CodeWars/5kyu/esolang_interpreters_ii/solution.swift

func interpreter(_ code: String, _ tape: String) -> String {
  func fromTape(stringTape: String) -> [Int] {
    return stringTape.map({ c in (c == "0") ? 0 : 1 })
  }

  func toTape(intTape: [Int]) -> String {
    return intTape.reduce("", { x, b in x + (b == 0 ? "0" : "1") })
  }

  func findComplement(direction: Int, startIndex: Int, l: Character, r: Character) -> Int {
    var depth = 0
    var i = startIndex

    repeat {
      let c = code[code.index(code.startIndex, offsetBy: i)]

      if c == l {
        depth += 1
      } else if c == r {
        depth -= 1
      }

      i += direction
    } while depth != 0

    return i
  }

  var bits = fromTape(stringTape: tape)
  var i = 0

  var codeIndex = 0
  while codeIndex >= 0 && codeIndex < code.count {
    switch code[code.index(code.startIndex, offsetBy: codeIndex)] {
      case ">":
        i += 1
      case "<":
        i -= 1
      case "*":
        bits[i] = (bits[i] + 1) % 2
      case "[":
        if bits[i] == 0 {
          codeIndex = findComplement(direction: 1, startIndex: codeIndex, l: "[", r: "]") - 1
        }
      case "]":
        if bits[i] != 0 {
          codeIndex = findComplement(direction: -1, startIndex: codeIndex, l: "]", r: "[")
        }
      default:
        break
    }

    codeIndex += 1

    if i < 0 || i >= bits.count {
      break
    }
  }

  return toTape(intTape: bits)
}