Parse and format posix raw file modes (including file type and special (suid/setgid/sticky) bits)

PosixFileModes.kt

private const val FILE_TYPES = "?pc?d?b?-?l?s???"

fun String.toPosixModeInt(): Int {
    require(length == 10) { "Should have 10 characters" }
    // type
    val type = when(get(0)) {
        'p' -> 1
        'c' -> 2
        'd' -> 4
        'b' -> 6
        '-' -> 8
        'l' -> 10
        's' -> 12
        else -> 0
    } shl 12
    return type + substring(1).chunked(3).reversed().mapIndexed(::parseModeChars).sum()
}

fun parseModeChars(position: Int, mode: String): Int {
    var value = when (mode[0]) {
        'r' -> 4 shl position * 3
        else -> 0
    }
    value += when (mode[1]) {
        'w' -> 2 shl position * 3
        else -> 0
    }
    return value + when (mode[2]) {
        'x' -> 1 shl position * 3
        'S' -> 1 shl (9 + position)
        's' -> (1 shl 9 + position) + (1 shl position * 3)
        'T' -> require(position == 0) {"Sticky bit in invalid position ($position)"}.let { 1 shl 9  }
        't' -> require(position == 0) {"Sticky bit in invalid position ($position)"}.let { (1 shl 9) + 1 }
        else -> 0
    }
}

fun Int.toPosixModeString(): String {
    val type = this shr 12 and 15
    val special = this shr 9 and 7
    return StringBuilder(10).also {
        // file type
        it.append(FILE_TYPES[type])
        // user
        it.appendModeChars(this shr 6 and 7, special and 4 != 0, 's')
        it.appendModeChars(this shr 3 and 7, special and 2 != 0, 's')
        it.appendModeChars(this shr 0 and 7, special and 1 != 0,  't')
    }.toString()
}

private fun StringBuilder.appendModeChars(bits: Int, special: Boolean, specialX: Char) {
    append(if (bits and 4 != 0) 'r' else '-')
    append(if (bits and 2 != 0) 'w' else '-')
    append(if (bits and 1 != 0) {
        if (special) specialX else 'x'
    } else {
        if (special) specialX.uppercaseChar() else '-'
    })
}