fix some value calculators and add docs

epsilon_value_calculators.go

@@ -8,6 +8,11 @@ import (
 
 // --- Definitions -----------------------
 
+// Structs implementing this interface are used to convert the average response time for a host
+// into a score that can be used to weight hosts in the epsilon greedy hostpool. Lower response
+// times should yield higher scores (we want to select the faster hosts more often) The default
+// LinearEpsilonValueCalculator just uses the reciprocal of the response time. In practice, any
+// decreasing function from the positive reals to the positive reals should work.
 type EpsilonValueCalculator interface {
 	CalcValueFromAvgResponseTime(float64) float64
 }
@@ -26,9 +31,10 @@ func (c *LinearEpsilonValueCalculator) CalcValueFromAvgResponseTime(v float64) f
 }
 
 func (c *LogEpsilonValueCalculator) CalcValueFromAvgResponseTime(v float64) float64 {
-	return math.Log(c.LinearEpsilonValueCalculator.CalcValueFromAvgResponseTime(v))
+	// we need to add 1 to v so that this will be defined on all positive floats
+	return c.LinearEpsilonValueCalculator.CalcValueFromAvgResponseTime(math.Log(v + 1.0))
 }
 
 func (c *PolynomialEpsilonValueCalculator) CalcValueFromAvgResponseTime(v float64) float64 {
-	return math.Pow(c.LinearEpsilonValueCalculator.CalcValueFromAvgResponseTime(v), c.Exp)
+	return c.LinearEpsilonValueCalculator.CalcValueFromAvgResponseTime(math.Pow(v, c.Exp))
 }