UTF8ToUTF16.swift

import Foundation

enum ConverterError: Error {
	case outOfBounds
}

func UTF8ToUTF16(_ data: Data) -> [unichar] {
	func decode(index: Int, length: Int) throws -> unichar {
		if index + length > data.count {
			throw ConverterError.outOfBounds
		}
		var rtn: unichar = 0
		var currentCode = data[index]
		for bitToClear in length..<8 {
			currentCode &= ~(1 << bitToClear)
		}
		rtn |= unichar(currentCode) << Int(6 * (length-1))
		for i in 1..<length {
			rtn |= unichar((data[index + i] & ~(1 << 7))) << Int(6 * (length-i-1))
		}
		return rtn
	}

	let matches:[(UInt8, Int)] = [
		(0b11111100, 6),
		(0b11111000, 5),
		(0b11110000, 4),
		(0b11100000, 3),
		(0b11000000, 2),
	]

	var index: Int = 0
	var rtn = [unichar]()
	mainLoop: while index < data.count {
		let byte = data[index]
		for match in matches {
			if (match.0 & byte) == match.0 {
				let length = match.1
				do { rtn.append(try decode(index: index, length: length))
				} catch { break mainLoop }
				index += length; continue mainLoop
			}
		}
		rtn.append(unichar(byte)); index += 1
	}
	return rtn
}

var str = "中文 Héllö World ⇾ ⇾"


let x = str.utf16.map { (i) -> Int in
	return Int(i)
}
print(x)

let result = UTF8ToUTF16(str.data(using: String.Encoding.utf8)!)
print(result)