Offsets.swift

// Speculation in regards to https://forums.swift.org/t/shorthand-for-offsetting-startindex-and-endindex/9397/133

/// A pair of offsets that can be used to slice a collection.
///
/// In order to have `RangeExpression` conformance, this would need
/// a phantom `Bound` type, which would unnecessarily restrict its
/// usage. In addition, the `RangeExpression.contains(_:)` method
/// couldn't be implemented without access to a collection with the
/// actual indices to resolve.
struct Offsets {
  /// The offset of the beginning of the slice.
  ///
  /// When positive, the offset is from a collection's `startIndex`.
  /// When negative, from the `endIndex`.
  var lowerOffset: Int

  /// The offset of the end of the slice.
  ///
  /// When positive, the offset is from a collection's `startIndex`.
  /// When negative, from the `endIndex`.
  /// When `nil`, the slice continues until the end of the collection.
  var upperOffset: Int?
  
  func relative<C>(to collection: C) -> Range<C.Index>
    where C : Collection
  {
    let start = lowerOffset < 0
      ? collection.index(collection.endIndex, offsetBy: lowerOffset, limitedBy: collection.startIndex) ?? collection.startIndex
      : collection.index(collection.startIndex, offsetBy: lowerOffset, limitedBy: collection.endIndex) ?? collection.endIndex
    
    let end: C.Index
    switch upperOffset {
    case let upper?:
      if lowerOffset.signum() == upper.signum() {
        end = collection.index(start, offsetBy: abs(upper - lowerOffset))
      } else {
        end = upper < 0
          ? collection.index(collection.endIndex, offsetBy: upper, limitedBy: collection.startIndex) ?? collection.startIndex
          : collection.index(collection.startIndex, offsetBy: upper, limitedBy: collection.endIndex) ?? collection.endIndex
      }
    case nil:
      end = collection.endIndex
    }
    
    return start < end
      ? start..<end
      : start..<start
  }
}

func offsets(
  from lower: Int = 0, to upper: Int? = nil
) -> Offsets {
  return Offsets(lowerOffset: lower, upperOffset: upper)
}

extension Collection {
  subscript(offsets: Offsets) -> SubSequence {
    return self[offsets.relative(to: self)]
  }
}

let numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
numbers[offsets(from: 2, to: 6)]    // [30, 40, 50, 60]
numbers[offsets(from: 2, to: -2)]   // [30, 40, 50, 60, 70, 80]
numbers[offsets(to: -2)]            // [10, 20, 30, 40, 50, 60, 70, 80]
numbers[offsets(to: 5)]             // [10, 20, 30, 40, 50]
numbers[offsets(from: 3)]           // [40, 50, 60, 70, 80, 90, 100]
numbers[offsets(from: -4)]          // [70, 80, 90, 100]

let str = "Hello, world!"
str[offsets(to: 3)]                 // "Hel"
str[offsets(from: 3)]               // "lo, world!"
str[offsets(to: -5)]                // "Hello, w"
str[offsets(from: -5)]              // "orld!"
str[offsets(from: 3, to: -5)]       // "lo, w"

let trim = offsets(from: 3, to: -3)
numbers[trim]                       // [40, 50, 60, 70]
str[trim]                           // "lo, wor"