package finance

// RSI calcula el Relative Strength Index.
// Usa el metodo de suavizado de Wilder (EMA con alpha = 1/period).
// Los primeros period elementos del resultado son 0.
func RSI(data []float64, period int) []float64 {
	n := len(data)
	result := make([]float64, n)
	if period <= 0 || n < period+1 {
		return result
	}
	var gainSum, lossSum float64
	for i := 1; i <= period; i++ {
		change := data[i] - data[i-1]
		if change > 0 {
			gainSum += change
		} else {
			lossSum -= change
		}
	}
	avgGain := gainSum / float64(period)
	avgLoss := lossSum / float64(period)
	if avgLoss == 0 {
		result[period] = 100
	} else {
		rs := avgGain / avgLoss
		result[period] = 100 - 100/(1+rs)
	}
	for i := period + 1; i < n; i++ {
		change := data[i] - data[i-1]
		var gain, loss float64
		if change > 0 {
			gain = change
		} else {
			loss = -change
		}
		avgGain = (avgGain*float64(period-1) + gain) / float64(period)
		avgLoss = (avgLoss*float64(period-1) + loss) / float64(period)
		if avgLoss == 0 {
			result[i] = 100
		} else {
			rs := avgGain / avgLoss
			result[i] = 100 - 100/(1+rs)
		}
	}
	return result
}