사용자 지정 반복자와 성능 팁

사용자 지정 반복자 입문

IteratorProtocol 예시

struct CountdownIterator: IteratorProtocol {
    var current: Int
    mutating func next() -> Int? {
        guard current >= 0 else { return nil }
        defer { current -= 1 }   // advance state
        return current
    }
}

var it = CountdownIterator(current: 3)
while let n = it.next() { print(n, terminator: " ") }  // 3 2 1 0
print("")

시퀀스 래퍼

struct Countdown: Sequence {
    let start: Int
    func makeIterator() -> CountdownIterator {
        CountdownIterator(current: start)
    }
}

for n in Countdown(start: 5) {
    print(n, terminator: " ")   // 5 4 3 2 1 0
}
print("")

AnyIterator 단축 방법

// Build an iterator from a closure
var i = 1
let anyIt = AnyIterator<Int> {
    guard i <= 3 else { return nil }
    defer { i += 1 }
    return i * 10
}
let numbers = AnySequence(anyIt)
for x in numbers { print(x, terminator: " ") } // 10 20 30
print("")

작업을 줄이는 지연 처리

let big = Array(1...10_000)
// Eager: builds two intermediate arrays
let eager = big.map { $0 * 2 }.filter { $0 % 3 == 0 }
// Lazy: no temporaries; work is done during collection
let lazyView = big.lazy.map { $0 * 2 }.filter { $0 % 3 == 0 }
let result = Array(lazyView.prefix(5)) // only first 5 actually computed
print(result)

성능 팁

반복자 규칙

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