Issue #11888: Add log2 function to math module. Patch written by Mark
Dickinson.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index d173bff..8815dd5 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -577,6 +577,55 @@
}
}
+/*
+ log2: log to base 2.
+
+ Uses an algorithm that should:
+ (a) produce exact results for powers of 2, and
+ (b) be monotonic, assuming that the system log is monotonic.
+*/
+
+static double
+m_log2(double x)
+{
+ if (!Py_IS_FINITE(x)) {
+ if (Py_IS_NAN(x))
+ return x; /* log2(nan) = nan */
+ else if (x > 0.0)
+ return x; /* log2(+inf) = +inf */
+ else {
+ errno = EDOM;
+ return Py_NAN; /* log2(-inf) = nan, invalid-operation */
+ }
+ }
+
+ if (x > 0.0) {
+ double m;
+ int e;
+ m = frexp(x, &e);
+ /* We want log2(m * 2**e) == log(m) / log(2) + e. Care is needed when
+ * x is just greater than 1.0: in that case e is 1, log(m) is negative,
+ * and we get significant cancellation error from the addition of
+ * log(m) / log(2) to e. The slight rewrite of the expression below
+ * avoids this problem.
+ */
+ if (x >= 1.0) {
+ return log(2.0 * m) / log(2.0) + (e - 1);
+ }
+ else {
+ return log(m) / log(2.0) + e;
+ }
+ }
+ else if (x == 0.0) {
+ errno = EDOM;
+ return -Py_HUGE_VAL; /* log2(0) = -inf, divide-by-zero */
+ }
+ else {
+ errno = EDOM;
+ return Py_NAN; /* log10(-inf) = nan, invalid-operation */
+ }
+}
+
static double
m_log10(double x)
{
@@ -1623,6 +1672,15 @@
If the base not specified, returns the natural logarithm (base e) of x.");
static PyObject *
+math_log2(PyObject *self, PyObject *arg)
+{
+ return loghelper(arg, m_log2, "log2");
+}
+
+PyDoc_STRVAR(math_log2_doc,
+"log2(x)\n\nReturn the base 2 logarithm of x.");
+
+static PyObject *
math_log10(PyObject *self, PyObject *arg)
{
return loghelper(arg, m_log10, "log10");
@@ -1894,6 +1952,7 @@
{"log", math_log, METH_VARARGS, math_log_doc},
{"log1p", math_log1p, METH_O, math_log1p_doc},
{"log10", math_log10, METH_O, math_log10_doc},
+ {"log2", math_log2, METH_O, math_log2_doc},
{"modf", math_modf, METH_O, math_modf_doc},
{"pow", math_pow, METH_VARARGS, math_pow_doc},
{"radians", math_radians, METH_O, math_radians_doc},