Issue #7117 (backport py3k float repr) continued:

Backport pystrtod.c from py3k.

Python/pystrtod.c

@@ -3,6 +3,60 @@
 #include <Python.h>
 #include <locale.h>
 
+/* Case-insensitive string match used for nan and inf detection; t should be
+   lower-case.  Returns 1 for a successful match, 0 otherwise. */
+
+static int
+case_insensitive_match(const char *s, const char *t)
+{
+	while(*t && Py_TOLOWER(*s) == *t) {
+		s++;
+		t++;
+	}
+	return *t ? 0 : 1;
+}
+
+/* _Py_parse_inf_or_nan: Attempt to parse a string of the form "nan", "inf" or
+   "infinity", with an optional leading sign of "+" or "-".  On success,
+   return the NaN or Infinity as a double and set *endptr to point just beyond
+   the successfully parsed portion of the string.  On failure, return -1.0 and
+   set *endptr to point to the start of the string. */
+
+double
+_Py_parse_inf_or_nan(const char *p, char **endptr)
+{
+	double retval;
+	const char *s;
+	int negate = 0;
+
+	s = p;
+	if (*s == '-') {
+		negate = 1;
+		s++;
+	}
+	else if (*s == '+') {
+		s++;
+	}
+	if (case_insensitive_match(s, "inf")) {
+		s += 3;
+		if (case_insensitive_match(s, "inity"))
+			s += 5;
+		retval = negate ? -Py_HUGE_VAL : Py_HUGE_VAL;
+	}
+#ifdef Py_NAN
+	else if (case_insensitive_match(s, "nan")) {
+		s += 3;
+		retval = negate ? -Py_NAN : Py_NAN;
+	}
+#endif
+	else {
+		s = p;
+		retval = -1.0;
+	}
+	*endptr = (char *)s;
+	return retval;
+}
+
 /**
  * PyOS_ascii_strtod:
  * @nptr:    the string to convert to a numeric value.
@@ -32,6 +86,33 @@
  * Return value: the #gdouble value.
  **/
 
+#ifndef PY_NO_SHORT_FLOAT_REPR
+
+double
+_PyOS_ascii_strtod(const char *nptr, char **endptr)
+{
+	double result;
+	_Py_SET_53BIT_PRECISION_HEADER;
+
+	assert(nptr != NULL);
+	/* Set errno to zero, so that we can distinguish zero results
+	   and underflows */
+	errno = 0;
+
+	_Py_SET_53BIT_PRECISION_START;
+	result = _Py_dg_strtod(nptr, endptr);
+	_Py_SET_53BIT_PRECISION_END;
+
+	if (*endptr == nptr)
+		/* string might represent and inf or nan */
+		result = _Py_parse_inf_or_nan(nptr, endptr);
+
+	return result;
+
+}
+
+#else
+
 /*
    Use system strtod;  since strtod is locale aware, we may
    have to first fix the decimal separator.
@@ -40,21 +121,8 @@
    correctly rounded results.
 */
 
-/* Case-insensitive string match used for nan and inf detection; t should be
-   lower-case.  Returns 1 for a successful match, 0 otherwise. */
-
-static int
-case_insensitive_match(const char *s, const char *t)
-{
-	while(*t && Py_TOLOWER(*s) == *t) {
-		s++;
-		t++;
-	}
-	return *t ? 0 : 1;
-}
-
 double
-PyOS_ascii_strtod(const char *nptr, char **endptr)
+_PyOS_ascii_strtod(const char *nptr, char **endptr)
 {
 	char *fail_pos;
 	double val = -1.0;
@@ -78,19 +146,19 @@ PyOS_ascii_strtod(const char *nptr, char **endptr)
 
 	decimal_point_pos = NULL;
 
+	/* Parse infinities and nans */
+	val = _Py_parse_inf_or_nan(nptr, endptr);
+	if (*endptr != nptr)
+		return val;
+
 	/* Set errno to zero, so that we can distinguish zero results
 	   and underflows */
 	errno = 0;
 
-	/* We process any leading whitespace and the optional sign manually,
+	/* We process the optional sign manually, then pass the remainder to
-	   then pass the remainder to the system strtod.  This ensures that
+	   the system strtod.  This ensures that the result of an underflow
-	   the result of an underflow has the correct sign. (bug #1725)  */
+	   has the correct sign. (bug #1725)  */
-
 	p = nptr;
-	/* Skip leading space */
-	while (Py_ISSPACE(*p))
-		p++;
-
 	/* Process leading sign, if present */
 	if (*p == '-') {
 		negate = 1;
@@ -100,31 +168,6 @@ PyOS_ascii_strtod(const char *nptr, char **endptr)
 		p++;
 	}
 
-	/* Parse infinities and nans */
-	if (*p == 'i' || *p == 'I') {
-		if (case_insensitive_match(p+1, "nf")) {
-			val = Py_HUGE_VAL;
-			if (case_insensitive_match(p+3, "inity"))
-				fail_pos = (char *)p+8;
-			else
-				fail_pos = (char *)p+3;
-			goto got_val;
-		}
-		else
-			goto invalid_string;
-	}
-#ifdef Py_NAN
-	if (*p == 'n' || *p == 'N') {
-		if (case_insensitive_match(p+1, "an")) {
-			val = Py_NAN;
-			fail_pos = (char *)p+3;
-			goto got_val;
-		}
-		else
-			goto invalid_string;
-	}
-#endif
-
 	/* Some platform strtods accept hex floats; Python shouldn't (at the
 	   moment), so we check explicitly for strings starting with '0x'. */
 	if (*p == '0' && (*(p+1) == 'x' || *(p+1) == 'X'))
@@ -175,7 +218,6 @@ PyOS_ascii_strtod(const char *nptr, char **endptr)
 		copy = (char *)PyMem_MALLOC(end - digits_pos +
 					    1 + decimal_point_len);
 		if (copy == NULL) {
-			if (endptr)
 			*endptr = (char *)nptr;
 			errno = ENOMEM;
 			return val;
@@ -214,28 +256,108 @@ PyOS_ascii_strtod(const char *nptr, char **endptr)
 	if (fail_pos == digits_pos)
 		goto invalid_string;
 
-  got_val:
 	if (negate && fail_pos != nptr)
 		val = -val;
-
-	if (endptr)
 	*endptr = fail_pos;
 
 	return val;
 
   invalid_string:
-	if (endptr)
 	*endptr = (char*)nptr;
 	errno = EINVAL;
 	return -1.0;
 }
 
+#endif
+
+double
+PyOS_ascii_strtod(const char *nptr, char **endptr)
+{
+	char *fail_pos;
+	const char *p;
+	double x;
+
+	/* _PyOS_ascii_strtod already does everything that we want,
+	   except that it doesn't parse leading whitespace */
+	p = nptr;
+	while (Py_ISSPACE(*p))
+		p++;
+	x = _PyOS_ascii_strtod(p, &fail_pos);
+	if (fail_pos == p)
+		fail_pos = (char *)nptr;
+	if (endptr)
+		*endptr = (char *)fail_pos;
+	return x;
+}
+
 double
 PyOS_ascii_atof(const char *nptr)
 {
 	return PyOS_ascii_strtod(nptr, NULL);
 }
 
+/* PyOS_string_to_double is the recommended replacement for the
+   PyOS_ascii_strtod and PyOS_ascii_atof functions.  It converts a
+   null-terminated byte string s (interpreted as a string of ASCII characters)
+   to a float.  The string should not have leading or trailing whitespace (in
+   contrast, PyOS_ascii_strtod allows leading whitespace but not trailing
+   whitespace).  The conversion is independent of the current locale.
+
+   If endptr is NULL, try to convert the whole string.  Raise ValueError and
+   return -1.0 if the string is not a valid representation of a floating-point
+   number.
+
+   If endptr is non-NULL, try to convert as much of the string as possible.
+   If no initial segment of the string is the valid representation of a
+   floating-point number then *endptr is set to point to the beginning of the
+   string, -1.0 is returned and again ValueError is raised.
+
+   On overflow (e.g., when trying to convert '1e500' on an IEEE 754 machine),
+   if overflow_exception is NULL then +-Py_HUGE_VAL is returned, and no Python
+   exception is raised.  Otherwise, overflow_exception should point to a
+   a Python exception, this exception will be raised, -1.0 will be returned,
+   and *endptr will point just past the end of the converted value.
+
+   If any other failure occurs (for example lack of memory), -1.0 is returned
+   and the appropriate Python exception will have been set.
+*/
+
+double
+PyOS_string_to_double(const char *s,
+		      char **endptr,
+		      PyObject *overflow_exception)
+{
+	double x, result=-1.0;
+	char *fail_pos;
+
+	errno = 0;
+	PyFPE_START_PROTECT("PyOS_string_to_double", return -1.0)
+	x = PyOS_ascii_strtod(s, &fail_pos);
+	PyFPE_END_PROTECT(x)
+
+	if (errno == ENOMEM) {
+		PyErr_NoMemory();
+		fail_pos = (char *)s;
+	}
+	else if (!endptr && (fail_pos == s || *fail_pos != '\0'))
+		PyErr_Format(PyExc_ValueError,
+			      "could not convert string to float: "
+			      "%.200s", s);
+	else if (fail_pos == s)
+		PyErr_Format(PyExc_ValueError,
+			      "could not convert string to float: "
+			      "%.200s", s);
+	else if (errno == ERANGE && fabs(x) >= 1.0 && overflow_exception)
+		PyErr_Format(overflow_exception,
+			      "value too large to convert to float: "
+			      "%.200s", s);
+	else
+		result = x;
+
+	if (endptr != NULL)
+		*endptr = fail_pos;
+	return result;
+}
 
 /* Given a string that may have a decimal point in the current
    locale, change it back to a dot.  Since the string cannot get
@@ -505,7 +627,7 @@ ensure_decimal_point(char* buffer, size_t buf_size, int precision)
 #define FLOAT_FORMATBUFLEN 120
 
 /**
- * _PyOS_ascii_formatd:
+ * PyOS_ascii_formatd:
  * @buffer: A buffer to place the resulting string in
  * @buf_size: The length of the buffer.
  * @format: The printf()-style format to use for the
@@ -523,12 +645,12 @@ ensure_decimal_point(char* buffer, size_t buf_size, int precision)
  * Return value: The pointer to the buffer with the converted string.
  * On failure returns NULL but does not set any Python exception.
  **/
-/* DEPRECATED, will be deleted in 2.8 and 3.2 */
+char *
-PyAPI_FUNC(char *)
+_PyOS_ascii_formatd(char       *buffer, 
-PyOS_ascii_formatd(char       *buffer, 
 		   size_t      buf_size, 
 		   const char *format, 
-		   double      d)
+		   double      d, 
+		   int         precision)
 {
 	char format_char;
 	size_t format_len = strlen(format);
@@ -537,11 +659,6 @@ PyOS_ascii_formatd(char       *buffer,
 	   also with at least one character past the decimal. */
 	char tmp_format[FLOAT_FORMATBUFLEN];
 
-	if (PyErr_WarnEx(PyExc_DeprecationWarning,
-			 "PyOS_ascii_formatd is deprecated, "
-			 "use PyOS_double_to_string instead", 1) < 0)
-		return NULL;
-
 	/* The last character in the format string must be the format char */
 	format_char = format[format_len - 1];
 
@@ -603,24 +720,24 @@ PyOS_ascii_formatd(char       *buffer,
 	   extra character would produce more significant digits that we
 	   really want. */
 	if (format_char == 'Z')
-		buffer = ensure_decimal_point(buffer, buf_size, -1);
+		buffer = ensure_decimal_point(buffer, buf_size, precision);
 
 	return buffer;
 }
 
-/* Precisions used by repr() and str(), respectively.
+char *
+PyOS_ascii_formatd(char       *buffer, 
+		   size_t      buf_size, 
+		   const char *format, 
+		   double      d)
+{
+	if (PyErr_WarnEx(PyExc_DeprecationWarning,
+			 "PyOS_ascii_formatd is deprecated, "
+			 "use PyOS_double_to_string instead", 1) < 0)
+		return NULL;
 
-   The repr() precision (17 significant decimal digits) is the minimal number
+	return _PyOS_ascii_formatd(buffer, buf_size, format, d, -1);
-   that is guaranteed to have enough precision so that if the number is read
+}
-   back in the exact same binary value is recreated.  This is true for IEEE
-   floating point by design, and also happens to work for all other modern
-   hardware.
-
-   The str() precision (12 significant decimal digits) is chosen so that in
-   most cases, the rounding noise created by various operations is suppressed,
-   while giving plenty of precision for practical use.
-
-*/
 
 PyAPI_FUNC(void)
 _PyOS_double_to_string(char *buf, size_t buf_len, double val,
@@ -738,31 +855,520 @@ _PyOS_double_to_string(char *buf, size_t buf_len, double val,
 }
 
 
+#ifdef PY_NO_SHORT_FLOAT_REPR
+
+/* The fallback code to use if _Py_dg_dtoa is not available. */
+
 PyAPI_FUNC(char *) PyOS_double_to_string(double val,
                                          char format_code,
                                          int precision,
                                          int flags,
-                                         int *ptype)
+                                         int *type)
 {
-	char buf[128];
+	char format[32];
-	Py_ssize_t len;
+	Py_ssize_t bufsize;
-	char *result;
+	char *buf;
+	int t, exp;
+	int upper = 0;
 
-	_PyOS_double_to_string(buf, sizeof(buf), val, format_code, precision,
+	/* Validate format_code, and map upper and lower case */
-			       flags, ptype);
+	switch (format_code) {
-	len = strlen(buf);
+	case 'e':          /* exponent */
-	if (len == 0) {
+	case 'f':          /* fixed */
+	case 'g':          /* general */
+		break;
+	case 'E':
+		upper = 1;
+		format_code = 'e';
+		break;
+	case 'F':
+		upper = 1;
+		format_code = 'f';
+		break;
+	case 'G':
+		upper = 1;
+		format_code = 'g';
+		break;
+	case 'r':          /* repr format */
+		/* Supplied precision is unused, must be 0. */
+		if (precision != 0) {
+			PyErr_BadInternalCall();
+			return NULL;
+		}
+		/* The repr() precision (17 significant decimal digits) is the
+		   minimal number that is guaranteed to have enough precision
+		   so that if the number is read back in the exact same binary
+		   value is recreated.  This is true for IEEE floating point
+		   by design, and also happens to work for all other modern
+		   hardware. */
+		precision = 17;
+		format_code = 'g';
+		break;
+	default:
 		PyErr_BadInternalCall();
 		return NULL;
 	}
 
-	/* Add 1 for the trailing 0 byte. */
+	/* Here's a quick-and-dirty calculation to figure out how big a buffer
-	result = PyMem_Malloc(len + 1);
+	   we need.  In general, for a finite float we need:
-	if (result == NULL) {
+
+	     1 byte for each digit of the decimal significand, and
+
+	     1 for a possible sign
+	     1 for a possible decimal point
+	     2 for a possible [eE][+-]
+	     1 for each digit of the exponent;  if we allow 19 digits
+	       total then we're safe up to exponents of 2**63.
+	     1 for the trailing nul byte
+
+	   This gives a total of 24 + the number of digits in the significand,
+	   and the number of digits in the significand is:
+
+	     for 'g' format: at most precision, except possibly
+	       when precision == 0, when it's 1.
+	     for 'e' format: precision+1
+	     for 'f' format: precision digits after the point, at least 1
+	       before.  To figure out how many digits appear before the point
+	       we have to examine the size of the number.  If fabs(val) < 1.0
+	       then there will be only one digit before the point.  If
+	       fabs(val) >= 1.0, then there are at most
+
+	         1+floor(log10(ceiling(fabs(val))))
+
+	       digits before the point (where the 'ceiling' allows for the
+	       possibility that the rounding rounds the integer part of val
+	       up).  A safe upper bound for the above quantity is
+	       1+floor(exp/3), where exp is the unique integer such that 0.5
+	       <= fabs(val)/2**exp < 1.0.  This exp can be obtained from
+	       frexp.
+
+	   So we allow room for precision+1 digits for all formats, plus an
+	   extra floor(exp/3) digits for 'f' format.
+
+	*/
+
+	if (Py_IS_NAN(val) || Py_IS_INFINITY(val))
+		/* 3 for 'inf'/'nan', 1 for sign, 1 for '\0' */
+		bufsize = 5;
+	else {
+		bufsize = 25 + precision;
+		if (format_code == 'f' && fabs(val) >= 1.0) {
+			frexp(val, &exp);
+			bufsize += exp/3;
+		}
+	}
+
+	buf = PyMem_Malloc(bufsize);
+	if (buf == NULL) {
 		PyErr_NoMemory();
 		return NULL;
 	}
-	strcpy(result, buf);
 
-	return result;
+	/* Handle nan and inf. */
+	if (Py_IS_NAN(val)) {
+		strcpy(buf, "nan");
+		t = Py_DTST_NAN;
+	} else if (Py_IS_INFINITY(val)) {
+		if (copysign(1., val) == 1.)
+			strcpy(buf, "inf");
+		else
+			strcpy(buf, "-inf");
+		t = Py_DTST_INFINITE;
+	} else {
+		t = Py_DTST_FINITE;
+		if (flags & Py_DTSF_ADD_DOT_0)
+			format_code = 'Z';
+
+		PyOS_snprintf(format, sizeof(format), "%%%s.%i%c",
+			      (flags & Py_DTSF_ALT ? "#" : ""), precision,
+			      format_code);
+		_PyOS_ascii_formatd(buf, bufsize, format, val, precision);
 	}
+
+	/* Add sign when requested.  It's convenient (esp. when formatting
+	 complex numbers) to include a sign even for inf and nan. */
+	if (flags & Py_DTSF_SIGN && buf[0] != '-') {
+		size_t len = strlen(buf);
+		/* the bufsize calculations above should ensure that we've got
+		   space to add a sign */
+		assert((size_t)bufsize >= len+2);
+		memmove(buf+1, buf, len+1);
+		buf[0] = '+';
+	}
+	if (upper) {
+		/* Convert to upper case. */
+		char *p1;
+		for (p1 = buf; *p1; p1++)
+			*p1 = Py_TOUPPER(*p1);
+	}
+
+	if (type)
+		*type = t;
+	return buf;
+}
+
+#else
+
+/* _Py_dg_dtoa is available. */
+
+/* I'm using a lookup table here so that I don't have to invent a non-locale
+   specific way to convert to uppercase */
+#define OFS_INF 0
+#define OFS_NAN 1
+#define OFS_E 2
+
+/* The lengths of these are known to the code below, so don't change them */
+static char *lc_float_strings[] = {
+	"inf",
+	"nan",
+	"e",
+};
+static char *uc_float_strings[] = {
+	"INF",
+	"NAN",
+	"E",
+};
+
+
+/* Convert a double d to a string, and return a PyMem_Malloc'd block of
+   memory contain the resulting string.
+
+   Arguments:
+     d is the double to be converted
+     format_code is one of 'e', 'f', 'g', 'r'.  'e', 'f' and 'g'
+       correspond to '%e', '%f' and '%g';  'r' corresponds to repr.
+     mode is one of '0', '2' or '3', and is completely determined by
+       format_code: 'e' and 'g' use mode 2; 'f' mode 3, 'r' mode 0.
+     precision is the desired precision
+     always_add_sign is nonzero if a '+' sign should be included for positive
+       numbers
+     add_dot_0_if_integer is nonzero if integers in non-exponential form
+       should have ".0" added.  Only applies to format codes 'r' and 'g'.
+     use_alt_formatting is nonzero if alternative formatting should be
+       used.  Only applies to format codes 'e', 'f' and 'g'.  For code 'g',
+       at most one of use_alt_formatting and add_dot_0_if_integer should
+       be nonzero.
+     type, if non-NULL, will be set to one of these constants to identify
+       the type of the 'd' argument:
+         Py_DTST_FINITE
+         Py_DTST_INFINITE
+         Py_DTST_NAN
+
+   Returns a PyMem_Malloc'd block of memory containing the resulting string,
+    or NULL on error. If NULL is returned, the Python error has been set.
+ */
+
+static char *
+format_float_short(double d, char format_code,
+		   int mode, Py_ssize_t precision,
+		   int always_add_sign, int add_dot_0_if_integer,
+		   int use_alt_formatting, char **float_strings, int *type)
+{
+	char *buf = NULL;
+	char *p = NULL;
+	Py_ssize_t bufsize = 0;
+	char *digits, *digits_end;
+	int decpt_as_int, sign, exp_len, exp = 0, use_exp = 0;
+	Py_ssize_t decpt, digits_len, vdigits_start, vdigits_end;
+	_Py_SET_53BIT_PRECISION_HEADER;
+
+	/* _Py_dg_dtoa returns a digit string (no decimal point or exponent).
+	   Must be matched by a call to _Py_dg_freedtoa. */
+	_Py_SET_53BIT_PRECISION_START;
+	digits = _Py_dg_dtoa(d, mode, precision, &decpt_as_int, &sign,
+			     &digits_end);
+	_Py_SET_53BIT_PRECISION_END;
+
+	decpt = (Py_ssize_t)decpt_as_int;
+	if (digits == NULL) {
+		/* The only failure mode is no memory. */
+		PyErr_NoMemory();
+		goto exit;
+	}
+	assert(digits_end != NULL && digits_end >= digits);
+	digits_len = digits_end - digits;
+
+	if (digits_len && !Py_ISDIGIT(digits[0])) {
+		/* Infinities and nans here; adapt Gay's output,
+		   so convert Infinity to inf and NaN to nan, and
+		   ignore sign of nan. Then return. */
+
+		/* ignore the actual sign of a nan */
+		if (digits[0] == 'n' || digits[0] == 'N')
+			sign = 0;
+
+		/* We only need 5 bytes to hold the result "+inf\0" . */
+		bufsize = 5; /* Used later in an assert. */
+		buf = (char *)PyMem_Malloc(bufsize);
+		if (buf == NULL) {
+			PyErr_NoMemory();
+			goto exit;
+		}
+		p = buf;
+
+		if (sign == 1) {
+			*p++ = '-';
+		}
+		else if (always_add_sign) {
+			*p++ = '+';
+		}
+		if (digits[0] == 'i' || digits[0] == 'I') {
+			strncpy(p, float_strings[OFS_INF], 3);
+			p += 3;
+
+			if (type)
+				*type = Py_DTST_INFINITE;
+		}
+		else if (digits[0] == 'n' || digits[0] == 'N') {
+			strncpy(p, float_strings[OFS_NAN], 3);
+			p += 3;
+
+			if (type)
+				*type = Py_DTST_NAN;
+		}
+		else {
+			/* shouldn't get here: Gay's code should always return
+			   something starting with a digit, an 'I',  or 'N' */
+			strncpy(p, "ERR", 3);
+			p += 3;
+			assert(0);
+		}
+		goto exit;
+	}
+
+	/* The result must be finite (not inf or nan). */
+	if (type)
+		*type = Py_DTST_FINITE;
+
+
+	/* We got digits back, format them.  We may need to pad 'digits'
+	   either on the left or right (or both) with extra zeros, so in
+	   general the resulting string has the form
+
+	     [<sign>]<zeros><digits><zeros>[<exponent>]
+
+	   where either of the <zeros> pieces could be empty, and there's a
+	   decimal point that could appear either in <digits> or in the
+	   leading or trailing <zeros>.
+
+	   Imagine an infinite 'virtual' string vdigits, consisting of the
+	   string 'digits' (starting at index 0) padded on both the left and
+	   right with infinite strings of zeros.  We want to output a slice
+
+	     vdigits[vdigits_start : vdigits_end]
+
+	   of this virtual string.  Thus if vdigits_start < 0 then we'll end
+	   up producing some leading zeros; if vdigits_end > digits_len there
+	   will be trailing zeros in the output.  The next section of code
+	   determines whether to use an exponent or not, figures out the
+	   position 'decpt' of the decimal point, and computes 'vdigits_start'
+	   and 'vdigits_end'. */
+	vdigits_end = digits_len;
+	switch (format_code) {
+	case 'e':
+		use_exp = 1;
+		vdigits_end = precision;
+		break;
+	case 'f':
+		vdigits_end = decpt + precision;
+		break;
+	case 'g':
+		if (decpt <= -4 || decpt >
+		    (add_dot_0_if_integer ? precision-1 : precision))
+			use_exp = 1;
+		if (use_alt_formatting)
+			vdigits_end = precision;
+		break;
+	case 'r':
+		/* convert to exponential format at 1e16.  We used to convert
+		   at 1e17, but that gives odd-looking results for some values
+		   when a 16-digit 'shortest' repr is padded with bogus zeros.
+		   For example, repr(2e16+8) would give 20000000000000010.0;
+		   the true value is 20000000000000008.0. */
+		if (decpt <= -4 || decpt > 16)
+			use_exp = 1;
+		break;
+	default:
+		PyErr_BadInternalCall();
+		goto exit;
+	}
+
+	/* if using an exponent, reset decimal point position to 1 and adjust
+	   exponent accordingly.*/
+	if (use_exp) {
+		exp = decpt - 1;
+		decpt = 1;
+	}
+	/* ensure vdigits_start < decpt <= vdigits_end, or vdigits_start <
+	   decpt < vdigits_end if add_dot_0_if_integer and no exponent */
+	vdigits_start = decpt <= 0 ? decpt-1 : 0;
+	if (!use_exp && add_dot_0_if_integer)
+		vdigits_end = vdigits_end > decpt ? vdigits_end : decpt + 1;
+	else
+		vdigits_end = vdigits_end > decpt ? vdigits_end : decpt;
+
+	/* double check inequalities */
+	assert(vdigits_start <= 0 &&
+	       0 <= digits_len &&
+	       digits_len <= vdigits_end);
+	/* decimal point should be in (vdigits_start, vdigits_end] */
+	assert(vdigits_start < decpt && decpt <= vdigits_end);
+
+	/* Compute an upper bound how much memory we need. This might be a few
+	   chars too long, but no big deal. */
+	bufsize =
+		/* sign, decimal point and trailing 0 byte */
+		3 +
+
+		/* total digit count (including zero padding on both sides) */
+		(vdigits_end - vdigits_start) +
+
+		/* exponent "e+100", max 3 numerical digits */
+		(use_exp ? 5 : 0);
+
+	/* Now allocate the memory and initialize p to point to the start of
+	   it. */
+	buf = (char *)PyMem_Malloc(bufsize);
+	if (buf == NULL) {
+		PyErr_NoMemory();
+		goto exit;
+	}
+	p = buf;
+
+	/* Add a negative sign if negative, and a plus sign if non-negative
+	   and always_add_sign is true. */
+	if (sign == 1)
+		*p++ = '-';
+	else if (always_add_sign)
+		*p++ = '+';
+
+	/* note that exactly one of the three 'if' conditions is true,
+	   so we include exactly one decimal point */
+	/* Zero padding on left of digit string */
+	if (decpt <= 0) {
+		memset(p, '0', decpt-vdigits_start);
+		p += decpt - vdigits_start;
+		*p++ = '.';
+		memset(p, '0', 0-decpt);
+		p += 0-decpt;
+	}
+	else {
+		memset(p, '0', 0-vdigits_start);
+		p += 0 - vdigits_start;
+	}
+
+	/* Digits, with included decimal point */
+	if (0 < decpt && decpt <= digits_len) {
+		strncpy(p, digits, decpt-0);
+		p += decpt-0;
+		*p++ = '.';
+		strncpy(p, digits+decpt, digits_len-decpt);
+		p += digits_len-decpt;
+	}
+	else {
+		strncpy(p, digits, digits_len);
+		p += digits_len;
+	}
+
+	/* And zeros on the right */
+	if (digits_len < decpt) {
+		memset(p, '0', decpt-digits_len);
+		p += decpt-digits_len;
+		*p++ = '.';
+		memset(p, '0', vdigits_end-decpt);
+		p += vdigits_end-decpt;
+	}
+	else {
+		memset(p, '0', vdigits_end-digits_len);
+		p += vdigits_end-digits_len;
+	}
+
+	/* Delete a trailing decimal pt unless using alternative formatting. */
+	if (p[-1] == '.' && !use_alt_formatting)
+		p--;
+
+	/* Now that we've done zero padding, add an exponent if needed. */
+	if (use_exp) {
+		*p++ = float_strings[OFS_E][0];
+		exp_len = sprintf(p, "%+.02d", exp);
+		p += exp_len;
+	}
+  exit:
+	if (buf) {
+		*p = '\0';
+		/* It's too late if this fails, as we've already stepped on
+		   memory that isn't ours. But it's an okay debugging test. */
+		assert(p-buf < bufsize);
+	}
+	if (digits)
+		_Py_dg_freedtoa(digits);
+
+	return buf;
+}
+
+
+PyAPI_FUNC(char *) PyOS_double_to_string(double val,
+					 char format_code,
+					 int precision,
+					 int flags,
+					 int *type)
+{
+	char **float_strings = lc_float_strings;
+	int mode;
+
+	/* Validate format_code, and map upper and lower case. Compute the
+	   mode and make any adjustments as needed. */
+	switch (format_code) {
+	/* exponent */
+	case 'E':
+		float_strings = uc_float_strings;
+		format_code = 'e';
+		/* Fall through. */
+	case 'e':
+		mode = 2;
+		precision++;
+		break;
+
+	/* fixed */
+	case 'F':
+		float_strings = uc_float_strings;
+		format_code = 'f';
+		/* Fall through. */
+	case 'f':
+		mode = 3;
+		break;
+
+	/* general */
+	case 'G':
+		float_strings = uc_float_strings;
+		format_code = 'g';
+		/* Fall through. */
+	case 'g':
+		mode = 2;
+		/* precision 0 makes no sense for 'g' format; interpret as 1 */
+		if (precision == 0)
+			precision = 1;
+		break;
+
+	/* repr format */
+	case 'r':
+		mode = 0;
+		/* Supplied precision is unused, must be 0. */
+		if (precision != 0) {
+			PyErr_BadInternalCall();
+			return NULL;
+		}
+		break;
+
+	default:
+		PyErr_BadInternalCall();
+		return NULL;
+	}
+
+	return format_float_short(val, format_code, mode, precision,
+				  flags & Py_DTSF_SIGN,
+				  flags & Py_DTSF_ADD_DOT_0,
+				  flags & Py_DTSF_ALT,
+				  float_strings, type);
+}
+#endif /* ifdef PY_NO_SHORT_FLOAT_REPR */