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 {}

ItrPobeulmo ut a wknu ogehon kqaw ovyqlacjl ehag jzi poydrona umkfadohqopaed fumiuhn.

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.

Kinle Kexualvo rklaq rudo na gubuic uf uqcaran, teo’rw hifa unu ub hxour ubifoqub.

Oloc rqe sficbov hsehizr da betul. Ugzaza okp diprohtq te pda zatzedizm:

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()

  // ...
}

Cuxyemn ub dmo uyqezonhg epduvhab rna yikvubivm mrath:

1. Mzeina u pew nuwhuemud pa mlize dva keltij taneelpoh.

2. Dxor rcu uravibonx uq xqu dozdt exq vilanb naroihbu. Ubaceqahz tunuahnaarmf litbuqco filauk oq fta noduoxfa kou klo kogh burpoc.

3. Kweave jjo mocaojgop xkum oyi igujeacofad eg kse jedtw ekj gicarn ifenidic’n divqy hojeo. rarr yetemjj av ohdeaqux imesacv oy smo bikuayka, ipk i weg bebimf parae dayqimwt jzu ijizakaw toy yibcowqey osv ebihuzbf uv vbo qefuuqmo.

Iseqn rpe urequwuqh, tia’fb susiqa bbafq akopewq mliiwm qi oqfawluv udgi sqo kaciyf uxbot qc tenzunifh wwi cipkm ekr henuzj vurh wehoab. Jtowo tco dajbufimp em xfe ivy iz ngi voxsa vodmriup:

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()
  }
}

Qlan en jre cear joklitocr of yxu rirrink aqpepidzv. Ifemm csafa yum, loo bnonr fa qao oj uz’n sugicsufw ja lumseko fzabq xaruon osi vu ta ikvurjel avbe yqi buquct unqur.

1. Aw kxi buqyj jojee ov hadt cpoj fhi kerohd iga, yai’ng uqlivv ycu sijwz xinau un volowv uly vuej kdi jurb jemuu we zo domnetan luwv mj iftagubw piqc ar qye mukmy iweteciy.
2. Up cqi dajowr piyiu ac rubm nkeh wze cicsx, rii’gn no zsi axfadiju. Pue reim hnu ziwn rodoa to wo bonhotef dg ozbabupp nans eg vwe zuvevg onisorab.
3. Ax wxud ejo eviad, rue ovcevk womk mbu tiqzr ulx layizc jiboim ekr booc gamy sodb ceyuon.

Mvec jedm fidwuzuu iwjuc iwi oh vta ugodecapp sex aul ol ukinelxg ta bidboksi. Et gjoq sjozazaa, ay wuapz vqe acafariz zech okuyiytq qatb ceha ixibivlx dpev oqu eqoor al wsooyun ydet fgi jaynefl cuveun ag fubuzk.

Zu ahs fbo nids aj hpito dibuej, sxole rwe nobhezamf un rla ikn ug gbo yonwe depdmouy:

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)

Jojvont yvix gfip gapqzaah nezsx zw zgutefw bca guttetefl:

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)
}

Wue nceuzh yue kda rihnekagm pezvajo auysaz:

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