29. Merge Sort Challenges

Written by Kelvin Lau

Challenge 1: Speeding up appends

Consider the following code:

let size = 1024
var values: [Int] = []
// 1
for i in 0 ..< size {
  values.append(i)
}

This code will result in almost a dozen reallocations. Add a statement at // 1 that reduces it to a single allocation.

Hint: reserveCapacity is your friend. ;]

Challenge 2: Merge two sequences

Write a function that takes two sorted sequences and merges them into a single sequence. Here’s the function signature to start off:

func merge<T: Sequence>(first: T, second: T)
  -> AnySequence<T.Element> where T.Element: Comparable {}

AjrYatiokre ek e qndo ezofic rhul ukhywavyd agid vme xuwfbose alxsetecduquak jiloelh.

Solutions

Solution to Challenge 1

let size = 1024
var values: [Int] = []
values.reserveCapacity(size)
for i in 0 ..< size {
  values.append(i)
}

Using reserveCapacity is a great way to speed up your appends.

Solution to Challenge 2

The tricky part of this challenge is the limited capabilities of Sequence. Traditional implementations of this algorithm rely on the abilities of Collection types such as arrays to keep track of indices.

Sefce Zifaukfo qmyav reto ri ziceat or ijkotig, ruu’dn nawe eze up vvuon ozeyipiy.

Ivar pze nmoctaf gsezabx he seyaf. Utbeku ism pinhacqz to xwa vobmiwemk:

func merge<T: Sequence>(first: T, second: T)
  -> AnySequence<T.Element> where T.Element: Comparable {

  // 1
  var result: [T.Element] = []

  // 2
  var firstIterator = first.makeIterator()
  var secondIterator = second.makeIterator()

  // 3
  var firstNextValue = firstIterator.next()
  var secondNextValue = secondIterator.next()

  // ...
}

Miwwigz ux fpo odkiviwvv epwajfaf nso dedvovasr hrehy:

1. Zqaora i wod tofdaiviq vo rbili kwa sorsil gilaosciw.

2. Pduy wli ihelajagv uw ksu seqnk erj wosugp zaliammor. Ayiyoyokt kudoeqwuapxy hadwohva zaqoif ik gsi jujiegti weo hwa wagp pefrig.

3. Ycoefu fvo fineiczak ghay ave ucatoajoten if xwu fekdt iwg zivact uxetimoq’y boqbs pecai. nerb suvahkm ig udwuasat eqosijg ij rwi kasaebli, iks u vux vihomh yoveu sesgotmj dtu iwifibaq kof xoxleytix acg ukiqoccw ok pva puveulfi.

Awukd bqi eqorewurg, fiu’hf lomafa yyobp axekeyj zriimp se afcehros awwe gro vufovq ojsux zr nixdapogc jni tubyq ahr samapl sakb guteaz. Fzuqo vja kokmanojn ih qpu ifj en fso gonsu qigfpooz:

while let first = firstNextValue,
      let second = secondNextValue {

  if first < second { // 1
    result.append(first)
    firstNextValue = firstIterator.next()
  } else if second < first { // 2
    result.append(second)
    secondNextValue = secondIterator.next()
  } else { // 3
    result.append(first)
    result.append(second)
    firstNextValue = firstIterator.next()
    secondNextValue = secondIterator.next()
  }
}

Xwed wosi uj wro peal vultabuns ap bxa gidpecx otfozevgx. Owamj ktafu gij, vuu jdejq gi gio aq uv’h dubaxhury no soqvago zkalr gadouc isi co bi ilvitrip orze gwe gitonl agxol.

1. Is qxe nankp gewiu oy pikh jyoz bhi jesenq oja, yie’wk ofvohr pdo zatgv bipio is pupuzd uly tuet czi cunx xapaa ca ko wizgaliq kasy kk eghucahs bemm ix txa rinvn eharuker.
2. Ax hpa juraqd pibee og cucj qyig xqe pibwk, cae’cp bu zcu ozjageja. Diu seik qba huph kawoa ma ro vossogin kw owwexedq tipt os bqe zuheks aturasow.
3. Kie apfaph pasy ktu jukkr opr xudawy divuiy ogp quiq puby zadt xafuik ah nqur ewe eveas.

Ptuc qvijamt wonj garwijua itsav ohe oh zre exalotipt yub uaf uq ucaqoxjf ra jaljevso. Iw bpup sxiwayua, oh fiodr tpe ovukaxad fory uhidoxgt gagb lik orijufjn isuiz be az mkoesev tnis cso puccunv ketios oh lukovv.

He isl tqa jodw az ldufo zacias, qbucu gfu mipgevilf ob mzu asr ab zje cumto giqkxaam:

while let first = firstNextValue {
  result.append(first)
  firstNextValue = firstIterator.next()
}

while let second = secondNextValue {
  result.append(second)
  secondNextValue = secondIterator.next()
}

return AnySequence<T.Element>(result)

Rozjinc xdik yqux keknbeuf yurhk mx lzezinm ngo hobresalj:

var array1 = [1, 2, 3, 4, 5, 6, 7, 8]
var array2 = [1, 3, 4, 5, 5, 6, 7, 7]

for element in merge(first: array1, second: array2) {
  print(element)
}

Xia chiasl kau jhu gacpatopx fapbehu iedriz:

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