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........
  r75117 | mark.dickinson | 2009-09-28 19:54:55 +0100 (Mon, 28 Sep 2009) | 3 lines

  Issue #3366:  Add gamma function to math module.
  (lgamma, erf and erfc to follow).
........
This commit is contained in:
Mark Dickinson 2009-09-28 19:21:11 +00:00
parent 40af630672
commit 12c4bdb0e8
5 changed files with 573 additions and 36 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
Doc/library/math.rst
View File

@ -278,6 +278,16 @@ Hyperbolic functions
Return the hyperbolic tangent of *x*.
Special functions
-----------------
.. function:: gamma(x)
Return the Gamma function at *x*.
.. versionadded:: 2.7
Constants
---------

146
Lib/test/math_testcases.txt Normal file
View File

@ -0,0 +1,146 @@
-- Testcases for functions in math.
--
-- Each line takes the form:
--
-- <testid> <function> <input_value> -> <output_value> <flags>
--
-- where:
--
-- <testid> is a short name identifying the test,
--
-- <function> is the function to be tested (exp, cos, asinh, ...),
--
-- <input_value> is a string representing a floating-point value
--
-- <output_value> is the expected (ideal) output value, again
-- represented as a string.
--
-- <flags> is a list of the floating-point flags required by C99
--
-- The possible flags are:
--
-- divide-by-zero : raised when a finite input gives a
-- mathematically infinite result.
--
-- overflow : raised when a finite input gives a finite result that
-- is too large to fit in the usual range of an IEEE 754 double.
--
-- invalid : raised for invalid inputs (e.g., sqrt(-1))
--
-- ignore-sign : indicates that the sign of the result is
-- unspecified; e.g., if the result is given as inf,
-- then both -inf and inf should be accepted as correct.
--
-- Flags may appear in any order.
--
-- Lines beginning with '--' (like this one) start a comment, and are
-- ignored. Blank lines, or lines containing only whitespace, are also
-- ignored.
-- Many of the values below were computed with the help of
-- version 2.4 of the MPFR library for multiple-precision
-- floating-point computations with correct rounding. All output
-- values in this file are (modulo yet-to-be-discovered bugs)
-- correctly rounded, provided that each input and output decimal
-- floating-point value below is interpreted as a representation of
-- the corresponding nearest IEEE 754 double-precision value. See the
-- MPFR homepage at http://www.mpfr.org for more information about the
-- MPFR project.
---------------------------
-- gamma: Gamma function --
---------------------------
-- special values
gam0000 gamma 0.0 -> inf divide-by-zero
gam0001 gamma -0.0 -> -inf divide-by-zero
gam0002 gamma inf -> inf
gam0003 gamma -inf -> nan invalid
gam0004 gamma nan -> nan
-- negative integers inputs are invalid
gam0010 gamma -1 -> nan invalid
gam0011 gamma -2 -> nan invalid
gam0012 gamma -1e16 -> nan invalid
gam0013 gamma -1e300 -> nan invalid
-- small positive integers give factorials
gam0020 gamma 1 -> 1
gam0021 gamma 2 -> 1
gam0022 gamma 3 -> 2
gam0023 gamma 4 -> 6
gam0024 gamma 5 -> 24
gam0025 gamma 6 -> 120
-- half integers
gam0030 gamma 0.5 -> 1.7724538509055161
gam0031 gamma 1.5 -> 0.88622692545275805
gam0032 gamma 2.5 -> 1.3293403881791370
gam0033 gamma 3.5 -> 3.3233509704478426
gam0034 gamma -0.5 -> -3.5449077018110322
gam0035 gamma -1.5 -> 2.3632718012073548
gam0036 gamma -2.5 -> -0.94530872048294190
gam0037 gamma -3.5 -> 0.27008820585226911
-- values near 0
gam0040 gamma 0.1 -> 9.5135076986687306
gam0041 gamma 0.01 -> 99.432585119150602
gam0042 gamma 1e-8 -> 99999999.422784343
gam0043 gamma 1e-16 -> 10000000000000000
gam0044 gamma 1e-30 -> 9.9999999999999988e+29
gam0045 gamma 1e-160 -> 1.0000000000000000e+160
gam0046 gamma 1e-308 -> 1.0000000000000000e+308
gam0047 gamma 5.6e-309 -> 1.7857142857142848e+308
gam0048 gamma 5.5e-309 -> inf overflow
gam0049 gamma 1e-309 -> inf overflow
gam0050 gamma 1e-323 -> inf overflow
gam0051 gamma 5e-324 -> inf overflow
gam0060 gamma -0.1 -> -10.686287021193193
gam0061 gamma -0.01 -> -100.58719796441078
gam0062 gamma -1e-8 -> -100000000.57721567
gam0063 gamma -1e-16 -> -10000000000000000
gam0064 gamma -1e-30 -> -9.9999999999999988e+29
gam0065 gamma -1e-160 -> -1.0000000000000000e+160
gam0066 gamma -1e-308 -> -1.0000000000000000e+308
gam0067 gamma -5.6e-309 -> -1.7857142857142848e+308
gam0068 gamma -5.5e-309 -> -inf overflow
gam0069 gamma -1e-309 -> -inf overflow
gam0070 gamma -1e-323 -> -inf overflow
gam0071 gamma -5e-324 -> -inf overflow
-- values near negative integers
gam0080 gamma -0.99999999999999989 -> -9007199254740992.0
gam0081 gamma -1.0000000000000002 -> 4503599627370495.5
gam0082 gamma -1.9999999999999998 -> 2251799813685248.5
gam0083 gamma -2.0000000000000004 -> -1125899906842623.5
gam0084 gamma -100.00000000000001 -> -7.5400833348831090e-145
gam0085 gamma -99.999999999999986 -> 7.5400833348840962e-145
-- large inputs
gam0100 gamma 170 -> 4.2690680090047051e+304
gam0101 gamma 171 -> 7.2574156153079990e+306
gam0102 gamma 171.624 -> 1.7942117599248104e+308
gam0103 gamma 171.625 -> inf overflow
gam0104 gamma 172 -> inf overflow
gam0105 gamma 2000 -> inf overflow
gam0106 gamma 1.7e308 -> inf overflow
-- inputs for which gamma(x) is tiny
gam0120 gamma -100.5 -> -3.3536908198076787e-159
gam0121 gamma -160.5 -> -5.2555464470078293e-286
gam0122 gamma -170.5 -> -3.3127395215386074e-308
gam0123 gamma -171.5 -> 1.9316265431711902e-310
gam0124 gamma -176.5 -> -1.1956388629358166e-321
gam0125 gamma -177.5 -> 4.9406564584124654e-324
gam0126 gamma -178.5 -> -0.0
gam0127 gamma -179.5 -> 0.0
gam0128 gamma -201.0001 -> 0.0
gam0129 gamma -202.9999 -> -0.0
gam0130 gamma -1000.5 -> -0.0
gam0131 gamma -1000000000.3 -> -0.0
gam0132 gamma -4503599627370495.5 -> 0.0
-- inputs that cause problems for the standard reflection formula,
-- thanks to loss of accuracy in 1-x
gam0140 gamma -63.349078729022985 -> 4.1777971677761880e-88
gam0141 gamma -127.45117632943295 -> 1.1831110896236810e-214

88
Lib/test/test_math.py
View File

@ -7,6 +7,7 @@ import math
import os
import sys
import random
import struct
eps = 1E-05
NAN = float('nan')
@ -29,8 +30,50 @@ if __name__ == '__main__':
else:
file = __file__
test_dir = os.path.dirname(file) or os.curdir
math_testcases = os.path.join(test_dir, 'math_testcases.txt')
test_file = os.path.join(test_dir, 'cmath_testcases.txt')
def to_ulps(x):
"""Convert a non-NaN float x to an integer, in such a way that
adjacent floats are converted to adjacent integers. Then
abs(ulps(x) - ulps(y)) gives the difference in ulps between two
floats.
The results from this function will only make sense on platforms
where C doubles are represented in IEEE 754 binary64 format.
"""
n = struct.unpack('q', struct.pack('<d', x))[0]
if n < 0:
n = ~(n+2**63)
return n
def parse_mtestfile(fname):
"""Parse a file with test values
-- starts a comment
blank lines, or lines containing only a comment, are ignored
other lines are expected to have the form
id fn arg -> expected [flag]*
"""
with open(fname) as fp:
for line in fp:
# strip comments, and skip blank lines
if '--' in line:
line = line[:line.index('--')]
if not line.strip():
continue
lhs, rhs = line.split('->')
id, fn, arg = lhs.split()
rhs_pieces = rhs.split()
exp = rhs_pieces[0]
flags = rhs_pieces[1:]
yield (id, fn, float(arg), float(exp), flags)
def parse_testfile(fname):
"""Parse a file with test values
@ -884,6 +927,51 @@ class MathTests(unittest.TestCase):
self.fail(message)
self.ftest("%s:%s(%r)" % (id, fn, ar), result, er)
@unittest.skipUnless(float.__getformat__("double").startswith("IEEE"),
"test requires IEEE 754 doubles")
def test_mtestfile(self):
ALLOWED_ERROR = 20 # permitted error, in ulps
fail_fmt = "{}:{}({!r}): expected {!r}, got {!r}"
failures = []
for id, fn, arg, expected, flags in parse_mtestfile(math_testcases):
func = getattr(math, fn)
if 'invalid' in flags or 'divide-by-zero' in flags:
expected = 'ValueError'
elif 'overflow' in flags:
expected = 'OverflowError'
try:
got = func(arg)
except ValueError:
got = 'ValueError'
except OverflowError:
got = 'OverflowError'
diff_ulps = None
if isinstance(got, float) and isinstance(expected, float):
if math.isnan(expected) and math.isnan(got):
continue
if not math.isnan(expected) and not math.isnan(got):
diff_ulps = to_ulps(expected) - to_ulps(got)
if diff_ulps <= ALLOWED_ERROR:
continue
if isinstance(got, str) and isinstance(expected, str):
if got == expected:
continue
fail_msg = fail_fmt.format(id, fn, arg, expected, got)
if diff_ulps is not None:
fail_msg += ' ({} ulps)'.format(diff_ulps)
failures.append(fail_msg)
if failures:
self.fail('Failures in test_mtestfile:\n ' +
'\n '.join(failures))
def test_main():
from doctest import DocFileSuite
suite = unittest.TestSuite()

3
Misc/NEWS
View File

@ -201,6 +201,9 @@ Library
Extension Modules
-----------------
- Issue #3366: Add gamma function to math module.
- Issue #6877: It is now possible to link the readline extension to the
libedit readline emulation on OSX 10.5 or later.

362
Modules/mathmodule.c
View File

@ -60,44 +60,265 @@ raised for division by zero and mod by zero.
extern double copysign(double, double);
#endif
/* Call is_error when errno != 0, and where x is the result libm
* returned. is_error will usually set up an exception and return
* true (1), but may return false (0) without setting up an exception.
*/
static int
is_error(double x)
{
int result = 1; /* presumption of guilt */
assert(errno); /* non-zero errno is a precondition for calling */
if (errno == EDOM)
PyErr_SetString(PyExc_ValueError, "math domain error");
/*
sin(pi*x), giving accurate results for all finite x (especially x
integral or close to an integer). This is here for use in the
reflection formula for the gamma function. It conforms to IEEE
754-2008 for finite arguments, but not for infinities or nans.
*/
else if (errno == ERANGE) {
/* ANSI C generally requires libm functions to set ERANGE
* on overflow, but also generally *allows* them to set
* ERANGE on underflow too. There's no consistency about
* the latter across platforms.
* Alas, C99 never requires that errno be set.
* Here we suppress the underflow errors (libm functions
* should return a zero on underflow, and +- HUGE_VAL on
* overflow, so testing the result for zero suffices to
* distinguish the cases).
*
* On some platforms (Ubuntu/ia64) it seems that errno can be
* set to ERANGE for subnormal results that do *not* underflow
* to zero. So to be safe, we'll ignore ERANGE whenever the
* function result is less than one in absolute value.
*/
if (fabs(x) < 1.0)
result = 0;
else
PyErr_SetString(PyExc_OverflowError,
"math range error");
static const double pi = 3.141592653589793238462643383279502884197;
static double
sinpi(double x)
{
double y, r;
int n;
/* this function should only ever be called for finite arguments */
assert(Py_IS_FINITE(x));
y = fmod(fabs(x), 2.0);
n = (int)round(2.0*y);
assert(0 <= n && n <= 4);
switch (n) {
case 0:
r = sin(pi*y);
break;
case 1:
r = cos(pi*(y-0.5));
break;
case 2:
/* N.B. -sin(pi*(y-1.0)) is *not* equivalent: it would give
-0.0 instead of 0.0 when y == 1.0. */
r = sin(pi*(1.0-y));
break;
case 3:
r = -cos(pi*(y-1.5));
break;
case 4:
r = sin(pi*(y-2.0));
break;
default:
assert(0); /* should never get here */
r = -1.23e200; /* silence gcc warning */
}
else
/* Unexpected math error */
PyErr_SetFromErrno(PyExc_ValueError);
return result;
return copysign(1.0, x)*r;
}
/* Implementation of the real gamma function. In extensive but non-exhaustive
random tests, this function proved accurate to within <= 10 ulps across the
entire float domain. Note that accuracy may depend on the quality of the
system math functions, the pow function in particular. Special cases
follow C99 annex F. The parameters and method are tailored to platforms
whose double format is the IEEE 754 binary64 format.
Method: for x > 0.0 we use the Lanczos approximation with parameters N=13
and g=6.024680040776729583740234375; these parameters are amongst those
used by the Boost library. Following Boost (again), we re-express the
Lanczos sum as a rational function, and compute it that way. The
coefficients below were computed independently using MPFR, and have been
double-checked against the coefficients in the Boost source code.
For x < 0.0 we use the reflection formula.
There's one minor tweak that deserves explanation: Lanczos' formula for
Gamma(x) involves computing pow(x+g-0.5, x-0.5) / exp(x+g-0.5). For many x
values, x+g-0.5 can be represented exactly. However, in cases where it
can't be represented exactly the small error in x+g-0.5 can be magnified
significantly by the pow and exp calls, especially for large x. A cheap
correction is to multiply by (1 + e*g/(x+g-0.5)), where e is the error
involved in the computation of x+g-0.5 (that is, e = computed value of
x+g-0.5 - exact value of x+g-0.5). Here's the proof:
Correction factor
-----------------
Write x+g-0.5 = y-e, where y is exactly representable as an IEEE 754
double, and e is tiny. Then:
pow(x+g-0.5,x-0.5)/exp(x+g-0.5) = pow(y-e, x-0.5)/exp(y-e)
= pow(y, x-0.5)/exp(y) * C,
where the correction_factor C is given by
C = pow(1-e/y, x-0.5) * exp(e)
Since e is tiny, pow(1-e/y, x-0.5) ~ 1-(x-0.5)*e/y, and exp(x) ~ 1+e, so:
C ~ (1-(x-0.5)*e/y) * (1+e) ~ 1 + e*(y-(x-0.5))/y
But y-(x-0.5) = g+e, and g+e ~ g. So we get C ~ 1 + e*g/y, and
pow(x+g-0.5,x-0.5)/exp(x+g-0.5) ~ pow(y, x-0.5)/exp(y) * (1 + e*g/y),
Note that for accuracy, when computing r*C it's better to do
r + e*g/y*r;
than
r * (1 + e*g/y);
since the addition in the latter throws away most of the bits of
information in e*g/y.
*/
#define LANCZOS_N 13
static const double lanczos_g = 6.024680040776729583740234375;
static const double lanczos_g_minus_half = 5.524680040776729583740234375;
static const double lanczos_num_coeffs[LANCZOS_N] = {
23531376880.410759688572007674451636754734846804940,
42919803642.649098768957899047001988850926355848959,
35711959237.355668049440185451547166705960488635843,
17921034426.037209699919755754458931112671403265390,
6039542586.3520280050642916443072979210699388420708,
1439720407.3117216736632230727949123939715485786772,
248874557.86205415651146038641322942321632125127801,
31426415.585400194380614231628318205362874684987640,
2876370.6289353724412254090516208496135991145378768,
186056.26539522349504029498971604569928220784236328,
8071.6720023658162106380029022722506138218516325024,
210.82427775157934587250973392071336271166969580291,
2.5066282746310002701649081771338373386264310793408
};
/* denominator is x*(x+1)*...*(x+LANCZOS_N-2) */
static const double lanczos_den_coeffs[LANCZOS_N] = {
0.0, 39916800.0, 120543840.0, 150917976.0, 105258076.0, 45995730.0,
13339535.0, 2637558.0, 357423.0, 32670.0, 1925.0, 66.0, 1.0};
/* gamma values for small positive integers, 1 though NGAMMA_INTEGRAL */
#define NGAMMA_INTEGRAL 23
static const double gamma_integral[NGAMMA_INTEGRAL] = {
1.0, 1.0, 2.0, 6.0, 24.0, 120.0, 720.0, 5040.0, 40320.0, 362880.0,
3628800.0, 39916800.0, 479001600.0, 6227020800.0, 87178291200.0,
1307674368000.0, 20922789888000.0, 355687428096000.0,
6402373705728000.0, 121645100408832000.0, 2432902008176640000.0,
51090942171709440000.0, 1124000727777607680000.0,
};
/* Lanczos' sum L_g(x), for positive x */
static double
lanczos_sum(double x)
{
double num = 0.0, den = 0.0;
int i;
assert(x > 0.0);
/* evaluate the rational function lanczos_sum(x). For large
x, the obvious algorithm risks overflow, so we instead
rescale the denominator and numerator of the rational
function by x**(1-LANCZOS_N) and treat this as a
rational function in 1/x. This also reduces the error for
larger x values. The choice of cutoff point (5.0 below) is
somewhat arbitrary; in tests, smaller cutoff values than
this resulted in lower accuracy. */
if (x < 5.0) {
for (i = LANCZOS_N; --i >= 0; ) {
num = num * x + lanczos_num_coeffs[i];
den = den * x + lanczos_den_coeffs[i];
}
}
else {
for (i = 0; i < LANCZOS_N; i++) {
num = num / x + lanczos_num_coeffs[i];
den = den / x + lanczos_den_coeffs[i];
}
}
return num/den;
}
static double
m_tgamma(double x)
{
double absx, r, y, z, sqrtpow;
/* special cases */
if (!Py_IS_FINITE(x)) {
if (Py_IS_NAN(x) || x > 0.0)
return x; /* tgamma(nan) = nan, tgamma(inf) = inf */
else {
errno = EDOM;
return Py_NAN; /* tgamma(-inf) = nan, invalid */
}
}
if (x == 0.0) {
errno = EDOM;
return 1.0/x; /* tgamma(+-0.0) = +-inf, divide-by-zero */
}
/* integer arguments */
if (x == floor(x)) {
if (x < 0.0) {
errno = EDOM; /* tgamma(n) = nan, invalid for */
return Py_NAN; /* negative integers n */
}
if (x <= NGAMMA_INTEGRAL)
return gamma_integral[(int)x - 1];
}
absx = fabs(x);
/* tiny arguments: tgamma(x) ~ 1/x for x near 0 */
if (absx < 1e-20) {
r = 1.0/x;
if (Py_IS_INFINITY(r))
errno = ERANGE;
return r;
}
/* large arguments: assuming IEEE 754 doubles, tgamma(x) overflows for
x > 200, and underflows to +-0.0 for x < -200, not a negative
integer. */
if (absx > 200.0) {
if (x < 0.0) {
return 0.0/sinpi(x);
}
else {
errno = ERANGE;
return Py_HUGE_VAL;
}
}
y = absx + lanczos_g_minus_half;
/* compute error in sum */
if (absx > lanczos_g_minus_half) {
/* note: the correction can be foiled by an optimizing
compiler that (incorrectly) thinks that an expression like
a + b - a - b can be optimized to 0.0. This shouldn't
happen in a standards-conforming compiler. */
double q = y - absx;
z = q - lanczos_g_minus_half;
}
else {
double q = y - lanczos_g_minus_half;
z = q - absx;
}
z = z * lanczos_g / y;
if (x < 0.0) {
r = -pi / sinpi(absx) / absx * exp(y) / lanczos_sum(absx);
r -= z * r;
if (absx < 140.0) {
r /= pow(y, absx - 0.5);
}
else {
sqrtpow = pow(y, absx / 2.0 - 0.25);
r /= sqrtpow;
r /= sqrtpow;
}
}
else {
r = lanczos_sum(absx) / exp(y);
r += z * r;
if (absx < 140.0) {
r *= pow(y, absx - 0.5);
}
else {
sqrtpow = pow(y, absx / 2.0 - 0.25);
r *= sqrtpow;
r *= sqrtpow;
}
}
if (Py_IS_INFINITY(r))
errno = ERANGE;
return r;
}
/*
@ -188,6 +409,46 @@ m_log10(double x)
}
/* Call is_error when errno != 0, and where x is the result libm
* returned. is_error will usually set up an exception and return
* true (1), but may return false (0) without setting up an exception.
*/
static int
is_error(double x)
{
int result = 1; /* presumption of guilt */
assert(errno); /* non-zero errno is a precondition for calling */
if (errno == EDOM)
PyErr_SetString(PyExc_ValueError, "math domain error");
else if (errno == ERANGE) {
/* ANSI C generally requires libm functions to set ERANGE
* on overflow, but also generally *allows* them to set
* ERANGE on underflow too. There's no consistency about
* the latter across platforms.
* Alas, C99 never requires that errno be set.
* Here we suppress the underflow errors (libm functions
* should return a zero on underflow, and +- HUGE_VAL on
* overflow, so testing the result for zero suffices to
* distinguish the cases).
*
* On some platforms (Ubuntu/ia64) it seems that errno can be
* set to ERANGE for subnormal results that do *not* underflow
* to zero. So to be safe, we'll ignore ERANGE whenever the
* function result is less than one in absolute value.
*/
if (fabs(x) < 1.0)
result = 0;
else
PyErr_SetString(PyExc_OverflowError,
"math range error");
}
else
/* Unexpected math error */
PyErr_SetFromErrno(PyExc_ValueError);
return result;
}
/*
math_1 is used to wrap a libm function f that takes a double
arguments and returns a double.
@ -252,6 +513,26 @@ math_1_to_whatever(PyObject *arg, double (*func) (double),
return (*from_double_func)(r);
}
/* variant of math_1, to be used when the function being wrapped is known to
set errno properly (that is, errno = EDOM for invalid or divide-by-zero,
errno = ERANGE for overflow). */
static PyObject *
math_1a(PyObject *arg, double (*func) (double))
{
double x, r;
x = PyFloat_AsDouble(arg);
if (x == -1.0 && PyErr_Occurred())
return NULL;
errno = 0;
PyFPE_START_PROTECT("in math_1a", return 0);
r = (*func)(x);
PyFPE_END_PROTECT(r);
if (errno && is_error(r))
return NULL;
return PyFloat_FromDouble(r);
}
/*
math_2 is used to wrap a libm function f that takes two double
arguments and returns a double.
@ -330,6 +611,12 @@ math_2(PyObject *args, double (*func) (double, double), char *funcname)
}\
PyDoc_STRVAR(math_##funcname##_doc, docstring);
#define FUNC1A(funcname, func, docstring) \
static PyObject * math_##funcname(PyObject *self, PyObject *args) { \
return math_1a(args, func); \
}\
PyDoc_STRVAR(math_##funcname##_doc, docstring);
#define FUNC2(funcname, func, docstring) \
static PyObject * math_##funcname(PyObject *self, PyObject *args) { \
return math_2(args, func, #funcname); \
@ -405,6 +692,8 @@ PyDoc_STRVAR(math_floor_doc,
"floor(x)\n\nReturn the floor of x as an int.\n"
"This is the largest integral value <= x.");
FUNC1A(gamma, m_tgamma,
"gamma(x)\n\nGamma function at x.")
FUNC1(log1p, log1p, 1,
"log1p(x)\n\nReturn the natural logarithm of 1+x (base e).\n\
The result is computed in a way which is accurate for x near zero.")
@ -1150,6 +1439,7 @@ static PyMethodDef math_methods[] = {
{"fmod", math_fmod, METH_VARARGS, math_fmod_doc},
{"frexp", math_frexp, METH_O, math_frexp_doc},
{"fsum", math_fsum, METH_O, math_fsum_doc},
{"gamma", math_gamma, METH_O, math_gamma_doc},
{"hypot", math_hypot, METH_VARARGS, math_hypot_doc},
{"isinf", math_isinf, METH_O, math_isinf_doc},
{"isnan", math_isnan, METH_O, math_isnan_doc},