Simple recursive descent parser

recursive-descent.swift

import Cocoa

let input = "15,7, 3 ,, [1,2,[3, 4, 5], 6],8"
//let input = "5,7,[1]"

var index = input.characters.startIndex
var lookahead = input[index]
var done = false

func eat() {
  index = input.index(after: index)
  if index != input.endIndex {
    lookahead = input[index]
  } else {
    done = true
  }
}

func bracket() -> String {
  var symbol = "["
  eat()

  while index != input.endIndex {
    switch lookahead {
    case "[":
      symbol += bracket()

    case "]":
      eat()
      return symbol + "]"

    default:
      symbol += String(lookahead)
      eat()
    }
  }

  fatalError("Missing closing ]")
}

func parseNext() -> String {
  var symbol = ""
  while index != input.endIndex {
    switch lookahead {
    case "[":
      symbol += bracket()

    case ",":
      eat()
      return symbol

    default:
      symbol += String(lookahead)
      eat()
    }
  }
  return symbol
}

var output: [String] = []
while !done {
  output.append(parseNext())
}

print(output)

// Output is: ["15", "7", " 3 ", "", " [1,2,[3, 4, 5], 6]", "8"]