feature: Mirr function

README.md

@@ -24,7 +24,7 @@ which are as follows:
 | rate                         |  ✅   |    Computes the rate of interest per period|
 | irr                          |      |    Computes the internal rate of return|
 | npv                          |  ✅   |   Computes the net present value of a series of cash flow|
-| mirr                         |      |    Computes the modified internal rate of return|
+| mirr                         |  ✅   |    Computes the modified internal rate of return|
 
 # Index  
 While the numpy-financial package contains a set of elementary financial functions, this pkg also contains some helper functions on top of it. Their usage and description can be found below:  
@@ -49,6 +49,8 @@ While the numpy-financial package contains a set of elementary financial functio
     + [Example(Nper-Loan)](#examplenper-loan)
   * [Rate(Interest Rate)](#rate)
 	+ [Example(Rate-Investment)](#examplerate-investment)
+  * [Mirr(Modified irr)](#mirr)
+	+ [Example(Mirr)](#examplemirr)
 
  Detailed documentation is available at [godoc](https://godoc.org/github.com/razorpay/go-financial).
 ## Amortisation(Generate Table)  
@@ -638,3 +640,48 @@ func main() {
 }
 ```
 [Run on go-playground](https://play.golang.org/p/H2uybe1dbRj)
+
+## Mirr
+
+```go  
+func Mirr(cashflows []decimal.Decimal, financeRate, reinvestRate decimal.Decimal) decimal.Decimal
+```
+
+Params:  
+```text
+ cashflows: periodic chasflow statement
+ financeRate: interest rate for inflows
+ reinvestRate: interst received for reinvesting outflows
+``` 
+
+Returns:
+```text
+mirr    : a value for the corresponding mirr
+```
+
+Mirr calculates the Modified internal rate of return for a given cashflow and finance and reinvestment rates
+
+### Example(Mirr)
+
+```go
+package main
+
+import (
+	"fmt"
+	gofinancial "github.com/razorpay/go-financial"
+	"github.com/shopspring/decimal"
+)
+
+func main() {
+	cashflow := []decimal.Decimal{decimal.NewFromInt(-1000), decimal.NewFromInt(-4000), decimal.NewFromInt(5000), decimal.NewFromInt(2000)}
+	financeRate :=  decimal.NewFromFloat(0.1)
+	reinvestRate := decimal.NewFromFloat(0.12)
+
+	mirr := gofinancial.Mirr(cashflow,financeRate,reinvestRate)
+
+	fmt.Printf("mirr: %v",mirr)
+	// Output:
+	// mirr: 0.1790856860348926
+}
+```
+[Run on go-playground](https://play.golang.org/p/ci6pBUkX4jy)

reducing_utils.go

@@ -371,3 +371,40 @@ func Rate(pv, fv, pmt decimal.Decimal, nper int64, when paymentperiod.Type, maxI
 	}
 	return nextIterRate, nil
 }
+
+/*
+Mirr calculates the Modified internal rate of return for a given cashflow and finance and reinvestment rates
+
+Params:
+ cashflows	: periodic chasflow statement
+ financeRate	: interest rate for inflows
+ reinvestRate	: interst received for reinvesting outflows
+*/
+func Mirr(cashflows []decimal.Decimal, financeRate, reinvestRate decimal.Decimal) decimal.Decimal {
+	var inflow, outflow []decimal.Decimal
+
+	for _, v := range cashflows {
+		if v.IsNegative() {
+			inflow = append(inflow, v)
+			outflow = append(outflow, decimal.Zero)
+		}
+		if v.IsPositive() {
+			outflow = append(outflow, v)
+			inflow = append(inflow, decimal.Zero)
+		}
+	}
+
+	one := decimal.NewFromFloat(1)
+	n_1 := decimal.NewFromInt(int64(len(cashflows) - 1))
+	_pv := Npv(financeRate, inflow)
+	_fv := Npv(reinvestRate, outflow).Mul(one.Add(reinvestRate).Pow(n_1))
+
+	// ! shopspring decimal struct based underroot is imprecise (hence math library)
+	_x1 := _fv.Div(_pv.Neg())
+	_go_x1, _ := _x1.Float64()
+	_go_x2, _ := one.Div(n_1).Float64()
+	_x3 := math.Pow(_go_x1, _go_x2)
+
+	_mirr := decimal.NewFromFloat(_x3).Sub(one)
+	return _mirr
+}

reducing_utils_test.go

@@ -422,3 +422,41 @@ func Test_Rate(t *testing.T) {
 		})
 	}
 }
+
+func Test_Mirr(t *testing.T) {
+	type args struct {
+		cashflow     []decimal.Decimal
+		financeRate  decimal.Decimal
+		reinvestRate decimal.Decimal
+	}
+	tests := []struct {
+		name string
+		args args
+		want decimal.Decimal
+	}{
+		{
+			name: "success", args: args{
+				cashflow:     []decimal.Decimal{decimal.NewFromInt(-1000), decimal.NewFromInt(-4000), decimal.NewFromInt(5000), decimal.NewFromInt(2000)},
+				financeRate:  decimal.NewFromFloat(0.1),
+				reinvestRate: decimal.NewFromFloat(0.12),
+			},
+			want: decimal.NewFromFloat(0.1790856860),
+		},
+		{
+			name: "success", args: args{
+				cashflow:     []decimal.Decimal{decimal.NewFromInt(-2000), decimal.NewFromInt(2000), decimal.NewFromInt(2000)},
+				financeRate:  decimal.NewFromFloat(0.3),
+				reinvestRate: decimal.NewFromFloat(0.1),
+			},
+			want: decimal.NewFromFloat(0.4491376746),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if res := Mirr(tt.args.cashflow, tt.args.financeRate, tt.args.reinvestRate); isAlmostEqual(res, tt.want, decimal.NewFromFloat(precision)) != nil {
+				t.Errorf("Mirr returned (%v), wanted (%v)", res, tt.want)
+			}
+		})
+	}
+}