otherwise the result can not be trusted.
 // There will be *length digits inside the buffer followed by a null terminator.
-// If the function returns true then
-//   v == (double) (buffer * 10^(point - length)).
-// The digits in the buffer are the shortest representation possible: no
-// 0.099999999999 instead of 0.1.
-// The last digit will be closest to the actual v. That is, even if several
-// digits might correctly yield 'v' when read again, the buffer will contain the
-// one closest to v.
-// The variable 'sign' will be '0' if the given number is positive, and '1'
-//   otherwise.
+// If the function returns true and mode equals
+//   - FAST_DTOA_SHORTEST, then
+//     the parameter requested_digits is ignored.
+//     The result satisfies
+//         v == (double) (buffer * 10^(point - length)).
+//     The digits in the buffer are the shortest representation possible. E.g.
+//     if 0.099999999999 and 0.1 represent the same double then "1" is returned
+//     with point = 0.
+//     The last digit will be closest to the actual v. That is, even if several
+//     digits might correctly yield 'v' when read again, the buffer will contain
+//     the one closest to v.
+//   - FAST_DTOA_PRECISION, then
+//     the buffer contains requested_digits digits.
+//     the difference v - (buffer * 10^(point-length)) is closest to zero for
+//     all possible representations of requested_digits digits.
+//     If there are two values that are equally close, then FastDtoa returns
+//     false.
+// For both modes the buffer must be large enough to hold the result.
 bool FastDtoa(double d,
+              FastDtoaMode mode,
+              int requested_digits,
               Vector<char> buffer,
               int* length,
-              int* point);
+              int* decimal_point);
 
 } }  // namespace v8::internal