add comments, change constant name, add ProbWin()

go/glicko2.go

@@ -16,7 +16,8 @@ import (
 	"math"
 )
 
-const dvolatility float64 = 0.3
+// system constant, determines delta volatility over time; should be [0.3,1.2]
+const kDVOL float64 = 0.3
 
 // Glicko2 state reprensentation (Glicko2 uses different scales from Glicko)
 type Glicko2 struct {
@@ -47,6 +48,11 @@ func newGlicko2(rating float64, deviation float64, volatility float64) *Glicko2
 	return g
 }
 
+// get the probability of beating opponent
+func (g *Glicko2) ProbWin(opponent *Glicko2) float64 {
+	return 1.0 / (1.0 + math.Pow(10.0, ((opponent.Rating()-g.Rating())/(400.0*math.Sqrt(1.0+0.0000100723986*((g.Deviation()*g.Deviation())+(opponent.Deviation()*opponent.Deviation())))))))
+}
+
 // Get the current rating.  This rating is a Glicko rating, not a Glicko2 rating.
 // For details, please see http://www.glicko.com/
 func (g *Glicko2) Rating() float64 {
@@ -127,12 +133,12 @@ func (g *Glicko2) Print() {
 	}
 }
 
-// util func
+// util func 'g'
 func calcG(deviation float64) float64 {
 	return 1.0 / (math.Sqrt(1.0 + 3.0*(deviation*deviation)/(math.Pi*math.Pi)))
 }
 
-// util func
+// util func 'E'
 func calcE(rating float64, rating_opponent float64, deviation_opponent float64) float64 {
 	return 1.0 / (1.0 + math.Exp(-calcG(deviation_opponent)*(rating-rating_opponent)))
 }
@@ -143,13 +149,26 @@ func (g *Glicko2) Update() {
 	// merge wins, losses, draws slices for convenience
 	results := append(append(g.wins, g.losses...), g.draws...)
 
-	// bail if no opponents set
+	// Note that if a player does not compete during the rating period, then
+	// only Step 6 applies.
 	if len(results) == 0 {
 		g.deviation = math.Sqrt((g.deviation * g.deviation) + (g.volatility * g.volatility))
 		return
 	}
 
-	// compute variance
+	// Step 1. Determine a rating and RD for each player at the onset of the
+	// rating period. The system constant which constrains the change in
+	// volatility over time, needs to be set prior to application of the system.
+	// Reasonable choices are between 0.3 and 1.2, though the system should be
+	// tested to decide which value results in greatest predictive accuracy
+	// ... (ratings already stored in instance)
+
+	// Step 2. For each player, convert the ratings and RD's onto the Glicko-2
+	// scale.
+	// ... (ratings already stored in G2 format)
+
+	// Step 3.  Compute the quantity v. This is the estimated variance of the
+	// team's/player's rating based only on game outcomes.
 	variance := 0.0
 	for _, r := range results {
 		g_i := calcG(r.deviation)
@@ -158,7 +177,9 @@ func (g *Glicko2) Update() {
 	}
 	variance = 1.0 / variance
 
-	// compute delta
+	// Step 4. Compute the quantity 'delta', the estimated improvement in rating
+	// by comparing the pre-period rating to the performance rating based only
+	// on game outcomes.
 	delta := 0.0
 	for _, r := range g.wins {
 		delta += calcG(r.deviation) * (1.0 - calcE(g.rating, r.rating, r.deviation))
@@ -171,7 +192,7 @@ func (g *Glicko2) Update() {
 	}
 	delta *= variance
 
-	// determine new volatility
+	// Step 5. Determine the new value of the volatility.
 	new_volatility := 0.0
 	a := math.Log((g.volatility * g.volatility))
 	x := 0.0
@@ -179,16 +200,16 @@ func (g *Glicko2) Update() {
 	for math.Abs(x-x_new) > 0.0000001 {
 		x = x_new
 		d := (g.deviation * g.deviation) + variance + math.Exp(x)
-		h1 := -(x-a)/(dvolatility*dvolatility) - 0.5*math.Exp(x)/d + 0.5*math.Exp(x)*(delta/d)*(delta/d)
-		h2 := -1.0/(dvolatility*dvolatility) - 0.5*math.Exp(x)*((g.deviation*g.deviation)+variance)/(d*d) + 0.5*(delta*delta)*math.Exp(x)*((g.deviation*g.deviation)+variance-math.Exp(x))/(d*d*d)
+		h1 := -(x-a)/(kDVOL*kDVOL) - 0.5*math.Exp(x)/d + 0.5*math.Exp(x)*(delta/d)*(delta/d)
+		h2 := -1.0/(kDVOL*kDVOL) - 0.5*math.Exp(x)*((g.deviation*g.deviation)+variance)/(d*d) + 0.5*(delta*delta)*math.Exp(x)*((g.deviation*g.deviation)+variance-math.Exp(x))/(d*d*d)
 		x_new = x - h1/h2
 	}
 	new_volatility = math.Exp(x_new / 2.0)
 
-	// update the rating deviation to the new pre-rating period value
+	// Step 6. Update the rating deviation to the new pre-rating period value.
 	pre_deviation := math.Sqrt((g.deviation * g.deviation) + (new_volatility * new_volatility))
 
-	// update the rating and deviation
+	// Step 7. Update the rating and RD to the new values.
 	new_deviation := 1.0 / (math.Sqrt(1.0/(pre_deviation*pre_deviation) + 1.0/variance))
 	new_rating := 0.0
 	for _, r := range g.wins {
@@ -203,11 +224,11 @@ func (g *Glicko2) Update() {
 	new_rating = new_rating * (new_deviation * new_deviation)
 	new_rating += g.rating
 
-	// wipe our result lists
-	g.ClearResults()
-
-	// copy new values
+	// Step 8. Convert ratings and RD's back to original scale.
 	g.deviation = new_deviation
 	g.volatility = new_volatility
 	g.rating = new_rating
+
+	// wipe our result lists
+	g.ClearResults()
 }