Author: Ronan Lamy <ronan.l...@gmail.com>
Branch:
Changeset: r84541:97a570bd5122
Date: 2014-12-18 22:35 +0100
http://bitbucket.org/pypy/pypy/changeset/97a570bd5122/
Log: Add rfloat.log2(), which tries to be exact for powers of two.
(grafted from d3c7a156cc70124031fe3b71400f921cd29de0d6)
diff --git a/rpython/rlib/rfloat.py b/rpython/rlib/rfloat.py
--- a/rpython/rlib/rfloat.py
+++ b/rpython/rlib/rfloat.py
@@ -281,6 +281,35 @@
return (u - 1.) * x / math.log(u)
return math.exp(x) - 1.
+def log2(x):
+ # Uses an algorithm that should:
+ # (a) produce exact results for powers of 2, and
+ # (b) be monotonic, assuming that the system log is monotonic.
+ if not isfinite(x):
+ if isnan(x):
+ return x # log2(nan) = nan
+ elif x > 0.0:
+ return x # log2(+inf) = +inf
+ else:
+ # log2(-inf) = nan, invalid-operation
+ raise ValueError("math domain error")
+
+ if x > 0.0:
+ if 0: # HAVE_LOG2
+ return math.log2(x)
+ m, e = math.frexp(x)
+ # 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 math.log(2.0 * m) / math.log(2.0) + (e - 1)
+ else:
+ return math.log(m) / math.log(2.0) + e
+ else:
+ raise ValueError("math domain error")
+
def round_away(x):
# round() from libm, which is not available on all platforms!
absx = abs(x)
diff --git a/rpython/rlib/test/test_rfloat.py b/rpython/rlib/test/test_rfloat.py
--- a/rpython/rlib/test/test_rfloat.py
+++ b/rpython/rlib/test/test_rfloat.py
@@ -265,3 +265,12 @@
if s.strip(): # empty s raises OperationError directly
py.test.raises(ParseStringError, string_to_float, s)
py.test.raises(ParseStringError, string_to_float, "")
+
+def test_log2():
+ from rpython.rlib import rfloat
+ assert rfloat.log2(1.0) == 0.0
+ assert rfloat.log2(2.0) == 1.0
+ assert rfloat.log2(2.0**1023) == 1023.0
+ assert 1.584 < rfloat.log2(3.0) < 1.585
+ py.test.raises(ValueError, rfloat.log2, 0)
+ py.test.raises(ValueError, rfloat.log2, -1)
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