DeltaDecoded.swift

import Foundation

extension Array where Element == Int {

    /// Returns a decoded version of a delta encoded array
    public func deltaDecoded() -> [Element] {
        let filteredArray = filterArray(self)

        var cummulativeSum = 0
        var decodedValues = [Element]()
        for value in filteredArray {
            cummulativeSum += value
            decodedValues.append(cummulativeSum)
        }
        return decodedValues
    }

    private func filterArray(_ array: [Int]) -> [Int] {
        var filtered = [Int]()
        var index = 0
        while index < array.count {
            if array[index] == -128 {
                if index < array.count - 1, array[index + 1] == -128 {
                    filtered.append(-128)
                    index += 1
                }
            } else {
                filtered.append(array[index])
            }
            index += 1
        }
        return filtered
    }
}


let result = [25626, -128, 131, -128, -1390, -100, -128, -24251, 84, -98, -128, 7275].deltaDecoded()
print(result)

// [25626, 25757, 24367, 24267, 16, 100, 2, 7277]