bpo-39310: Add math.ulp(x) (GH-17965)
Add math.ulp(): return the value of the least significant bit
of a float.
diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index c9f2a38..c4c1800 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -226,6 +226,8 @@
* ``math.nextafter(x, 0.0)`` goes towards zero.
* ``math.nextafter(x, math.copysign(math.inf, x))`` goes away from zero.
+ See also :func:`math.ulp`.
+
.. versionadded:: 3.9
.. function:: perm(n, k=None)
@@ -284,6 +286,30 @@
:class:`~numbers.Integral` (usually an integer). Delegates to
:meth:`x.__trunc__() <object.__trunc__>`.
+.. function:: ulp(x)
+
+ Return the value of the least significant bit of the float *x*:
+
+ * If *x* is a NaN (not a number), return *x*.
+ * If *x* is negative, return ``ulp(-x)``.
+ * If *x* is a positive infinity, return *x*.
+ * If *x* is equal to zero, return the smallest positive
+ *denormalized* representable float (smaller than the minimum positive
+ *normalized* float, :data:`sys.float_info.min <sys.float_info>`).
+ * If *x* is equal to the largest positive representable float,
+ return the value of the least significant bit of *x*, such that the first
+ float smaller than *x* is ``x - ulp(x)``.
+ * Otherwise (*x* is a positive finite number), return the value of the least
+ significant bit of *x*, such that the first float bigger than *x*
+ is ``x + ulp(x)``.
+
+ ULP stands for "Unit in the Last Place".
+
+ See also :func:`math.nextafter` and :data:`sys.float_info.epsilon
+ <sys.float_info>`.
+
+ .. versionadded:: 3.9
+
Note that :func:`frexp` and :func:`modf` have a different call/return pattern
than their C equivalents: they take a single argument and return a pair of