/*
Package Goxirr is a simple implementation of a function for calculating
the Internal Rate of Return for irregular cash flow (XIRR).
*/
package goxirr

import (
	"math"
	"time"
)

//A Transaction represents a single transaction from a series of irregular payments.
type Transaction struct {
	Date time.Time
	Cash 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 _, t := range transactions {
		years = append(years, (t.Date.Sub(transactions[0].Date).Hours()/24)/365)
	}

	residual := 1.0
	step := 0.05
	guess := 0.05
	epsilon := 0.0001
	limit := 10000

	for math.Abs(residual) > epsilon && limit > 0 {
		limit--
		residual = 0.0

		for i, t := range transactions {
			residual += t.Cash / math.Pow(guess, years[i])
		}

		if math.Abs(residual) > epsilon {
			if residual > 0 {
				guess += step
			} else {
				guess -= step
				step /= 2.0
			}
		}
	}

	return math.Round(((guess-1)*100)*100) / 100
}