Merge pull request #12 from Kijewski/pr-faster-floating-point-encoding

Faster double encoding
diff --git a/.github/workflows/compile_test.yml b/.github/workflows/compile_test.yml
index 9928efc..140aece 100644
--- a/.github/workflows/compile_test.yml
+++ b/.github/workflows/compile_test.yml
@@ -12,8 +12,8 @@
           - macos-latest
           - windows-latest
         python: [
-          '3.5',
-          # '3.6', '3.7', '3.8',  # it takes too much GitHub action time to run tests on all versions in between
+          '3.6',
+          # '3.5', '3.7', '3.8',  # it takes too much GitHub action time to run tests on all versions in between
           '3.9',
         ]
         include:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1a97231..8149835 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+### 1.5.0
+
+* Faster floating-point number encoding using [Junekey Jeon's Dragonbox algorithm](https://github.com/abolz/Drachennest/blob/77f4889a4cd9d7f0b9da82a379f14beabcfba13e/src/dragonbox.cc) implemented by Alexander Bolz
+* Removed a lot of configuration options from pyjson5.Options()
+
 ### 1.4.9
 
 * Faster floating-point number decoding using [fast_double_parser](https://github.com/lemire/fast_double_parser) by Daniel Lemire
diff --git a/Makefile b/Makefile
index 4b878d7..ec84964 100644
--- a/Makefile
+++ b/Makefile
@@ -10,7 +10,8 @@
 INCLUDES := \
     src/VERSION src/DESCRIPTION \
     src/_decoder_recursive_select.hpp src/_unicode_cat_of.hpp \
-    src/_escape_dct.hpp src/_stack_heap_string.hpp src/native.hpp
+    src/_escape_dct.hpp src/_stack_heap_string.hpp src/native.hpp \
+    src/dragonbox.cc
 
 FILES := Makefile MANIFEST.in pyjson5.pyx README.rst setup.py ${INCLUDES}
 
diff --git a/docs/performance.rst b/docs/performance.rst
index 5756899..9563649 100644
--- a/docs/performance.rst
+++ b/docs/performance.rst
@@ -25,9 +25,71 @@
 The output is always ASCII regardless if you call :func:`pyjson5.encode` or :func:`pyjson5.encode_bytes`.
 
 * Dataset: https://github.com/zemirco/sf-city-lots-json
-* Version: 3.9.1 (default, Dec  8 2020, 07:51:42)
+* Python 3.9.1+ (default, Feb  5 2021, 13:46:56) 
 * CPU: AMD Ryzen 7 2700 @ 3.7GHz
-* :func:`pyjson5.encode`: **3.21** s ± 25.1 per loop *(lower is better)*
-* :func:`json.dumps`: **3.82** s ± 20.4 ms per loop
-* :func:`json.dumps` + :func:`xml.sax.saxutils.escape`: **3.95** s ± 18.3 ms per loop
-* The encoder works correcty: ``obj == json.loads(pyjson5.encode(obj, floatformat='%.16e'))``
+* :func:`pyjson5.encode`: **1.37** s ± 19.2 per loop *(lower is better)*
+* :func:`json.dumps`: **3.66** s ± 72.6 ms per loop
+* :func:`json.dumps` + :func:`xml.sax.saxutils.escape`: **4.01** s ± 21.3 ms per loop
+* The encoder works correcty: ``obj == json.loads(pyjson5.encode(obj))``
+
+
+Benchmark
+---------
+
+Using `Ultrajson's benchmark <https://github.com/ultrajson/ultrajson/blob/197a7fd4d8bbf0a8355852017c8b25aab26b6777/tests/benchmark.py>`_
+you can tell for which kind of data PyJSON5 is fast, and for which data it is slow in comparison *(higher is better)*:
+
++-----------------------------------------------------------+-------------+------------+------------+------------+
+|                                                           |  json       | pyjson5    | ujson      | orjson     |
++===========================================================+=============+============+============+============+
+| **Array with 256 doubles**                                |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |       6,425 |     81,202 |     28,966 |     83,836 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |      16,759 |     34,801 |     34,794 |     80,655 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| **Array with 256 strings**                                |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |      36,969 |     73,165 |     35,574 |    113,082 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |      42,730 |     38,542 |     38,386 |     60,732 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| **Array with 256 UTF-8 strings**                          |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |       3,458 |      3,134 |      4,024 |     31,677 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |       2,428 |      2,498 |      2,491 |      1,750 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| **Array with 256 True values**                            |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |     130,441 |    282,703 |    131,279 |    423,371 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |     220,657 |    262,690 |    264,485 |    262,283 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| **Array with 256 dict{string, int} pairs**                |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |      11,621 |     10,014 |     18,148 |     73,905 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |      17,802 |     19,406 |     19,391 |     23,478 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| **Dict with 256 arrays with 256 dict{string, int} pairs** |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |          40 |         38 |         68 |        213 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |          43 |         49 |         48 |         51 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| **Medium complex object**                                 |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |       8,704 |     11,922 |     15,319 |     49,677 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |      12,567 |     14,042 |     13,985 |     19,481 |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| **Complex object**                                        |             |            |            |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| encode                                                    |         672 |        909 |        731 |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+| decode                                                    |         462 |        700 |        700 |            |
++-----------------------------------------------------------+-------------+------------+------------+------------+
+
+* `ujson <https://github.com/ultrajson/ultrajson>`_ == 4.0.3.dev9
+* `orjson <https://github.com/ijl/orjson>`_ == 3.5.1
diff --git a/requirements.txt b/requirements.txt
index 9000539..9c1dc39 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,6 +3,6 @@
 cython == 0.*, >= 0.29.14
 more_itertools == 8.*, >= 8.0.2
 sphinx == 2.*, >= 2.2.2
-sphinx_autodoc_typehints == 1.*, >= 1.10.3
+sphinx_autodoc_typehints == 1.*, >= 1.10.3, < 1.11
 sphinx_rtd_theme == 0.*, >= 0.4.3
 wheel == 0.*, >= 0.33.6
diff --git a/src/VERSION b/src/VERSION
index 340028b..c5c7371 100644
--- a/src/VERSION
+++ b/src/VERSION
@@ -1 +1 @@
-"1.4.9.post1"
+"1.5.0"
diff --git a/src/_encoder.pyx b/src/_encoder.pyx
index 675034d..80dd2c4 100644
--- a/src/_encoder.pyx
+++ b/src/_encoder.pyx
@@ -286,49 +286,46 @@
     return True
 
 
-cdef int _encode_format_string(WriterRef writer, object data, object formatter_string) except -1:
+cdef int _encode_format_string(WriterRef writer, object data, object fmt) except -1:
+    cdef object formatted
     cdef const char *string
     cdef Py_ssize_t length = 0  # silence warning
 
-    if expect(formatter_string is None, False):
-        _raise_unstringifiable(data)
-
-    formatter_string = PyUnicode_Format(formatter_string, data)
-    string = PyUnicode_AsUTF8AndSize(formatter_string, &length)
+    formatted = PyUnicode_Format(fmt, data)
+    string = PyUnicode_AsUTF8AndSize(formatted, &length)
     writer.append_s(writer, string, length)
 
     return True
 
 
 cdef int _encode_float(WriterRef writer, object data) except -1:
-    cdef object formatter_string
     cdef double value = PyFloat_AsDouble(data)
     cdef int classification = fpclassify(value)
+    cdef char buf[64]
+    cdef char *end
+    cdef char *string
+    cdef Py_ssize_t length
 
     if classification == FP_NORMAL:
-        return _encode_format_string(writer, data, (<Options> writer.options).floatformat)
-
-    if classification in (FP_SUBNORMAL, FP_ZERO):
-        writer.append_c(writer, b'0')
-        return True
-
-    if classification == FP_NAN:
-        formatter_string = (<Options> writer.options).nan
+        end = Dtoa(buf, PyFloat_AsDouble(data))
+        length = end - buf
+        string = buf
+    elif classification in (FP_SUBNORMAL, FP_ZERO):
+        string = b'0.0'
+        length = 3
+    elif classification == FP_NAN:
+        string = b'NaN'
+        length = 3
     else:
         # classification == FP_INFINITE
         if value > 0.0:
-            formatter_string = (<Options> writer.options).posinfinity
+            string = b'Infinity'
+            length = 8
         else:
-            formatter_string = (<Options> writer.options).neginfinity
+            string = b'-Infinity'
+            length = 9
 
-    if expect(formatter_string is None, False):
-        _raise_unstringifiable(data)
-
-    writer.append_s(
-        writer,
-        <const char*> PyUnicode_1BYTE_DATA(formatter_string),
-        PyUnicode_GET_LENGTH(formatter_string),
-    )
+    writer.append_s(writer, string, length)
     return True
 
 
@@ -339,7 +336,7 @@
         else:
             writer.append_s(writer, 'false', 5)
     else:
-        _encode_format_string(writer, data, (<Options> writer.options).intformat)
+        _encode_format_string(writer, data, DEFAULT_INTFORMAT)
     return True
 
 
@@ -347,7 +344,7 @@
     if not isinstance(data, Decimal):
         return False
 
-    _encode_format_string(writer, data, (<Options> writer.options).decimalformat)
+    _encode_format_string(writer, data, DEFAULT_DECIMALFORMAT)
     return True
 
 
diff --git a/src/_encoder_options.pyx b/src/_encoder_options.pyx
index 7454c3e..16d5bee 100644
--- a/src/_encoder_options.pyx
+++ b/src/_encoder_options.pyx
@@ -1,9 +1,5 @@
 cdef object DEFAULT_TOJSON = False
-cdef object DEFAULT_POSINFINITY = 'Infinity'
-cdef object DEFAULT_NEGINFINITY = '-Infinity'
-cdef object DEFAULT_NAN = 'NaN'
 cdef object DEFAULT_INTFORMAT = '%d'
-cdef object DEFAULT_FLOATFORMAT = '%.6e'
 cdef object DEFAULT_DECIMALFORMAT = '%s'
 cdef object DEFAULT_MAPPINGTYPES = (Mapping,)
 cdef object DEFAULT_QUOTATIONMARK = '"'
@@ -45,12 +41,6 @@
     return ', '.join(filter(bool, (
         _option_from_ascii('quotationmark', self.quotationmark, DEFAULT_QUOTATIONMARK),
         _option_from_ascii('tojson', self.tojson, None),
-        _option_from_ascii('posinfinity', self.posinfinity, DEFAULT_POSINFINITY),
-        _option_from_ascii('neginfinity', self.neginfinity, DEFAULT_NEGINFINITY),
-        _option_from_ascii('intformat', self.intformat, DEFAULT_INTFORMAT),
-        _option_from_ascii('floatformat', self.floatformat, DEFAULT_FLOATFORMAT),
-        _option_from_ascii('decimalformat', self.decimalformat, DEFAULT_DECIMALFORMAT),
-        _option_from_ascii('nan', self.nan, DEFAULT_NAN),
     )))
 
 
@@ -73,30 +63,6 @@
         * **str:** A special method to call on objects to return a custom JSON encoded string. Must return ASCII data!
         * **False:** No such member exists. (Default.)
         * **None:** Use default.
-    posinfinity : str|False|None
-        * **str:** String to represent positive infinity. Must be ASCII.
-        * **False:** Throw an exception if ``float('+inf')`` is encountered.
-        * **None:** Use default: ``"Infinity"``.
-    neginfinity : str|False|None
-        * **str:** String to represent negative infinity. Must be ASCII.
-        * **False:** Throw an exception if ``float('-inf')`` is encountered.
-        * **None:** Use default: ``"-Infinity"``.
-    nan : str|False|None
-        * **str:** String to represent not-a-number. Must be ASCII.
-        * **False:** Throw an exception if ``float('NaN')`` is encountered.
-        * **None:** Use default: ``"NaN"``.
-    intformat : str|False|None
-        * **str:** Format string to use with ``int``.
-        * **False:** Throw an exception if an ``int`` is encountered.
-        * **None:** Use default: ``"%d"``.
-    floatformat : str|False|None
-        * **str:** Format string to use with ``float``.
-        * **False:** Throw an exception if a ``float`` is encountered.
-        * **None:** Use default: ``"%.6e"``.
-    decimalformat : str|False|None
-        * **str:** Format string to use with ``Decimal``.
-        * **False:** Throw an exception if a ``Decimal`` is encountered.
-        * **None:** Use default: ``"%s"``.
     mappingtypes : Iterable[type]|False|None
         * **Iterable[type]:** Classes that should be encoded to objects. Must be iterable over their keys, and implement ``__getitem__``.
         * **False:** There are no objects. Any object will be encoded as list of keys as in list(obj).
@@ -109,30 +75,6 @@
     '''The creation argument ``tojson``.
     ``None`` if ``False`` was specified.
     '''
-    cdef readonly unicode posinfinity
-    '''The creation argument ``posinfinity``.
-    ``None`` if ``False`` was specified.
-    '''
-    cdef readonly unicode neginfinity
-    '''The creation argument ``neginfinity``.
-    ``None`` if ``False`` was specified.
-    '''
-    cdef readonly unicode nan
-    '''The creation argument ``nan``.
-    ``None`` if ``False`` was specified.
-    '''
-    cdef readonly unicode intformat
-    '''The creation argument ``intformat``.
-    ``None`` if ``False`` was specified.
-    '''
-    cdef readonly unicode floatformat
-    '''The creation argument ``floatformat``.
-    ``None`` if ``False`` was specified.
-    '''
-    cdef readonly unicode decimalformat
-    '''The creation argument ``decimalformat``.
-    ``None`` if ``False`` was specified.
-    '''
     cdef readonly tuple mappingtypes
     '''The creation argument ``mappingtypes``.
     ``()`` if ``False`` was specified.
@@ -142,12 +84,7 @@
         cdef object args = tuple(filter(bool, (
             _options_reduce_arg('quotationmark', self.quotationmark, DEFAULT_QUOTATIONMARK),
             _options_reduce_arg('tojson', self.tojson, None),
-            _options_reduce_arg('posinfinity', self.posinfinity, DEFAULT_POSINFINITY),
-            _options_reduce_arg('neginfinity', self.neginfinity, DEFAULT_NEGINFINITY),
-            _options_reduce_arg('intformat', self.intformat, DEFAULT_INTFORMAT),
-            _options_reduce_arg('floatformat', self.floatformat, DEFAULT_FLOATFORMAT),
-            _options_reduce_arg('decimalformat', self.decimalformat, DEFAULT_DECIMALFORMAT),
-            _options_reduce_arg('nan', self.nan, DEFAULT_NAN),
+            _options_reduce_arg('mappingtypes', self.mappingtypes, DEFAULT_MAPPINGTYPES),
         )))
         return (_UnpickleOptions if args else Options), args
 
@@ -170,7 +107,7 @@
     def __cinit__(self, *,
                   quotationmark=None,
                   tojson=None, posinfinity=None, neginfinity=None, nan=None,
-                  decimalformat=None, intformat=None, floatformat=None,
+                  decimalformat=None, intformat=None,
                   mappingtypes=None):
         cdef object cls
         cdef object ex
@@ -179,51 +116,15 @@
             quotationmark = DEFAULT_QUOTATIONMARK
         if tojson is None:
             tojson = DEFAULT_TOJSON
-        if posinfinity is None:
-            posinfinity = DEFAULT_POSINFINITY
-        if neginfinity is None:
-            neginfinity = DEFAULT_NEGINFINITY
-        if nan is None:
-            nan = DEFAULT_NAN
-        if intformat is None:
-            intformat = DEFAULT_INTFORMAT
-        if floatformat is None:
-            floatformat = DEFAULT_FLOATFORMAT
-        if decimalformat is None:
-            decimalformat = DEFAULT_DECIMALFORMAT
         if mappingtypes is None:
             mappingtypes = DEFAULT_MAPPINGTYPES
 
         self.quotationmark = _options_ascii(quotationmark)
         self.tojson = _options_ascii(tojson, False)
-        self.posinfinity = _options_ascii(posinfinity)
-        self.neginfinity = _options_ascii(neginfinity)
-        self.intformat = _options_ascii(intformat)
-        self.floatformat = _options_ascii(floatformat)
-        self.decimalformat = _options_ascii(decimalformat)
-        self.nan = _options_ascii(nan)
 
         if self.quotationmark is None or PyUnicode_GET_LENGTH(self.quotationmark) != 1:
             raise TypeError('quotationmark must be one ASCII character.')
 
-        if intformat is not None:
-            try:
-                PyUnicode_Format(intformat, TEST_INT)
-            except Exception as ex:
-                raise ValueError('intformat is not a valid format string') from ex
-
-        if floatformat is not None:
-            try:
-                PyUnicode_Format(floatformat, TEST_FLOAT)
-            except Exception as ex:
-                raise ValueError('floatformat is not a valid format string') from ex
-
-        if decimalformat is not None:
-            try:
-                PyUnicode_Format(decimalformat, TEST_DECIMAL)
-            except Exception as ex:
-                raise ValueError('decimalformat is not a valid format string') from ex
-
         if mappingtypes is False:
             self.mappingtypes = ()
         else:
@@ -264,12 +165,6 @@
 
     PyDict_SetDefault(kw, 'quotationmark', (<Options> arg).quotationmark)
     PyDict_SetDefault(kw, 'tojson', (<Options> arg).tojson)
-    PyDict_SetDefault(kw, 'posinfinity', (<Options> arg).posinfinity)
-    PyDict_SetDefault(kw, 'neginfinity', (<Options> arg).neginfinity)
-    PyDict_SetDefault(kw, 'nan', (<Options> arg).nan)
-    PyDict_SetDefault(kw, 'intformat', (<Options> arg).intformat)
-    PyDict_SetDefault(kw, 'floatformat', (<Options> arg).floatformat)
-    PyDict_SetDefault(kw, 'decimalformat', (<Options> arg).decimalformat)
     PyDict_SetDefault(kw, 'mappingtypes', (<Options> arg).mappingtypes)
 
     return Options(**kw)
diff --git a/src/_imports.pyx b/src/_imports.pyx
index 75d9926..7af9493 100644
--- a/src/_imports.pyx
+++ b/src/_imports.pyx
@@ -130,6 +130,10 @@
     const char *parse_number(const char *p, double *outDouble)
 
 
+cdef extern from 'src/dragonbox.cc' namespace 'dragonbox' nogil:
+    char *Dtoa(char* buffer, double value)
+
+
 cdef extern from 'Python.h':
     enum:
         PyUnicode_WCHAR_KIND
diff --git a/src/dragonbox.cc b/src/dragonbox.cc
new file mode 100644
index 0000000..1855316
--- /dev/null
+++ b/src/dragonbox.cc
@@ -0,0 +1,1527 @@
+// Copyright 2020 Junekey Jeon
+// Copyright 2020 Alexander Bolz
+//
+// Distributed under the Boost Software License, Version 1.0.
+//  (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+
+//--------------------------------------------------------------------------------------------------
+// This file contains an implementation of Junekey Jeon's Dragonbox algorithm.
+//
+// It is a simplified version of the reference implementation found here:
+// https://github.com/jk-jeon/dragonbox
+//
+// The reference implementation also works with single-precision floating-point numbers and
+// has options to configure the rounding mode.
+//--------------------------------------------------------------------------------------------------
+
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#if defined(_MSC_VER)
+#include <intrin.h>
+#endif
+
+#ifndef DRAGONBOX_ASSERT
+#define DRAGONBOX_ASSERT(X)
+#endif
+
+namespace dragonbox {
+namespace {
+
+//==================================================================================================
+//
+//==================================================================================================
+
+template <typename Dest, typename Source>
+static inline Dest ReinterpretBits(Source source)
+{
+    static_assert(sizeof(Dest) == sizeof(Source), "size mismatch");
+
+    Dest dest;
+    std::memcpy(&dest, &source, sizeof(Source));
+    return dest;
+}
+
+struct Double
+{
+    static_assert(std::numeric_limits<double>::is_iec559
+               && std::numeric_limits<double>::digits == 53
+               && std::numeric_limits<double>::max_exponent == 1024,
+        "IEEE-754 double-precision implementation required");
+
+    using value_type = double;
+    using bits_type = uint64_t;
+
+//  static constexpr int32_t   MaxDigits10     = std::numeric_limits<value_type>::max_digits10;
+    static constexpr int32_t   SignificandSize = std::numeric_limits<value_type>::digits; // = p   (includes the hidden bit)
+    static constexpr int32_t   ExponentBias    = std::numeric_limits<value_type>::max_exponent - 1 + (SignificandSize - 1);
+//  static constexpr int32_t   MaxExponent     = std::numeric_limits<value_type>::max_exponent - 1 - (SignificandSize - 1);
+//  static constexpr int32_t   MinExponent     = std::numeric_limits<value_type>::min_exponent - 1 - (SignificandSize - 1);
+    static constexpr bits_type MaxIeeeExponent = bits_type{2 * std::numeric_limits<value_type>::max_exponent - 1};
+    static constexpr bits_type HiddenBit       = bits_type{1} << (SignificandSize - 1);   // = 2^(p-1)
+    static constexpr bits_type SignificandMask = HiddenBit - 1;                           // = 2^(p-1) - 1
+    static constexpr bits_type ExponentMask    = MaxIeeeExponent << (SignificandSize - 1);
+    static constexpr bits_type SignMask        = ~(~bits_type{0} >> 1);
+
+    bits_type bits;
+
+    explicit Double(bits_type bits_) : bits(bits_) {}
+    explicit Double(value_type value) : bits(ReinterpretBits<bits_type>(value)) {}
+
+    bits_type PhysicalSignificand() const {
+        return bits & SignificandMask;
+    }
+
+    bits_type PhysicalExponent() const {
+        return (bits & ExponentMask) >> (SignificandSize - 1);
+    }
+
+    bool IsFinite() const {
+        return (bits & ExponentMask) != ExponentMask;
+    }
+
+    bool IsInf() const {
+        return (bits & ExponentMask) == ExponentMask && (bits & SignificandMask) == 0;
+    }
+
+    bool IsNaN() const {
+        return (bits & ExponentMask) == ExponentMask && (bits & SignificandMask) != 0;
+    }
+
+    bool IsZero() const {
+        return (bits & ~SignMask) == 0;
+    }
+
+    bool SignBit() const {
+        return (bits & SignMask) != 0;
+    }
+};
+
+//==================================================================================================
+//
+//==================================================================================================
+
+// Returns floor(x / 2^n).
+//
+// Technically, right-shift of negative integers is implementation defined...
+// Should easily be optimized into SAR (or equivalent) instruction.
+static inline int32_t FloorDivPow2(int32_t x, int32_t n)
+{
+#if 0
+    return x < 0 ? ~(~x >> n) : (x >> n);
+#else
+    return x >> n;
+#endif
+}
+
+static inline int32_t FloorLog2Pow10(int32_t e)
+{
+    DRAGONBOX_ASSERT(e >= -1233);
+    DRAGONBOX_ASSERT(e <=  1233);
+    return FloorDivPow2(e * 1741647, 19);
+}
+
+static inline int32_t FloorLog10Pow2(int32_t e)
+{
+    DRAGONBOX_ASSERT(e >= -1500);
+    DRAGONBOX_ASSERT(e <=  1500);
+    return FloorDivPow2(e * 1262611, 22);
+}
+
+static inline int32_t FloorLog10ThreeQuartersPow2(int32_t e)
+{
+    DRAGONBOX_ASSERT(e >= -1500);
+    DRAGONBOX_ASSERT(e <=  1500);
+    return FloorDivPow2(e * 1262611 - 524031, 22);
+}
+
+//==================================================================================================
+//
+//==================================================================================================
+
+namespace {
+struct uint64x2 {
+    uint64_t hi;
+    uint64_t lo;
+};
+}
+
+static inline uint64x2 ComputePow10(int32_t k)
+{
+    static constexpr int32_t kMin = -292;
+    static constexpr int32_t kMax =  326;
+    static constexpr uint64x2 Pow10[kMax - kMin + 1] = {
+        {0xFF77B1FCBEBCDC4F, 0x25E8E89C13BB0F7B},
+        {0x9FAACF3DF73609B1, 0x77B191618C54E9AD},
+        {0xC795830D75038C1D, 0xD59DF5B9EF6A2418},
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+        {0x9EC95D1463E8A506, 0xF4363804324A40AA},
+        {0xC67BB4597CE2CE48, 0xB143C6053EDCD0D5},
+        {0xF81AA16FDC1B81DA, 0xDD94B7868E94050A},
+        {0x9B10A4E5E9913128, 0xCA7CF2B4191C8326},
+        {0xC1D4CE1F63F57D72, 0xFD1C2F611F63A3F0},
+        {0xF24A01A73CF2DCCF, 0xBC633B39673C8CEC},
+        {0x976E41088617CA01, 0xD5BE0503E085D813},
+        {0xBD49D14AA79DBC82, 0x4B2D8644D8A74E18},
+        {0xEC9C459D51852BA2, 0xDDF8E7D60ED1219E},
+        {0x93E1AB8252F33B45, 0xCABB90E5C942B503},
+        {0xB8DA1662E7B00A17, 0x3D6A751F3B936243},
+        {0xE7109BFBA19C0C9D, 0x0CC512670A783AD4},
+        {0x906A617D450187E2, 0x27FB2B80668B24C5},
+        {0xB484F9DC9641E9DA, 0xB1F9F660802DEDF6},
+        {0xE1A63853BBD26451, 0x5E7873F8A0396973},
+        {0x8D07E33455637EB2, 0xDB0B487B6423E1E8},
+        {0xB049DC016ABC5E5F, 0x91CE1A9A3D2CDA62},
+        {0xDC5C5301C56B75F7, 0x7641A140CC7810FB},
+        {0x89B9B3E11B6329BA, 0xA9E904C87FCB0A9D},
+        {0xAC2820D9623BF429, 0x546345FA9FBDCD44},
+        {0xD732290FBACAF133, 0xA97C177947AD4095},
+        {0x867F59A9D4BED6C0, 0x49ED8EABCCCC485D},
+        {0xA81F301449EE8C70, 0x5C68F256BFFF5A74},
+        {0xD226FC195C6A2F8C, 0x73832EEC6FFF3111},
+        {0x83585D8FD9C25DB7, 0xC831FD53C5FF7EAB},
+        {0xA42E74F3D032F525, 0xBA3E7CA8B77F5E55},
+        {0xCD3A1230C43FB26F, 0x28CE1BD2E55F35EB},
+        {0x80444B5E7AA7CF85, 0x7980D163CF5B81B3},
+        {0xA0555E361951C366, 0xD7E105BCC332621F},
+        {0xC86AB5C39FA63440, 0x8DD9472BF3FEFAA7},
+        {0xFA856334878FC150, 0xB14F98F6F0FEB951},
+        {0x9C935E00D4B9D8D2, 0x6ED1BF9A569F33D3},
+        {0xC3B8358109E84F07, 0x0A862F80EC4700C8},
+        {0xF4A642E14C6262C8, 0xCD27BB612758C0FA},
+        {0x98E7E9CCCFBD7DBD, 0x8038D51CB897789C},
+        {0xBF21E44003ACDD2C, 0xE0470A63E6BD56C3},
+        {0xEEEA5D5004981478, 0x1858CCFCE06CAC74},
+        {0x95527A5202DF0CCB, 0x0F37801E0C43EBC8},
+        {0xBAA718E68396CFFD, 0xD30560258F54E6BA},
+        {0xE950DF20247C83FD, 0x47C6B82EF32A2069},
+        {0x91D28B7416CDD27E, 0x4CDC331D57FA5441},
+        {0xB6472E511C81471D, 0xE0133FE4ADF8E952},
+        {0xE3D8F9E563A198E5, 0x58180FDDD97723A6},
+        {0x8E679C2F5E44FF8F, 0x570F09EAA7EA7648},
+        {0xB201833B35D63F73, 0x2CD2CC6551E513DA},
+        {0xDE81E40A034BCF4F, 0xF8077F7EA65E58D1},
+        {0x8B112E86420F6191, 0xFB04AFAF27FAF782},
+        {0xADD57A27D29339F6, 0x79C5DB9AF1F9B563},
+        {0xD94AD8B1C7380874, 0x18375281AE7822BC},
+        {0x87CEC76F1C830548, 0x8F2293910D0B15B5},
+        {0xA9C2794AE3A3C69A, 0xB2EB3875504DDB22},
+        {0xD433179D9C8CB841, 0x5FA60692A46151EB},
+        {0x849FEEC281D7F328, 0xDBC7C41BA6BCD333},
+        {0xA5C7EA73224DEFF3, 0x12B9B522906C0800},
+        {0xCF39E50FEAE16BEF, 0xD768226B34870A00},
+        {0x81842F29F2CCE375, 0xE6A1158300D46640},
+        {0xA1E53AF46F801C53, 0x60495AE3C1097FD0},
+        {0xCA5E89B18B602368, 0x385BB19CB14BDFC4},
+        {0xFCF62C1DEE382C42, 0x46729E03DD9ED7B5},
+        {0x9E19DB92B4E31BA9, 0x6C07A2C26A8346D1},
+        {0xC5A05277621BE293, 0xC7098B7305241885},
+        {0xF70867153AA2DB38, 0xB8CBEE4FC66D1EA7},
+    };
+
+    DRAGONBOX_ASSERT(k >= kMin);
+    DRAGONBOX_ASSERT(k <= kMax);
+    return Pow10[static_cast<uint32_t>(k - kMin)];
+}
+
+// Returns whether value is divisible by 2^e2
+static inline bool MultipleOfPow2(uint64_t value, int32_t e2)
+{
+    DRAGONBOX_ASSERT(e2 >= 0);
+    return e2 < 64 && (value & ((uint64_t{1} << e2) - 1)) == 0;
+}
+
+// Returns whether value is divisible by 5^e5
+static inline bool MultipleOfPow5(uint64_t value, int32_t e5)
+{
+    struct MulCmp {
+        uint64_t mul;
+        uint64_t cmp;
+    };
+
+    static constexpr MulCmp Mod5[] = {
+        {0x0000000000000001u, 0xFFFFFFFFFFFFFFFFu}, // 5^0
+        {0xCCCCCCCCCCCCCCCDu, 0x3333333333333333u}, // 5^1
+        {0x8F5C28F5C28F5C29u, 0x0A3D70A3D70A3D70u}, // 5^2
+        {0x1CAC083126E978D5u, 0x020C49BA5E353F7Cu}, // 5^3
+        {0xD288CE703AFB7E91u, 0x0068DB8BAC710CB2u}, // 5^4
+        {0x5D4E8FB00BCBE61Du, 0x0014F8B588E368F0u}, // 5^5
+        {0x790FB65668C26139u, 0x000431BDE82D7B63u}, // 5^6
+        {0xE5032477AE8D46A5u, 0x0000D6BF94D5E57Au}, // 5^7
+        {0xC767074B22E90E21u, 0x00002AF31DC46118u}, // 5^8
+        {0x8E47CE423A2E9C6Du, 0x0000089705F4136Bu}, // 5^9
+        {0x4FA7F60D3ED61F49u, 0x000001B7CDFD9D7Bu}, // 5^10
+        {0x0FEE64690C913975u, 0x00000057F5FF85E5u}, // 5^11
+        {0x3662E0E1CF503EB1u, 0x000000119799812Du}, // 5^12
+        {0xA47A2CF9F6433FBDu, 0x0000000384B84D09u}, // 5^13
+        {0x54186F653140A659u, 0x00000000B424DC35u}, // 5^14
+        {0x7738164770402145u, 0x0000000024075F3Du}, // 5^15
+        {0xE4A4D1417CD9A041u, 0x000000000734ACA5u}, // 5^16
+        {0xC75429D9E5C5200Du, 0x000000000170EF54u}, // 5^17
+        {0xC1773B91FAC10669u, 0x000000000049C977u}, // 5^18
+        {0x26B172506559CE15u, 0x00000000000EC1E4u}, // 5^19
+        {0xD489E3A9ADDEC2D1u, 0x000000000002F394u}, // 5^20
+        {0x90E860BB892C8D5Du, 0x000000000000971Du}, // 5^21char* buffer, double value
+        {0x502E79BF1B6F4F79u, 0x0000000000001E39u}, // 5^22
+        {0xDCD618596BE30FE5u, 0x000000000000060Bu}, // 5^23
+        {0x2C2AD1AB7BFA3661u, 0x0000000000000135u}, // 5^24
+    };
+
+    DRAGONBOX_ASSERT(e5 >= 0);
+    DRAGONBOX_ASSERT(e5 <= 24);
+    const MulCmp m5 = Mod5[static_cast<unsigned>(e5)];
+
+    return value * m5.mul <= m5.cmp;
+}
+
+namespace {
+struct FloatingDecimal64 {
+    uint64_t significand;
+    int32_t exponent;
+};
+}
+
+static inline FloatingDecimal64 ToDecimal64_asymmetric_interval(int32_t e2)
+{
+    // NB:
+    // accept_lower_endpoint = true
+    // accept_upper_endpoint = true
+
+    static constexpr int32_t P = Double::SignificandSize;
+
+    // Compute k and beta
+    const int32_t minus_k = FloorLog10ThreeQuartersPow2(e2);
+    const int32_t beta_minus_1 = e2 + FloorLog2Pow10(-minus_k);
+
+    // Compute xi and zi
+    const uint64x2 pow10 = ComputePow10(-minus_k);
+
+    const uint64_t lower_endpoint = (pow10.hi - (pow10.hi >> (P + 1))) >> (64 - P - beta_minus_1);
+    const uint64_t upper_endpoint = (pow10.hi + (pow10.hi >> (P + 0))) >> (64 - P - beta_minus_1);
+
+    // If we don't accept the left endpoint (but we do!) or
+	// if the left endpoint is not an integer, increase it
+    const bool lower_endpoint_is_integer = (2 <= e2 && e2 <= 3);
+
+    const uint64_t xi = lower_endpoint + !lower_endpoint_is_integer;
+    const uint64_t zi = upper_endpoint;
+
+    // Try bigger divisor
+    uint64_t q = zi / 10;
+    if (q * 10 >= xi)
+    {
+        return {q, minus_k + 1};
+    }
+
+    // Otherwise, compute the round-up of y
+    q = ((pow10.hi >> (64 - (P + 1) - beta_minus_1)) + 1) / 2;
+
+    // When tie occurs, choose one of them according to the rule
+    if (e2 == -77)
+    {
+        q -= (q % 2 != 0); // Round to even.
+    }
+    else
+    {
+        q += (q < xi);
+    }
+
+    return {q, minus_k};
+}
+
+static inline uint32_t ComputeDelta(uint64x2 pow10, int32_t beta_minus_1)
+{
+    DRAGONBOX_ASSERT(beta_minus_1 >= 0);
+    DRAGONBOX_ASSERT(beta_minus_1 <= 63);
+    return static_cast<uint32_t>(pow10.hi >> (64 - 1 - beta_minus_1));
+}
+
+#if defined(__SIZEOF_INT128__)
+static inline uint64x2 Mul128(uint64_t x, uint64_t y) // 1 mulx
+{
+    __extension__ using uint128_t = unsigned __int128;
+
+    const uint128_t p = uint128_t{x} * y;
+
+    const uint64_t hi = static_cast<uint64_t>(p >> 64);
+    const uint64_t lo = static_cast<uint64_t>(p);
+    return {hi, lo};
+}
+#elif defined(_MSC_VER) && defined(_M_X64)
+static inline uint64x2 Mul128(uint64_t x, uint64_t y)
+{
+    uint64_t hi = 0;
+    uint64_t lo = _umul128(x, y, &hi);
+    return {hi, lo};
+}
+#else
+static inline uint32_t Lo32(uint64_t x)
+{
+    return static_cast<uint32_t>(x);
+}
+
+static inline uint32_t Hi32(uint64_t x)
+{
+    return static_cast<uint32_t>(x >> 32);
+}
+
+static inline uint64x2 Mul128(uint64_t a, uint64_t b)
+{
+    const uint64_t b00 = uint64_t{Lo32(a)} * Lo32(b);
+    const uint64_t b01 = uint64_t{Lo32(a)} * Hi32(b);
+    const uint64_t b10 = uint64_t{Hi32(a)} * Lo32(b);
+    const uint64_t b11 = uint64_t{Hi32(a)} * Hi32(b);
+
+    const uint64_t mid1 = b10 + Hi32(b00);
+    const uint64_t mid2 = b01 + Lo32(mid1);
+
+    const uint64_t hi = b11 + Hi32(mid1) + Hi32(mid2);
+    const uint64_t lo = Lo32(b00) | uint64_t{Lo32(mid2)} << 32;
+    return {hi, lo};
+}
+#endif
+
+// Returns (x * y) / 2^128
+static inline uint64_t MulShift(uint64_t x, uint64x2 y) // 2 mulx
+{
+    uint64x2 p1 = Mul128(x, y.hi);
+    uint64x2 p0 = Mul128(x, y.lo);
+    p1.lo += p0.hi;
+    p1.hi += p1.lo < p0.hi;
+    return p1.hi;
+}
+
+static inline bool MulParity(uint64_t two_f, uint64x2 pow10, int32_t beta_minus_1) // 1 mulx, 1 mul
+{
+    DRAGONBOX_ASSERT(beta_minus_1 >= 1);
+    DRAGONBOX_ASSERT(beta_minus_1 <= 63);
+
+    const uint64_t p01 = two_f * pow10.hi;
+    const uint64_t p10 = Mul128(two_f, pow10.lo).hi;
+    const uint64_t mid = p01 + p10;
+
+    return (mid & (uint64_t{1} << (64 - beta_minus_1))) != 0;
+}
+
+static inline bool IsIntegralEndpoint(uint64_t two_f, int32_t e2, int32_t minus_k)
+{
+    if (e2 < -2)
+        return false;
+    if (e2 <= 9)
+        return true;
+    if (e2 <= 86)
+        return MultipleOfPow5(two_f, minus_k);
+
+    return false;
+}
+
+static inline bool IsIntegralMidpoint(uint64_t two_f, int32_t e2, int32_t minus_k)
+{
+    if (e2 < -4)
+        return MultipleOfPow2(two_f, minus_k - e2 + 1);
+    if (e2 <= 9)
+        return true;
+    if (e2 <= 86)
+        return MultipleOfPow5(two_f, minus_k);
+
+    return false;
+}
+
+static inline FloatingDecimal64 ToDecimal64(const uint64_t ieee_significand, const uint64_t ieee_exponent)
+{
+    static constexpr int32_t Kappa = 2;
+    static constexpr uint32_t BigDivisor   = 1000; // 10^(kappa + 1)
+    static constexpr uint32_t SmallDivisor = 100;  // 10^(kappa)
+
+    //
+    // Step 1:
+    // integer promotion & Schubfach multiplier calculation.
+    //
+
+    uint64_t m2;
+    int32_t  e2;
+    if (ieee_exponent != 0)
+    {
+        m2 = Double::HiddenBit | ieee_significand;
+        e2 = static_cast<int32_t>(ieee_exponent) - Double::ExponentBias;
+
+        if /*unlikely*/ (0 <= -e2 && -e2 < Double::SignificandSize && MultipleOfPow2(m2, -e2))
+        {
+            // Small integer.
+            return {m2 >> -e2, 0};
+        }
+
+        if /*unlikely*/ (ieee_significand == 0 && ieee_exponent > 1)
+        {
+            // Shorter interval case; proceed like Schubfach.
+            return ToDecimal64_asymmetric_interval(e2);
+        }
+    }
+    else
+    {
+        // Subnormal case; interval is always regular.
+        m2 = ieee_significand;
+        e2 = 1 - Double::ExponentBias;
+    }
+
+    const bool is_even = (m2 % 2 == 0);
+    const bool accept_lower = is_even;
+    const bool accept_upper = is_even;
+
+    // Compute k and beta.
+    const int32_t minus_k = FloorLog10Pow2(e2) - Kappa;
+    const int32_t beta_minus_1 = e2 + FloorLog2Pow10(-minus_k);
+    DRAGONBOX_ASSERT(beta_minus_1 >= 6);
+    DRAGONBOX_ASSERT(beta_minus_1 <= 9);
+
+    const uint64x2 pow10 = ComputePow10(-minus_k);
+
+    // Compute delta
+    // 10^kappa <= delta < 10^(kappa + 1)
+    //      100 <= delta < 1000
+    const uint32_t delta = ComputeDelta(pow10, beta_minus_1);
+    DRAGONBOX_ASSERT(delta >= SmallDivisor);
+    DRAGONBOX_ASSERT(delta <  BigDivisor  );
+
+    const uint64_t two_fl = 2 * m2 - 1;
+    const uint64_t two_fc = 2 * m2;
+    const uint64_t two_fr = 2 * m2 + 1; // (54 bits)
+
+    // Compute zi
+    //  (54 + 9 = 63 bits)
+    const uint64_t zi = MulShift(two_fr << beta_minus_1, pow10); // 2 mulx
+
+    //
+    // Step 2:
+    // Try larger divisor.
+    //
+
+    uint64_t q = zi / BigDivisor;
+//  uint64_t q = Mul128(zi, 0x83126E978D4FDF3Cu).hi >> 9; // 1 mulx
+    uint32_t r = static_cast<uint32_t>(zi) - BigDivisor * static_cast<uint32_t>(q); // r = zi % BigDivisor
+    // 0 <= r < 1000
+
+    if /*likely ~50% ?!*/ (r < delta)
+    {
+        // Exclude the right endpoint if necessary
+        if /*likely*/ (r != 0 || accept_upper || !IsIntegralEndpoint(two_fr, e2, minus_k))
+        {
+            return {q, minus_k + Kappa + 1};
+        }
+
+        DRAGONBOX_ASSERT(q != 0);
+        --q;
+        r = BigDivisor;
+    }
+    else if /*unlikely*/ (r == delta)
+    {
+        // Compare fractional parts.
+        // Check conditions in the order different from the paper
+        // to take advantage of short-circuiting
+        if ((accept_lower && IsIntegralEndpoint(two_fl, e2, minus_k)) || MulParity(two_fl, pow10, beta_minus_1)) // 1 mulx, 1 mul
+        {
+            return {q, minus_k + Kappa + 1};
+        }
+    }
+    else /*likely ~50% ?!*/ // (r > deltai)
+    {
+    }
+
+    //
+    // Step 3:
+    // Find the significand with the smaller divisor
+    //
+
+    q *= 10; // 1 hmul
+
+    // 0 <= r <= 1000
+
+    const uint32_t dist = r - (delta / 2) + (SmallDivisor / 2);
+
+    const uint32_t dist_q = dist / 100; // 1 mul
+//  const uint32_t dist_r = dist % 100;
+    q += dist_q;
+
+//  if /*likely*/ (dist_r == 0)
+    if /*likely*/ (dist == dist_q * 100) // 1 mul32
+    {
+//      const bool approx_y_parity = ((dist ^ (SmallDivisor / 2)) & 1) != 0;
+        const bool approx_y_parity = (dist & 1) != 0;
+
+        // Check z^(f) >= epsilon^(f)
+        // We have either yi == zi - epsiloni or yi == (zi - epsiloni) - 1,
+        // where yi == zi - epsiloni if and only if z^(f) >= epsilon^(f)
+        // Since there are only 2 possibilities, we only need to care about the
+        // parity. Also, zi and r should have the same parity since the divisor
+        // is an even number
+        if /*likely*/ (MulParity(two_fc, pow10, beta_minus_1) != approx_y_parity) // 1 mulx, 1 mul
+        {
+            --q;
+        }
+        // If z^(f) >= epsilon^(f), we might have a tie
+        // when z^(f) == epsilon^(f), or equivalently, when y is an integer
+        else if (q % 2 != 0 && IsIntegralMidpoint(two_fc, e2, minus_k))
+        {
+            --q;
+        }
+    }
+
+    return {q, minus_k + Kappa};
+}
+
+//==================================================================================================
+// ToChars
+//==================================================================================================
+
+static inline void Utoa_2Digits(char* buf, uint32_t digits)
+{
+    static constexpr char Digits100[200] = {
+        '0','0','0','1','0','2','0','3','0','4','0','5','0','6','0','7','0','8','0','9',
+        '1','0','1','1','1','2','1','3','1','4','1','5','1','6','1','7','1','8','1','9',
+        '2','0','2','1','2','2','2','3','2','4','2','5','2','6','2','7','2','8','2','9',
+        '3','0','3','1','3','2','3','3','3','4','3','5','3','6','3','7','3','8','3','9',
+        '4','0','4','1','4','2','4','3','4','4','4','5','4','6','4','7','4','8','4','9',
+        '5','0','5','1','5','2','5','3','5','4','5','5','5','6','5','7','5','8','5','9',
+        '6','0','6','1','6','2','6','3','6','4','6','5','6','6','6','7','6','8','6','9',
+        '7','0','7','1','7','2','7','3','7','4','7','5','7','6','7','7','7','8','7','9',
+        '8','0','8','1','8','2','8','3','8','4','8','5','8','6','8','7','8','8','8','9',
+        '9','0','9','1','9','2','9','3','9','4','9','5','9','6','9','7','9','8','9','9',
+    };
+
+    DRAGONBOX_ASSERT(digits <= 99);
+    std::memcpy(buf, &Digits100[2 * digits], 2 * sizeof(char));
+}
+
+static inline int32_t TrailingZeros_2Digits(uint32_t digits)
+{
+    static constexpr int8_t TrailingZeros100[100] = {
+        2, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    };
+
+    DRAGONBOX_ASSERT(digits <= 99);
+    return TrailingZeros100[digits];
+}
+
+static inline int32_t Utoa_8Digits_skip_trailing_zeros(char* buf, uint32_t digits)
+{
+    DRAGONBOX_ASSERT(digits >= 1);
+    DRAGONBOX_ASSERT(digits <= 99999999);
+
+    const uint32_t q = digits / 10000;
+    const uint32_t r = digits % 10000;
+
+    const uint32_t qH = q / 100;
+    const uint32_t qL = q % 100;
+    Utoa_2Digits(buf + 0, qH);
+    Utoa_2Digits(buf + 2, qL);
+
+    if (r == 0)
+    {
+        return TrailingZeros_2Digits(qL == 0 ? qH : qL) + (qL == 0 ? 6 : 4);
+    }
+    else
+    {
+        const uint32_t rH = r / 100;
+        const uint32_t rL = r % 100;
+        Utoa_2Digits(buf + 4, rH);
+        Utoa_2Digits(buf + 6, rL);
+
+        return TrailingZeros_2Digits(rL == 0 ? rH : rL) + (rL == 0 ? 2 : 0);
+    }
+}
+
+static inline int32_t PrintDecimalDigitsBackwards(char* buf, uint64_t output64)
+{
+    int32_t tz = 0; // number of trailing zeros removed.
+    int32_t nd = 0; // number of decimal digits processed.
+
+    // At most 17 digits remaining
+
+    if (output64 >= 100000000)
+    {
+        const uint64_t q = output64 / 100000000;
+        const uint32_t r = static_cast<uint32_t>(output64 % 100000000);
+        output64 = q;
+        buf -= 8;
+        if (r != 0)
+        {
+            tz = Utoa_8Digits_skip_trailing_zeros(buf, r);
+            DRAGONBOX_ASSERT(tz >= 0);
+            DRAGONBOX_ASSERT(tz <= 7);
+        }
+        else
+        {
+            tz = 8;
+        }
+        nd = 8;
+    }
+
+    // At most 9 digits remaining.
+    DRAGONBOX_ASSERT(output64 <= UINT32_MAX);
+    uint32_t output = static_cast<uint32_t>(output64);
+
+    if (output >= 10000)
+    {
+        const uint32_t q = output / 10000;
+        const uint32_t r = output % 10000;
+        output = q;
+        buf -= 4;
+        if (r != 0)
+        {
+            const uint32_t rH = r / 100;
+            const uint32_t rL = r % 100;
+            Utoa_2Digits(buf + 0, rH);
+            Utoa_2Digits(buf + 2, rL);
+            if (tz == nd)
+            {
+                tz += TrailingZeros_2Digits(rL == 0 ? rH : rL) + (rL == 0 ? 2 : 0);
+            }
+        }
+        else
+        {
+            if (tz == nd)
+                tz += 4;
+            else
+                std::memset(buf, '0', 4); // (actually not required...)
+        }
+        nd += 4;
+    }
+
+    // At most 5 digits remaining.
+
+    if (output >= 100)
+    {
+        const uint32_t q = output / 100;
+        const uint32_t r = output % 100;
+        output = q;
+        buf -= 2;
+        Utoa_2Digits(buf, r);
+        if (tz == nd)
+        {
+            tz += TrailingZeros_2Digits(r);
+        }
+        nd += 2;
+
+        if (output >= 100)
+        {
+            const uint32_t q = output / 100;
+            const uint32_t r = output % 100;
+            output = q;
+            buf -= 2;
+            Utoa_2Digits(buf, r);
+            if (tz == nd)
+            {
+                tz += TrailingZeros_2Digits(r);
+            }
+            nd += 2;
+        }
+    }
+
+    // At most 2 digits remaining.
+
+    DRAGONBOX_ASSERT(output >= 1);
+    DRAGONBOX_ASSERT(output <= 99);
+
+    if (output >= 10)
+    {
+        const uint32_t q = output;
+        buf -= 2;
+        Utoa_2Digits(buf, q);
+        if (tz == nd)
+        {
+            tz += TrailingZeros_2Digits(q);
+        }
+//      nd += 2;
+    }
+    else
+    {
+        const uint32_t q = output;
+        DRAGONBOX_ASSERT(q >= 1);
+        DRAGONBOX_ASSERT(q <= 9);
+        *--buf = static_cast<char>('0' + q);
+    }
+
+    return tz;
+}
+
+static inline int32_t DecimalLength(uint64_t v)
+{
+    DRAGONBOX_ASSERT(v >= 1);
+    DRAGONBOX_ASSERT(v <= 99999999999999999ull);
+
+    if (static_cast<uint32_t>(v >> 32) != 0)
+    {
+        if (v >= 10000000000000000ull) { return 17; }
+        if (v >= 1000000000000000ull) { return 16; }
+        if (v >= 100000000000000ull) { return 15; }
+        if (v >= 10000000000000ull) { return 14; }
+        if (v >= 1000000000000ull) { return 13; }
+        if (v >= 100000000000ull) { return 12; }
+        if (v >= 10000000000ull) { return 11; }
+        return 10;
+    }
+
+    const uint32_t v32 = static_cast<uint32_t>(v);
+    if (v32 >= 1000000000u) { return 10; }
+    if (v32 >= 100000000u) { return 9; }
+    if (v32 >= 10000000u) { return 8; }
+    if (v32 >= 1000000u) { return 7; }
+    if (v32 >= 100000u) { return 6; }
+    if (v32 >= 10000u) { return 5; }
+    if (v32 >= 1000u) { return 4; }
+    if (v32 >= 100u) { return 3; }
+    if (v32 >= 10u) { return 2; }
+    return 1;
+}
+
+static inline char* FormatDigits(char* buffer, uint64_t digits, int32_t decimal_exponent, bool force_trailing_dot_zero = false)
+{
+    static constexpr int32_t MinFixedDecimalPoint = -6;
+    static constexpr int32_t MaxFixedDecimalPoint =  17;
+    static_assert(MinFixedDecimalPoint <= -1, "internal error");
+    static_assert(MaxFixedDecimalPoint >= 17, "internal error");
+
+    DRAGONBOX_ASSERT(digits >= 1);
+    DRAGONBOX_ASSERT(digits <= 99999999999999999ull);
+    DRAGONBOX_ASSERT(decimal_exponent >= -999);
+    DRAGONBOX_ASSERT(decimal_exponent <=  999);
+
+    int32_t num_digits = DecimalLength(digits);
+    const int32_t decimal_point = num_digits + decimal_exponent;
+
+    const bool use_fixed = MinFixedDecimalPoint <= decimal_point && decimal_point <= MaxFixedDecimalPoint;
+
+    // Prepare the buffer.
+    // Avoid calling memset/memcpy with variable arguments below...
+
+    std::memset(buffer +  0, '0', 16);
+    std::memset(buffer + 16, '0', 16);
+    static_assert(MinFixedDecimalPoint >= -30, "internal error");
+    static_assert(MaxFixedDecimalPoint <=  32, "internal error");
+
+    int32_t decimal_digits_position;
+    if (use_fixed)
+    {
+        if (decimal_point <= 0)
+        {
+            // 0.[000]digits
+            decimal_digits_position = 2 - decimal_point;
+        }
+        else
+        {
+            // dig.its
+            // digits[000]
+            decimal_digits_position = 0;
+        }
+    }
+    else
+    {
+        // dE+123 or d.igitsE+123
+        decimal_digits_position = 1;
+    }
+
+    char* digits_end = buffer + decimal_digits_position + num_digits;
+
+    const int32_t tz = PrintDecimalDigitsBackwards(digits_end, digits);
+    digits_end -= tz;
+    num_digits -= tz;
+//  decimal_exponent += tz; // => decimal_point unchanged.
+
+    if (use_fixed)
+    {
+        if (decimal_point <= 0)
+        {
+            // 0.[000]digits
+            buffer[1] = '.';
+            buffer = digits_end;
+        }
+        else if (decimal_point < num_digits)
+        {
+            // dig.its
+#if defined(_MSC_VER) && !defined(__clang__)
+            // VC does not inline the memmove call below. (Even if compiled with /arch:AVX2.)
+            // However, memcpy will be inlined.
+            uint8_t tmp[16];
+            char* const src = buffer + decimal_point;
+            char* const dst = src + 1;
+            std::memcpy(tmp, src, 16);
+            std::memcpy(dst, tmp, 16);
+#else
+            std::memmove(buffer + decimal_point + 1, buffer + decimal_point, 16);
+#endif
+            buffer[decimal_point] = '.';
+            buffer = digits_end + 1;
+        }
+        else
+        {
+            // digits[000]
+            buffer += decimal_point;
+            if (force_trailing_dot_zero)
+            {
+                std::memcpy(buffer, ".0", 2);
+                buffer += 2;
+            }
+        }
+    }
+    else
+    {
+        // Copy the first digit one place to the left.
+        buffer[0] = buffer[1];
+        if (num_digits == 1)
+        {
+            // dE+123
+            ++buffer;
+        }
+        else
+        {
+            // d.igitsE+123
+            buffer[1] = '.';
+            buffer = digits_end;
+        }
+
+        const int32_t scientific_exponent = decimal_point - 1;
+//      SF_ASSERT(scientific_exponent != 0);
+
+        std::memcpy(buffer, scientific_exponent < 0 ? "e-" : "e+", 2);
+        buffer += 2;
+
+        const uint32_t k = static_cast<uint32_t>(scientific_exponent < 0 ? -scientific_exponent : scientific_exponent);
+        if (k < 10)
+        {
+            *buffer++ = static_cast<char>('0' + k);
+        }
+        else if (k < 100)
+        {
+            Utoa_2Digits(buffer, k);
+            buffer += 2;
+        }
+        else
+        {
+            const uint32_t q = k / 100;
+            const uint32_t r = k % 100;
+            *buffer++ = static_cast<char>('0' + q);
+            Utoa_2Digits(buffer, r);
+            buffer += 2;
+        }
+    }
+
+    return buffer;
+}
+
+static inline char* ToChars(char* buffer, double value, bool force_trailing_dot_zero = false)
+{
+    const Double v(value);
+
+    const uint64_t significand = v.PhysicalSignificand();
+    const uint64_t exponent = v.PhysicalExponent();
+
+//    if (exponent != Double::MaxIeeeExponent) // [[likely]]
+//    {
+//        // Finite
+
+        buffer[0] = '-';
+        buffer += v.SignBit();
+
+//        if (exponent != 0 || significand != 0) // [[likely]]
+//        {
+//            // != 0
+//
+            const auto dec = ToDecimal64(significand, exponent);
+            return FormatDigits(buffer, dec.significand, dec.exponent, force_trailing_dot_zero);
+//        }
+//        else
+//        {
+//            std::memcpy(buffer, "0.0 ", 4);
+//            buffer += force_trailing_dot_zero ? 3 : 1;
+//            return buffer;
+//        }
+//    }
+//
+//    if (significand == 0)
+//    {
+//        buffer[0] = '-';
+//        buffer += v.SignBit();
+//
+//        std::memcpy(buffer, "inf ", 4);
+//        return buffer + 3;
+//    }
+//    else
+//    {
+//        std::memcpy(buffer, "nan ", 4);
+//        return buffer + 3;
+//    }
+}
+
+//==================================================================================================
+//
+//==================================================================================================
+
+char* Dtoa(char* buffer, double value)
+{
+    return ToChars(buffer, value, true);
+}
+
+} // namespace
+} // dragonbox