How to stitch and sort array in Swift

Issue #672

Supposed we want to stitch magazines array into books array. The requirement is to sort them by publishedDate, but must keep preferredOrder of books. One way to solve this is to declare an enum to hold all possible cases, and then do a sort that check every possible combination

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struct Book {
let preferredOrder: Int
let publishedDate: Date
}

struct Magazine {
let publishedDate: Date
}

enum StitchItem {
case book(Book)
case magazine(Magazine)
}

func stitch(_ books: [Book], magazines: [Magazine]) -> [StitchItem] {
let items = books.map({ StitchItem.book($0) }) + magazines.map({ StitchItem.magazine($0) })
return items.sorted(by: { book, magazine in
switch (book, magazine) {
case let (.book(b1), .book(b2)):
return b1.preferredOrder < b2.preferredOrder
case let (.book(book), .magazine(magazine)):
if book.publishedDate == magazine.publishedDate {
return true
} else {
return book.publishedDate < magazine.publishedDate
}
case let (.magazine(magazine), .book(book)):
if book.publishedDate == magazine.publishedDate {
return false
} else {
return book.publishedDate < magazine.publishedDate
}
case let (.magazine(m1), .magazine(m2)):
return m1.publishedDate < m2.publishedDate
}
})
}

The above sort function declares the intention but Swift just sort instead of trying to fully meet our requirements.

A manual solution is to sort each array first then use while loop to insert.

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func stitch(_ books: [Book], magazines: [Magazine]) -> [StitchItem] {
let books = books
.sorted(by: { $0.preferredOrder < $1.preferredOrder })
let magazines = magazines
.sorted(by: sortmagazines)

var bookIndex = 0
var magazineIndex = 0
var results: [StitchItem] = []
while (bookIndex < books.count && magazineIndex < magazines.count) {
let book = books[bookIndex]
let magazine = magazines[magazineIndex]
if book.publishedDate < magazine.publishedDate {
results.append(StitchItem.book(book))
bookIndex += 1
} else {
results.append(StitchItem.magazine(magazine))
magazineIndex += 1
}
}

while (bookIndex < books.count) {
let book = books[bookIndex]
results.append(StitchItem.book(book))
bookIndex += 1
}

while (magazineIndex < magazines.count) {
let magazine = magazines[magazineIndex]
results.append(StitchItem.magazine(magazine))
magazineIndex += 1
}

return results
}

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