8

I'm trying to decode a protobuff encoded message, so I need to convert the first byte (the key) in the protobuff message into bits, so I can find the field number. How do I convert a UInt8 (the byte) into an array of bits?

Pseudo Code

private func findFieldNum(from byte: UInt8) -> Int {
    //Byte is 0001 1010
    var fieldNumBits = byte[1] ++ byte[2] ++ byte[3] ++ byte[4] //concatentates bits to get 0011
    getFieldNum(from: fieldNumBits) //Converts 0011 to field number, 2^1 + 2^0 = 3
}

I saw this question, which converts an array of bits into array of bytes.

Amanda
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5 Answers5

18

Here's a basic function to get a Bit array from a byte:

func bits(fromByte byte: UInt8) -> [Bit] {
    var byte = byte
    var bits = [Bit](repeating: .zero, count: 8)
    for i in 0..<8 {
        let currentBit = byte & 0x01
        if currentBit != 0 {
            bits[i] = .one
        }

        byte >>= 1
    }

    return bits
}

Here, Bit is a custom enum type that I have defined as follows:

enum Bit: UInt8, CustomStringConvertible {
    case zero, one

    var description: String {
        switch self {
        case .one:
            return "1"
        case .zero:
            return "0"
        }
    }
}

With this setup, the output of the following code:

let byte: UInt8 = 0x1f

print(bits(fromByte: byte))

would be:

[1, 1, 1, 1, 1, 0, 0, 0]
mohak
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  • Do you have to write a byte as a hexadecimal or is it possible to write it as a binary like 00011010? – Amanda Jun 29 '17 at 13:42
  • You can write in binary with the `0b` suffix. e.g. `0b10001111` – mohak Jun 29 '17 at 14:57
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    This works but returns the reverse of the original bit ordering (as expressed by a binary literal such as `0b11111000`). – Nikolay Suvandzhiev Feb 21 '19 at 15:55
  • @NikolaySuvandzhiev The array returned by `bits()` has the 0th bit on the 0 index, 1st bit on the 1 index. This makes semantic sense, right? The issue is that the print function treats the bit array like a normal list and prints its elements starting with the 0 index whereas the literal is usually written with 7th bit (MSb) first. – mohak Dec 04 '19 at 10:19
  • Reverse order: `Array(bits(fromByte: byte).reversed())` – SoftDesigner Mar 27 '21 at 09:32
3

Improving on mohak's answer. With a generic function or an extension to cater for more than just UInt8.

enum Bit: UInt8, CustomStringConvertible {
    case zero, one

    var description: String {
        switch self {
        case .one:
            return "1"
        case .zero:
            return "0"
        }
    }
}

func bits<T: FixedWidthInteger>(fromBytes bytes: T) -> [Bit] {
    // Make variable
    var bytes = bytes
    // Fill an array of bits with zeros to the fixed width integer length
    var bits = [Bit](repeating: .zero, count: T.bitWidth)
    // Run through each bit (LSB first)
    for i in 0..<T.bitWidth {
        let currentBit = bytes & 0x01
        if currentBit != 0 {
            bits[i] = .one
        }

        bytes >>= 1
    }

    return bits
}

extension FixedWidthInteger {
    var bits: [Bit] {
        // Make variable
        var bytes = self
        // Fill an array of bits with zeros to the fixed width integer length
        var bits = [Bit](repeating: .zero, count: self.bitWidth)
        // Run through each bit (LSB first)
        for i in 0..<self.bitWidth {
            let currentBit = bytes & 0x01
            if currentBit != 0 {
                bits[i] = .one
            }

            bytes >>= 1
        }

        return bits
    }
}
jdelaune
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0

You can do some bitwise operations to get the value of the bits at indices 1-4.

// An example byte
let a: UInt8 = 0b00011010

// Extract the bits (using 0b01111000 as a bitmask) 
// in indices 1 to 4, then shift right by 3 places
// to remove indices 5 to 7
var b = (a & 0b01111000) >> 3 

// In this case, b is now 3 (or 11 in binary)
print(b)
paulvs
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0

A shorter one:

enum Bit { case zero, one }

func bit(_ i: Int, of uint8: UInt8) -> Bit {
  let first8PowersOf2 = (0...7).map { return UInt8(1) << $0 }
  return (uint8 & first8PowersOf2[i] != 0) ? Bit.one : Bit.zero
}

func bitsFrom(_ uint8: UInt8) -> [Bit] {
  return Array((0...7)).reversed().map { bit($0, of: uint8) }
}

We can use it as:

let fromUInt8  = bitsFrom(42)
let fromBinary = bitsFrom(0b01011010)
let fromHexa   = bitsFrom(0xFF)
kanobius
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0

Swift 4,5 works fine for me :

func bits(fromByte byte: UInt8) -> [Bool] {
  var byte = byte
  var bits = [Bool](repeating: false, count: 8)
  for i in 0..<8 {
      let currentBit = byte & 0x01
      if currentBit != 0 {
          bits[i] = true
      }
      byte >>= 1
  }
  return bits
}
user
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Krzysztof Majewski
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