Collection slicing

main.swift

extension Collection {
  func slice(from: Int = 0, to: Int = Int.max, increment: Int = 1) -> [Element] {
    let endIdx: Index
    let startIdx: Index

    if to >= count {
      endIdx = endIndex
    } else if to < 0 { 
      endIdx = index(endIndex, offsetBy: to, limitedBy: startIndex) ?? startIndex
    } else {
      endIdx = index(startIndex, offsetBy: to)
    }

    if from == 0 {
      startIdx = startIndex
    } else {
      startIdx = index(startIndex, offsetBy: from)
    }

    if increment == 1 {
      return Array(self[startIdx..<endIdx])
    }

    var res = [Element]()
    var curIdx = startIdx

    while curIdx != endIdx {
      res.append(self[curIdx])
      curIdx = index(curIdx, offsetBy: increment, limitedBy: endIdx) ?? endIdx
    }

    return res
  }
}

extension String {
  func substring(from: Int, to: Int = Int.max) -> String {
    return String(slice(from: from, to: to))
  }
}

print("hello🤣 world".substring(from: 5))
print((1...100).slice(from: 1, increment: 2))
print((1...100).slice(from: 1, to: -2, increment: 2))