SequenceとCollectionの階層

let seq = [1, 2, 3] as AnySequence<Int>    // wrap as AnySequence for demo
for x in seq { print(x, terminator: " ") }
print("")

let arr = ["a","b","c"]          // Array is a Collection
print(arr.startIndex == 0)            // true for Array
print(arr.endIndex == 3)              // endIndex is "one past last"
for i in arr.indices { print(arr[i], terminator: " ") }
print("")
// Iterate again (multi-pass)
for ch in arr { print(ch, terminator: " ") }
print("")

let text = "Swift"                         // String is a BidirectionalCollection
var i = text.index(before: text.endIndex)       // move backward
print(text[i])                                   // last character
// Array is RandomAccessCollection: jumping indices is cheap
let nums = [10,20,30,40]
print(nums[nums.startIndex.advanced(by: 2)])     // 30

struct Countdown: Sequence, IteratorProtocol {
    var current: Int
    mutating func next() -> Int? {
        guard current >= 0 else { return nil }
        defer { current -= 1 }
        return current
    }
}
var cd = Countdown(current: 3)
for n in cd { print(n, terminator: " ") }   // 3 2 1 0
print("")
// cd is exhausted; iterating again yields nothing (single-pass behavior)