package financial

import "math"

// IRRCalculatorImpl implements IRRCalculator interface
type IRRCalculatorImpl struct{}

// NewIRRCalculator creates a new IRR calculator
func NewIRRCalculator() IRRCalculator {
	return &IRRCalculatorImpl{}
}

// CalculateIRR calculates Internal Rate of Return using Newton-Raphson method
// IRR is the discount rate where NPV = 0
func (irr *IRRCalculatorImpl) CalculateIRR(capex, annualCashFlow float64, years int) float64 {
	if annualCashFlow <= 0 {
		return 0.0 // No positive cash flow
	}

	// Newton-Raphson iteration
	irrRate := 0.1 // Initial guess: 10%
	tolerance := 0.0001
	maxIterations := 100

	for i := 0; i < maxIterations; i++ {
		npv := -capex
		derivative := 0.0

		for year := 1; year <= years; year++ {
			yearFloat := float64(year)
			discountFactor := math.Pow(1+irrRate, yearFloat)
			npv += annualCashFlow / discountFactor
			derivative -= yearFloat * annualCashFlow / math.Pow(1+irrRate, yearFloat+1)
		}

		if math.Abs(npv) < tolerance {
			return irrRate * 100 // Return as percentage
		}

		if derivative != 0 {
			irrRate = irrRate - npv/derivative
		} else {
			break
		}

		// Prevent negative IRR
		if irrRate < -0.99 {
			irrRate = -0.99
		}
	}

	return irrRate * 100 // Return as percentage
}