Apply DiffMatchPatch Diffs to a String in pure Swift

String+StringFromDelta.swift

import Foundation

extension String {
    func string(withDelta deltaString: String) throws -> String {
        var outString = String()
        var indexPointer = self.startIndex
        let inStringCount = self.utf16.count
        try deltaString.components(separatedBy: "\t").forEach { string in
            guard let firstChar = string.first else { return }
            let param = string[string.index(after: string.startIndex)...]
            switch firstChar {
            case "+":
                guard let decoded = param.removingPercentEncoding else { throw DeltaError.invalidCharacter }
                outString += decoded
            case "=", "-":
                guard let n = Int(param), n > 0 else { throw DeltaError.invalidNumber }
                let newPointer = String.Index(encodedOffset: indexPointer.encodedOffset + n)
                guard newPointer.encodedOffset <= inStringCount else { throw DeltaError.deltaLenghtOutOfBounds }
                if firstChar == "=" {
                    outString += self[indexPointer..<newPointer]
                }
                indexPointer = newPointer
            default:
                throw DeltaError.invalidOperation
            }
        }
        return outString
    }
    
    enum DeltaError: Swift.Error {
        case invalidCharacter
        case invalidNumber
        case deltaLenghtOutOfBounds
        case invalidOperation
    }
}