Added precision mode to fast-dtoa.

Review URL: http://codereview.chromium.org/2000004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5491 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/conversions.cc

@@ -956,8 +956,9 @@ static char* CreateExponentialRepresentation(char* decimal_rep,
 
 
 char* DoubleToExponentialCString(double value, int f) {
+  const int kMaxDigitsAfterPoint = 20;
   // f might be -1 to signal that f was undefined in JavaScript.
-  ASSERT(f >= -1 && f <= 20);
+  ASSERT(f >= -1 && f <= kMaxDigitsAfterPoint);
 
   bool negative = false;
   if (value < 0) {
@@ -969,29 +970,60 @@ char* DoubleToExponentialCString(double value, int f) {
   int decimal_point;
   int sign;
   char* decimal_rep = NULL;
+  bool used_gay_dtoa = false;
+  // f corresponds to the digits after the point. There is always one digit
+  // before the point. The number of requested_digits equals hence f + 1.
+  // And we have to add one character for the null-terminator.
+  const int kV8DtoaBufferCapacity = kMaxDigitsAfterPoint + 1 + 1;
+  // Make sure that the buffer is big enough, even if we fall back to the
+  // shortest representation (which happens when f equals -1).
+  ASSERT(kBase10MaximalLength <= kMaxDigitsAfterPoint + 1);
+  char v8_dtoa_buffer[kV8DtoaBufferCapacity];
+  int decimal_rep_length;
+
   if (f == -1) {
-    decimal_rep = dtoa(value, 0, 0, &decimal_point, &sign, NULL);
-    f = StrLength(decimal_rep) - 1;
+    if (DoubleToAscii(value, DTOA_SHORTEST, 0,
+                      Vector<char>(v8_dtoa_buffer, kV8DtoaBufferCapacity),
+                      &sign, &decimal_rep_length, &decimal_point)) {
+      f = decimal_rep_length - 1;
+      decimal_rep = v8_dtoa_buffer;
+    } else {
+      decimal_rep = dtoa(value, 0, 0, &decimal_point, &sign, NULL);
+      decimal_rep_length = StrLength(decimal_rep);
+      f = decimal_rep_length - 1;
+      used_gay_dtoa = true;
+    }
   } else {
-    decimal_rep = dtoa(value, 2, f + 1, &decimal_point, &sign, NULL);
+    if (DoubleToAscii(value, DTOA_PRECISION, f + 1,
+                      Vector<char>(v8_dtoa_buffer, kV8DtoaBufferCapacity),
+                      &sign, &decimal_rep_length, &decimal_point)) {
+      decimal_rep = v8_dtoa_buffer;
+    } else {
+      decimal_rep = dtoa(value, 2, f + 1, &decimal_point, &sign, NULL);
+      decimal_rep_length = StrLength(decimal_rep);
+      used_gay_dtoa = true;
+    }
   }
-  int decimal_rep_length = StrLength(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);
+  if (used_gay_dtoa) {
+    freedtoa(decimal_rep);
+  }
 
   return result;
 }
 
 
 char* DoubleToPrecisionCString(double value, int p) {
-  ASSERT(p >= 1 && p <= 21);
+  const int kMinimalDigits = 1;
+  const int kMaximalDigits = 21;
+  ASSERT(p >= kMinimalDigits && p <= kMaximalDigits);
+  USE(kMinimalDigits);
 
   bool negative = false;
   if (value < 0) {
@@ -1002,8 +1034,22 @@ char* DoubleToPrecisionCString(double value, int p) {
   // 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 = StrLength(decimal_rep);
+  char* decimal_rep = NULL;
+  bool used_gay_dtoa = false;
+  // Add one for the terminating null character.
+  const int kV8DtoaBufferCapacity = kMaximalDigits + 1;
+  char v8_dtoa_buffer[kV8DtoaBufferCapacity];
+  int decimal_rep_length;
+
+  if (DoubleToAscii(value, DTOA_PRECISION, p,
+                    Vector<char>(v8_dtoa_buffer, kV8DtoaBufferCapacity),
+                    &sign, &decimal_rep_length, &decimal_point)) {
+    decimal_rep = v8_dtoa_buffer;
+  } else {
+    decimal_rep = dtoa(value, 2, p, &decimal_point, &sign, NULL);
+    decimal_rep_length = StrLength(decimal_rep);
+    used_gay_dtoa = true;
+  }
   ASSERT(decimal_rep_length <= p);
 
   int exponent = decimal_point - 1;
@@ -1047,7 +1093,9 @@ char* DoubleToPrecisionCString(double value, int p) {
     result = builder.Finalize();
   }
 
-  freedtoa(decimal_rep);
+  if (used_gay_dtoa) {
+    freedtoa(decimal_rep);
+  }
   return result;
 }
 

src/dtoa.cc

@@ -65,11 +65,12 @@ bool DoubleToAscii(double v, DtoaMode mode, int requested_digits,
 
   switch (mode) {
     case DTOA_SHORTEST:
-      return FastDtoa(v, buffer, length, point);
+      return FastDtoa(v, FAST_DTOA_SHORTEST, 0, buffer, length, point);
     case DTOA_FIXED:
       return FastFixedDtoa(v, requested_digits, buffer, length, point);
-    default:
-      break;
+    case DTOA_PRECISION:
+      return FastDtoa(v, FAST_DTOA_PRECISION, requested_digits,
+                      buffer, length, point);
   }
   return false;
 }

src/fast-dtoa.h

@@ -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
 

test/cctest/SConscript

@@ -35,6 +35,7 @@ Import('context object_files')
 SOURCES = {
   'all': [
     'gay-fixed.cc',
+    'gay-precision.cc',
     'gay-shortest.cc',
     'test-accessors.cc',
     'test-alloc.cc',

test/cctest/gay-precision.h

+// Copyright 2006-2008 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
+// 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.
+
+#ifndef GAY_PRECISION_H_
+#define GAY_PRECISION_H_
+
+namespace v8 {
+namespace internal {
+
+struct PrecomputedPrecision {
+  double v;
+  int number_digits;
+  const char* representation;
+  int decimal_point;
+};
+
+// Returns precomputed values of dtoa. The strings have been generated using
+// Gay's dtoa in mode "precision".
+Vector<const PrecomputedPrecision> PrecomputedPrecisionRepresentations();
+
+} }  // namespace v8::internal
+
+#endif  // GAY_PRECISION_H_

test/cctest/test-fast-dtoa.cc

@@ -9,13 +9,26 @@
 #include "diy-fp.h"
 #include "double.h"
 #include "fast-dtoa.h"
+#include "gay-precision.h"
 #include "gay-shortest.h"
 
 using namespace v8::internal;
 
 static const int kBufferSize = 100;
 
-TEST(FastDtoaVariousDoubles) {
+
+// Removes trailing '0' digits.
+static void TrimRepresentation(Vector<char> representation) {
+  int len = strlen(representation.start());
+  int i;
+  for (i = len - 1; i >= 0; --i) {
+    if (representation[i] != '0') break;
+  }
+  representation[i + 1] = '\0';
+}
+
+
+TEST(FastDtoaShortestVariousDoubles) {
   char buffer_container[kBufferSize];
   Vector<char> buffer(buffer_container, kBufferSize);
   int length;
@@ -23,38 +36,45 @@ TEST(FastDtoaVariousDoubles) {
   int status;
 
   double min_double = 5e-324;
-  status = FastDtoa(min_double, buffer, &length, &point);
+  status = FastDtoa(min_double, FAST_DTOA_SHORTEST, 0,
+                    buffer, &length, &point);
   CHECK(status);
   CHECK_EQ("5", buffer.start());
   CHECK_EQ(-323, point);
 
   double max_double = 1.7976931348623157e308;
-  status = FastDtoa(max_double, buffer, &length, &point);
+  status = FastDtoa(max_double, FAST_DTOA_SHORTEST, 0,
+                    buffer, &length, &point);
   CHECK(status);
   CHECK_EQ("17976931348623157", buffer.start());
   CHECK_EQ(309, point);
 
-  status = FastDtoa(4294967272.0, buffer, &length, &point);
+  status = FastDtoa(4294967272.0, FAST_DTOA_SHORTEST, 0,
+                    buffer, &length, &point);
   CHECK(status);
   CHECK_EQ("4294967272", buffer.start());
   CHECK_EQ(10, point);
 
-  status = FastDtoa(4.1855804968213567e298, buffer, &length, &point);
+  status = FastDtoa(4.1855804968213567e298, FAST_DTOA_SHORTEST, 0,
+                    buffer, &length, &point);
   CHECK(status);
   CHECK_EQ("4185580496821357", buffer.start());
   CHECK_EQ(299, point);
 
-  status = FastDtoa(5.5626846462680035e-309, buffer, &length, &point);
+  status = FastDtoa(5.5626846462680035e-309, FAST_DTOA_SHORTEST, 0,
+                    buffer, &length, &point);
   CHECK(status);
   CHECK_EQ("5562684646268003", buffer.start());
   CHECK_EQ(-308, point);
 
-  status = FastDtoa(2147483648.0, buffer, &length, &point);
+  status = FastDtoa(2147483648.0, FAST_DTOA_SHORTEST, 0,
+                    buffer, &length, &point);
   CHECK(status);
   CHECK_EQ("2147483648", buffer.start());
   CHECK_EQ(10, point);
 
-  status = FastDtoa(3.5844466002796428e+298, buffer, &length, &point);
+  status = FastDtoa(3.5844466002796428e+298, FAST_DTOA_SHORTEST, 0,
+                    buffer, &length, &point);
   if (status) {  // Not all FastDtoa variants manage to compute this number.
     CHECK_EQ("35844466002796428", buffer.start());
     CHECK_EQ(299, point);
@@ -62,7 +82,7 @@ TEST(FastDtoaVariousDoubles) {
 
   uint64_t smallest_normal64 = V8_2PART_UINT64_C(0x00100000, 00000000);
   double v = Double(smallest_normal64).value();
-  status = FastDtoa(v, buffer, &length, &point);
+  status = FastDtoa(v, FAST_DTOA_SHORTEST, 0, buffer, &length, &point);
   if (status) {
     CHECK_EQ("22250738585072014", buffer.start());
     CHECK_EQ(-307, point);
@@ -70,7 +90,7 @@ TEST(FastDtoaVariousDoubles) {
 
   uint64_t largest_denormal64 = V8_2PART_UINT64_C(0x000FFFFF, FFFFFFFF);
   v = Double(largest_denormal64).value();
-  status = FastDtoa(v, buffer, &length, &point);
+  status = FastDtoa(v, FAST_DTOA_SHORTEST, 0, buffer, &length, &point);
   if (status) {
     CHECK_EQ("2225073858507201", buffer.start());
     CHECK_EQ(-307, point);
@@ -78,6 +98,107 @@ TEST(FastDtoaVariousDoubles) {
 }
 
 
+TEST(FastDtoaPrecisionVariousDoubles) {
+  char buffer_container[kBufferSize];
+  Vector<char> buffer(buffer_container, kBufferSize);
+  int length;
+  int point;
+  int status;
+
+  status = FastDtoa(1.0, FAST_DTOA_PRECISION, 3, buffer, &length, &point);
+  CHECK(status);
+  CHECK_GE(3, length);
+  TrimRepresentation(buffer);
+  CHECK_EQ("1", buffer.start());
+  CHECK_EQ(1, point);
+
+  status = FastDtoa(1.5, FAST_DTOA_PRECISION, 10, buffer, &length, &point);
+  if (status) {
+    CHECK_GE(10, length);
+    TrimRepresentation(buffer);
+    CHECK_EQ("15", buffer.start());
+    CHECK_EQ(1, point);
+  }
+
+  double min_double = 5e-324;
+  status = FastDtoa(min_double, FAST_DTOA_PRECISION, 5,
+                    buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("49407", buffer.start());
+  CHECK_EQ(-323, point);
+
+  double max_double = 1.7976931348623157e308;
+  status = FastDtoa(max_double, FAST_DTOA_PRECISION, 7,
+                    buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("1797693", buffer.start());
+  CHECK_EQ(309, point);
+
+  status = FastDtoa(4294967272.0, FAST_DTOA_PRECISION, 14,
+                    buffer, &length, &point);
+  if (status) {
+    CHECK_GE(14, length);
+    TrimRepresentation(buffer);
+    CHECK_EQ("4294967272", buffer.start());
+    CHECK_EQ(10, point);
+  }
+
+  status = FastDtoa(4.1855804968213567e298, FAST_DTOA_PRECISION, 17,
+                    buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("41855804968213567", buffer.start());
+  CHECK_EQ(299, point);
+
+  status = FastDtoa(5.5626846462680035e-309, FAST_DTOA_PRECISION, 1,
+                    buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("6", buffer.start());
+  CHECK_EQ(-308, point);
+
+  status = FastDtoa(2147483648.0, FAST_DTOA_PRECISION, 5,
+                    buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("21475", buffer.start());
+  CHECK_EQ(10, point);
+
+  status = FastDtoa(3.5844466002796428e+298, FAST_DTOA_PRECISION, 10,
+                    buffer, &length, &point);
+  CHECK(status);
+  CHECK_GE(10, length);
+  TrimRepresentation(buffer);
+  CHECK_EQ("35844466", buffer.start());
+  CHECK_EQ(299, point);
+
+  uint64_t smallest_normal64 = V8_2PART_UINT64_C(0x00100000, 00000000);
+  double v = Double(smallest_normal64).value();
+  status = FastDtoa(v, FAST_DTOA_PRECISION, 17, buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("22250738585072014", buffer.start());
+  CHECK_EQ(-307, point);
+
+  uint64_t largest_denormal64 = V8_2PART_UINT64_C(0x000FFFFF, FFFFFFFF);
+  v = Double(largest_denormal64).value();
+  status = FastDtoa(v, FAST_DTOA_PRECISION, 17, buffer, &length, &point);
+  CHECK(status);
+  CHECK_GE(20, length);
+  TrimRepresentation(buffer);
+  CHECK_EQ("22250738585072009", buffer.start());
+  CHECK_EQ(-307, point);
+
+  v = 3.3161339052167390562200598e-237;
+  status = FastDtoa(v, FAST_DTOA_PRECISION, 18, buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("331613390521673906", buffer.start());
+  CHECK_EQ(-236, point);
+
+  v = 7.9885183916008099497815232e+191;
+  status = FastDtoa(v, FAST_DTOA_PRECISION, 4, buffer, &length, &point);
+  CHECK(status);
+  CHECK_EQ("7989", buffer.start());
+  CHECK_EQ(192, point);
+}
+
+
 TEST(FastDtoaGayShortest) {
   char buffer_container[kBufferSize];
   Vector<char> buffer(buffer_container, kBufferSize);
@@ -94,7 +215,7 @@ TEST(FastDtoaGayShortest) {
     const PrecomputedShortest current_test = precomputed[i];
     total++;
     double v = current_test.v;
-    status = FastDtoa(v, buffer, &length, &point);
+    status = FastDtoa(v, FAST_DTOA_SHORTEST, 0, buffer, &length, &point);
     CHECK_GE(kFastDtoaMaximalLength, length);
     if (!status) continue;
     if (length == kFastDtoaMaximalLength) needed_max_length = true;
@@ -105,3 +226,43 @@ TEST(FastDtoaGayShortest) {
   CHECK_GT(succeeded*1.0/total, 0.99);
   CHECK(needed_max_length);
 }
+
+
+TEST(FastDtoaGayPrecision) {
+  char buffer_container[kBufferSize];
+  Vector<char> buffer(buffer_container, kBufferSize);
+  bool status;
+  int length;
+  int point;
+  int succeeded = 0;
+  int total = 0;
+  // Count separately for entries with less than 15 requested digits.
+  int succeeded_15 = 0;
+  int total_15 = 0;
+
+  Vector<const PrecomputedPrecision> precomputed =
+      PrecomputedPrecisionRepresentations();
+  for (int i = 0; i < precomputed.length(); ++i) {
+    const PrecomputedPrecision current_test = precomputed[i];
+    double v = current_test.v;
+    int number_digits = current_test.number_digits;
+    total++;
+    if (number_digits <= 15) total_15++;
+    status = FastDtoa(v, FAST_DTOA_PRECISION, number_digits,
+                      buffer, &length, &point);
+    CHECK_GE(number_digits, length);
+    if (!status) continue;
+    succeeded++;
+    if (number_digits <= 15) succeeded_15++;
+    TrimRepresentation(buffer);
+    CHECK_EQ(current_test.decimal_point, point);
+    CHECK_EQ(current_test.representation, buffer.start());
+  }
+  // The precomputed numbers contain many entries with many requested
+  // digits. These have a high failure rate and we therefore expect a lower
+  // success rate than for the shortest representation.
+  CHECK_GT(succeeded*1.0/total, 0.85);
+  // However with less than 15 digits almost the algorithm should almost always
+  // succeed.
+  CHECK_GT(succeeded_15*1.0/total_15, 0.9999);
+}