uses libiconv.2.tbd, swift5.2

Iconv.swift

import Darwin.POSIX.iconv

class Iconv {
    enum Error: Swift.Error {
        case illegalByteSequence
        case invalidArgument
        case invalidResult
        case unknownError(errno: Int32)
    }

    private let context: iconv_t

    init(from fromCode: String) {
        context = iconv_open("UTF-8", fromCode)
    }

    deinit {
        iconv_close(context)
    }

    func decode(_ data: Data, tmpBufferSize: Int = 2048) throws -> String {
        try data.withUnsafeBytes { dataPtr -> String in
            var rawResultBuffer: [Int8] = []

            let tmpBuffer = UnsafeMutablePointer<Int8>.allocate(capacity: tmpBufferSize)
            defer {
                tmpBuffer.deallocate()
            }

            var inBuffer = UnsafeMutablePointer(mutating: dataPtr.baseAddress?.assumingMemoryBound(to: Int8.self))
            var inBytesLeft = dataPtr.count

            while inBytesLeft > 0 {
                var outBuffer: UnsafeMutablePointer<Int8>? = tmpBuffer
                var outBytesLeft = tmpBufferSize

                if iconv(context, &inBuffer, &inBytesLeft, &outBuffer, &outBytesLeft) == -1 {
                    switch errno {
                    case EILSEQ:
                        throw Error.illegalByteSequence
                    case E2BIG:
                        break
                    case EINVAL:
                        throw Error.invalidArgument
                    default:
                        throw Error.unknownError(errno: errno)
                    }
                }

                rawResultBuffer.append(contentsOf: UnsafeMutableBufferPointer(start: tmpBuffer, count: tmpBufferSize - outBytesLeft))
            }

            guard let result = String(utf8String: &rawResultBuffer) else {
                throw Error.invalidResult
            }

            return result
        }
    }
}