peekCharacter using Scanner in Swift

Scanner.swift

extension Scanner {
    var peekCharacter: Character? {
        if isAtEnd { return nil }
        return string[fallbackAwareCurrentIndex]
    }

    private var fallbackAwareCurrentIndex: String.Index {
        get {
            if #available(iOS 13, macCatalyst 13, macOS 10.15, *) {
                return currentIndex
            } else {
                // https://github.com/apple/swift-corelibs-foundation/blob/2a5bc4d8a0b073532e60410682f5eb8f00144870/Sources/Foundation/ScannerAPI.swift#L25
                let string = self.string
                var index = string.toUTF16Index(scanLocation)

                var delta = 0
                while index != string.endIndex && index.samePosition(in: string) == nil {
                    delta += 1
                    index = string.toUTF16Index(scanLocation + delta)
                }

                return index
            }
        }
        set {
            if #available(iOS 13, macCatalyst 13, macOS 10.15, *) {
                currentIndex = newValue
            } else {
                scanLocation = string.toUTF16Offset(newValue)
            }
        }
    }
}

// https://fossies.org/linux/swift-swift/stdlib/public/core/StringBridge.swift
extension String {
    func toUTF16Index(_ offset: Int) -> Index {
        utf16.index(utf16.startIndex, offsetBy: offset)
    }

    func toUTF16Offset(_ idx: Index) -> Int {
        utf16.distance(from: utf16.startIndex, to: idx)
    }
}