From 31de7837e1a880f355da85409f6b2b3023d10e48 Mon Sep 17 00:00:00 2001
From: Maksim Syomochkin <maksim77ster@gmail.com>
Date: Fri, 17 Jan 2020 13:14:42 +0300
Subject: [PATCH] Add test && docs

---
 xirr.go      | 27 +++++++++++++++-----
 xirr_test.go | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 92 insertions(+), 7 deletions(-)
 create mode 100644 xirr_test.go

diff --git a/xirr.go b/xirr.go
index f9424ea..c5645b1 100644
--- a/xirr.go
+++ b/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
 }
diff --git a/xirr_test.go b/xirr_test.go
new file mode 100644
index 0000000..bbe884b
--- /dev/null
+++ b/xirr_test.go
@@ -0,0 +1,72 @@
+package goxirr
+
+import (
+	"fmt"
+	"testing"
+	"time"
+)
+
+func ExampleXirr() {
+	firstDate := time.Date(2019, time.January, 1, 0, 0, 0, 0, time.UTC)
+	t1 := Transaction{
+		Date: firstDate,
+		Cash: -100,
+	}
+	t2 := Transaction{
+		Date: firstDate.Add(time.Hour * 24 * 365),
+		Cash: 112,
+	}
+
+	tas := Transactions{t1, t2}
+	fmt.Println(Xirr(tas))
+	// Output: 12
+}
+
+func TestXirr(t *testing.T) {
+	type args struct {
+		transactions []Transaction
+	}
+
+	var case1, case2, case3 args
+	case1.transactions = append(case1.transactions, Transaction{
+		Date: time.Date(2019, time.January, 1, 0, 0, 0, 0, time.UTC),
+		Cash: -100,
+	}, Transaction{
+		Date: time.Date(2020, time.January, 1, 0, 0, 0, 0, time.UTC),
+		Cash: 200,
+	})
+
+	case2.transactions = append(case2.transactions, Transaction{
+		Date: time.Date(2019, time.January, 1, 0, 0, 0, 0, time.UTC),
+		Cash: -100,
+	}, Transaction{
+		Date: time.Date(2020, time.January, 1, 0, 0, 0, 0, time.UTC),
+		Cash: 100,
+	})
+
+	case3.transactions = append(case3.transactions, Transaction{
+		Date: time.Date(2019, time.January, 1, 0, 0, 0, 0, time.UTC),
+		Cash: -100,
+	}, Transaction{
+		Date: time.Date(2020, time.January, 1, 0, 0, 0, 0, time.UTC),
+		Cash: 112,
+	})
+
+	tests := []struct {
+		name string
+		args args
+		want float64
+	}{
+		{name: "100%", args: case1, want: 100},
+		{name: "0%", args: case2, want: 0.0},
+		{name: "12%", args: case3, want: 12},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := Xirr(tt.args.transactions); got != tt.want {
+				t.Errorf("Xirr() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}