Advent of Code Day 3.swift

import Foundation

enum GeometryErr: Error {
  case InvalidDirection
  case InvalidDistance
}

enum Direction {
  case Up(_ distance: Int)
  case Down(_ distance: Int)
  case Left(_ distance: Int)
  case Right(_ distance: Int)

  init(_ s: String) throws {
    guard let distance = Int(String(s.suffix(s.count - 1))) else {
      throw GeometryErr.InvalidDistance
    }
    switch s.first {
    case "U": self = .Up(distance)
    case "D": self = .Down(distance)
    case "L": self = .Left(distance)
    case "R": self = .Right(distance)
    default: throw GeometryErr.InvalidDirection
    }
  }
}

struct Coordinate: Hashable {
  let x: Int
  let y: Int
}

func travel(wire input: String) throws -> [Coordinate] {
  let direction =
    try
      input
      .split(separator: ",")
      .map { String($0) }
      .map(Direction.init)

  var position = (0, 0)
  var coordinates: [Coordinate] = []
  direction.forEach { dir in
    switch dir {
    case .Up(let d):
      for n in 0..<d { coordinates.append(Coordinate(x: position.1, y: position.0 + n)) }
      position.0 += d
    case .Down(let d):
      for n in 0..<d { coordinates.append(Coordinate(x: position.1, y: position.0 - n)) }
      position.0 -= d
    case .Left(let d):
      for n in 0..<d { coordinates.append(Coordinate(x: position.1 - n, y: position.0)) }
      position.1 -= d
    case .Right(let d):
      for n in 0..<d { coordinates.append(Coordinate(x: position.1 + n, y: position.0)) }
      position.1 += d
    }
  }
  return coordinates
}

let red =
  "R1005,U370,L335,D670,R236,D634,L914,U15,R292,D695,L345,D183,R655,U438,R203,U551,L540,U51,R834,D563,L882,D605,L832,U663,R899,D775,L740,U764,L810,U442,R379,D951,L821,D703,R526,D624,L100,D796,R375,U129,L957,D41,R361,D504,R358,D320,L392,D842,R509,D612,L92,U788,L361,D757,R428,U257,L663,U956,L748,U938,R588,D942,R819,D732,R562,D331,L164,U801,R872,U872,L909,U260,R899,D278,R822,U968,L937,D594,L786,D34,R102,D650,R920,D539,R925,U436,R347,U686,L596,D608,R730,U5,R462,U831,R277,U411,R730,D828,L169,D276,L669,U167,R55,D879,L329,U258,R585,D134,R977,D609,L126,U848,L601,U624,R577,D421,L880,D488,R505,U385,L103,D693,L110,D338,R809,D864,L80,U413,R412,D134,L519,D988,R83,U580,R593,U435,R843,D953,R11,D655,R569,D237,R987,U894,L445,U974,L746,U450,R99,U69,R84,U258,L248,D581,R215,U306,R480,U126,R275,D353,R493,D800,L386,D876,L957,D722,L967,D612,L716,D901,R394,U764,R274,D686,L746,D957,R747,U517,L575,D961,R842,D753,L345,D59,L215,U413,R610,D166,L646,U107,L926,D848,R445,U297,L376,U869,L345,D529,R620,D353,R682,D908,R378,D221,R64,D911,L245,D364,R123,D555,L928,U412,R771,D543,L97,D477,R500,D125,R578,U150,R291,D252,R948,D576,L838,D144,L289,D677,L307,U692,R802,D743,R57,U839,R896,D110,R34,D508,L595,U658,L769,U47,L292,U66,R217,D8,L835,D479,L71,D24,R429,U64,R305,D406,R23,U819,R478,D7,L561,D503,R349,U104,L749,D123,R548,D421,R336,D837,R464,D908,L94,U988,L137,D757,L42,U842,R260,D406,L31,U965,L178,U973,L29,U276,L887,U920,L133,U243,R537,U282,R194,D152,R693,D509,L771,D365,L319,D378,L61,D849,R379"
let green =
  "L998,U242,R333,U631,L507,U313,R286,U714,R709,U585,R393,D893,R404,D448,R882,U246,L190,U238,R672,D184,L275,D120,R352,D584,L626,U413,L288,D942,R770,D551,L926,D242,R568,U48,R108,D349,R750,D323,L529,D703,L672,U775,L700,D465,L528,D596,R990,U366,L747,D270,L723,D469,L548,D47,L873,D678,R782,D187,L397,U975,R967,D224,L295,D86,L159,U610,L767,U641,L885,D623,L160,D509,R517,D981,L376,D604,R251,D140,L938,D358,L984,U63,R513,D54,L718,U90,L343,D982,L575,D692,L508,D361,L297,D880,L46,D875,R40,D97,R819,U919,R319,U152,R161,U553,L388,D100,R481,U306,L201,U706,L173,D657,L632,D182,R477,D332,R678,D683,L983,D584,R941,U801,R485,D376,R218,D432,R780,D617,R560,D618,R466,U456,L952,D72,R339,U16,L543,U176,L423,D770,L714,U621,L850,U929,R132,D908,R993,U440,R539,U374,L945,D443,L326,D651,L269,U321,R925,D777,R431,U273,R811,D63,R683,D540,L3,D617,R359,U332,L736,D98,L859,D994,R131,U71,L156,D661,R879,D303,L581,U407,L166,U878,L831,D871,R953,D137,L903,U200,R34,D857,R448,D412,L311,D212,R527,D707,R641,D775,L987,D814,L38,D96,R647,U868,L98,U882,L838,D308,R840,U161,R83,U424,L420,U934,R353,D287,R559,D665,R695,D888,R859,U992,L283,D525,L449,U255,L889,D296,R72,D899,R316,D3,L308,D404,L356,D333,R645,U274,R336,U258,R599,U746,L142,U21,R301,D890,L290,D624,R565,U117,L927,U412,L687,U480,R674,U372,L382,D134,L372,D892,R307,U217,L20,D535,L876,D548,L19,U590,R906,D816,R465,U768,R882,U980,L557,D788,R645,U684,L255,D803,L374,U759,L693,D92,L256,U772,R591,D126,R57,U363,R347,U191,L760,U223,R591,D507,R232,U251,R471,D912,R227"

let redPath = try travel(wire: red)
let greenPath = try travel(wire: green)

let intersection = Set<Coordinate>(redPath).intersection(Set<Coordinate>(greenPath))
let taxicabDistance =
  intersection
    .map { abs($0.x) + abs($0.y) }
    .sorted()
    .dropFirst()
    .first

let bestTravel =
  intersection
    .map { coordinate -> Int in
      let first = redPath.firstIndex(of: coordinate)!
      let second = greenPath.firstIndex(of: coordinate)!
      return first + second
    }.sorted()
    .dropFirst()
    .first

print(taxicabDistance!)
print(bestTravel!)