Add test && docs

xirr.go

@@ -1,3 +1,7 @@
+/*
+Goxirr is a simple implementation of a function for calculating
+the Internal Rate of Return for irregular cash flow (XIRR).
+*/
 package goxirr
 
 import (
@@ -5,16 +9,22 @@ import (
 	"time"
 )
 
+//A Transaction represents a single transaction from a series of irregular payments.
 type Transaction struct {
 	Date time.Time
 	Cash float64
 }
 
-func Xirr(transactions []Transaction) float64 {
+//Transactions represents a cash flow consisting of individual transactions
+type Transactions []Transaction
+
+//Xirr return the Internal Rate of Return (IRR) for an irregular series of cash flows (XIRR)
+func Xirr(transactions Transactions) float64 {
 	var years []float64
-	for _, ta := range transactions {
-		years = append(years, (ta.Date.Sub(transactions[0].Date).Hours()/24)/365)
+	for _, t := range transactions {
+		years = append(years, (t.Date.Sub(transactions[0].Date).Hours()/24)/365)
 	}
+
 	residual := 1.0
 	step := 0.05
 	guess := 0.05
@@ -22,11 +32,13 @@ func Xirr(transactions []Transaction) float64 {
 	limit := 10000
 
 	for math.Abs(residual) > epsilon && limit > 0 {
-		limit -= 1
+		limit--
 		residual = 0.0
-		for i, trans := range transactions {
-			residual += trans.Cash / math.Pow(guess, years[i])
+
+		for i, t := range transactions {
+			residual += t.Cash / math.Pow(guess, years[i])
 		}
+
 		if math.Abs(residual) > epsilon {
 			if residual > 0 {
 				guess += step
@@ -36,5 +48,6 @@ func Xirr(transactions []Transaction) float64 {
 			}
 		}
 	}
-	return (guess - 1) * 100
+
+	return math.Round(((guess-1)*100)*100) / 100
 }