Swift String extensions for shorthand (Ruby-like) indexing

StringIndexOffsetExtensions_Playground.swift

//: Playground - noun: a place where people can play

import Swift

var str = "Hello, playground"


extension String
{
	func index(atOffset offset:String.IndexDistance) -> String.Index {
		return index(offset >= 0 ? self.startIndex : self.endIndex, offsetBy: offset)
	}
	
	subscript(atOffset offset:String.IndexDistance) -> Character {
		return self[self.index(atOffset: offset)]
	}
	
	subscript(withinOffsets offsetRange:Range<String.IndexDistance>) -> String {
		return self[fromOffset: offsetRange.lowerBound, toOffset: offsetRange.upperBound]
	}
	subscript(fromOffset lowerOffset:String.IndexDistance, toOffset upperOffset:String.IndexDistance) -> String {
		let indexes = (
			lower: self.index(atOffset: lowerOffset),
			upper: self.index(atOffset: upperOffset)
		)
		return self[indexes.lower ..< indexes.upper]
	}
	
	subscript(withinOffsets offsetRange:ClosedRange<String.IndexDistance>) -> String {
		return self[fromOffset: offsetRange.lowerBound, throughOffset: offsetRange.upperBound]
	}
	subscript(fromOffset lowerOffset:String.IndexDistance, throughOffset upperOffset:String.IndexDistance) -> String {
		let indexes = (
			lower: self.index(atOffset: lowerOffset),
			upper: self.index(atOffset: upperOffset)
		)
		return self[indexes.lower ... indexes.upper]
	}
}

str[atOffset: -1] // "d"
str[atOffset: -2] // "n"
str[atOffset: 0] // "H"
str[atOffset: 1] // "e"

str[withinOffsets: 1 ... 10] // "ello, play"
str[withinOffsets: -10 ... -1] // "playground"
str[fromOffset: 1, throughOffset: -1] // "ello, playground"

str[withinOffsets: 1 ..< 10] // "ello, pla"
str[withinOffsets: -10 ..< -1] // "playgroun"
str[fromOffset: 1, toOffset: -1] // "ello, playgroun"