Sequences with IteratorProtocol examples

collection_protocols_sequences.playground

import Foundation

// MARK: - Sequences
struct ConstantIterator: IteratorProtocol {
    typealias Element = Int
    
    mutating func next() -> Element? {
        return 1
    }
}

//Example from infinity of 1
//var iterator = ConstantIterator()
//while let x = iterator.next() {
//    print(x)
//}

struct FibsIterator: IteratorProtocol {
    var state = (0,1)
    
    mutating func next() -> Int? {
        let upcomingNumber = state.1
        state = (state.1, state.0 + state.1)
        return upcomingNumber
    }
}

//Example from 1 to Int.max
//var iterator =  FibsIterator()
//while let x = iterator.next() {
//    print(x)
//}

struct FibSequence: Sequence {
     func makeIterator() -> FibsIterator {
        return FibsIterator()
    }
}

//Example from 1 to prefix element
for i in FibSequence().prefix(10) {
    print(i)
}
//1,1,2,3,5,8,13,21,34,55

struct PrefixIterator: IteratorProtocol {
    typealias Element = Substring
    let string: String
    var offset: String.Index
    
    init(string: String) {
        self.string = string
        offset = string.startIndex
    }
    
    mutating func next() -> Element? {
        guard offset < string.endIndex else { return nil }
        offset = string.index(after: offset)
        return string[..<offset]
    }
}


struct PrefixSequence: Sequence {
    let string: String
    
    func makeIterator() -> PrefixIterator {
        return PrefixIterator(string: string)
    }
}

// Example of string slising
for prefix in PrefixSequence(string: "Hello") {
    print(prefix)
}
//H,He,Hel,Hell,Hello

// Example of mapping
print(PrefixSequence(string: "Hello").map { $0.uppercased() })
//["H", "HE", "HEL", "HELL", "HELLO"]