added erf to util.math to support python 2.6

diff --git a/p2pool/util/math.py b/p2pool/util/math.py
index 8d4843b..efc4741 100644 (file)
--- a/p2pool/util/math.py
+++ b/p2pool/util/math.py
@@ -72,11 +72,31 @@ def perfect_round(x):
     else:
         return a2 + 1
 
+def erf(x):
+    # save the sign of x
+    sign = 1
+    if x < 0: 
+        sign = -1
+    x = abs(x)
+
+    # constants
+    a1 =  0.254829592
+    a2 = -0.284496736
+    a3 =  1.421413741
+    a4 = -1.453152027
+    a5 =  1.061405429
+    p  =  0.3275911
+
+    # A&S formula 7.1.26
+    t = 1.0/(1.0 + p*x)
+    y = 1.0 - (((((a5*t + a4)*t) + a3)*t + a2)*t + a1)*t*math.exp(-x*x)
+    return sign*y # erf(-x) = -erf(x)
+
 def ierf(z, steps=10):
     guess = 0
     for i in xrange(steps):
         d = 2*math.e**(-guess**2)/math.sqrt(math.pi)
-        guess = guess - (math.erf(guess) - z)/d
+        guess = guess - (erf(guess) - z)/d
     return guess
 
 def binomial_conf_interval(x, n, conf=0.95):