How To Calculate Average Directional Index (ADX) in Swift?

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func calculateADX(highs: [Double], lows: [Double], closes: [Double]) -> [Double] {
    var plusDM = [Double]()
    var minusDM = [Double]()
    var tr = [Double]()
    var dx = [Double]()
    var adx = [Double]()

    for i in 1..<highs.count {
        let high = highs[i]
        let low = lows[i]
        let previousHigh = highs[i - 1]
        let previousLow = lows[i - 1]
        let previousClose = closes[i - 1]

        let trueRange = max(high - low, abs(high - previousClose), abs(low - previousClose))
        tr.append(trueRange)

        let plusDirectionalMovement = high - previousHigh
        let minusDirectionalMovement = previousLow - low

        if plusDirectionalMovement > minusDirectionalMovement && plusDirectionalMovement > 0 {
            plusDM.append(plusDirectionalMovement)
        } else {
            plusDM.append(0)
        }

        if minusDirectionalMovement > plusDirectionalMovement && minusDirectionalMovement > 0 {
            minusDM.append(minusDirectionalMovement)
        } else {
            minusDM.append(0)
        }

        if i > 14 {
            let trSum = tr.suffix(14).reduce(0, +)
            let plusDMSum = plusDM.suffix(14).reduce(0, +)
            let minusDMSum = minusDM.suffix(14).reduce(0, +)

            let plusDI = (plusDMSum / trSum) * 100
            let minusDI = (minusDMSum / trSum) * 100
            let directionalIndex = abs(plusDI - minusDI) / (plusDI + minusDI)

            if dx.count < 14 {
                dx.append(directionalIndex)
            } else {
                let previousADX = adx.last ?? 0
                let currentADX = ((previousADX * 13) + directionalIndex) / 14
                adx.append(currentADX)
            }
        }
    }

    return adx
}