Subscripts e índices personalizados, revisados

struct TimesTable {
    let multiplier: Int
    subscript(index: Int) -> Int { multiplier * index } // read-only
}
let t = TimesTable(multiplier: 3)
print(t[4]) // 12

struct SafeArray<T> {
    private var storage: [T]
    init(_ xs: [T]) { self.storage = xs }
    subscript(i: Int) -> T? {
        get { (0..<storage.count).contains(i) ? storage[i] : nil }
        set {
            guard let v = newValue, (0..<storage.count).contains(i) else { return }
            storage[i] = v
        }
    }
}
var s = SafeArray([10,20,30])
print(s[1] ?? -1) // 20
s[1] = 99
print(s[1] ?? -1) // 99
s[5] = 7          // ignored (out of range)

struct Matrix {
    let rows: Int, cols: Int
    private var grid: [Int]
    init(_ r: Int, _ c: Int, fill: Int = 0) {
        rows = r; cols = c; grid = Array(repeating: fill, count: r * c)
    }
    private func valid(_ r: Int, _ c: Int) -> Bool { r >= 0 && r < rows && c >= 0 && c < cols }
    subscript(row r: Int, col c: Int) -> Int {
        get { precondition(valid(r,c), "Index out of range"); return grid[r*cols + c] }
        set { precondition(valid(r,c), "Index out of range"); grid[r*cols + c] = newValue }
    }
}
var m = Matrix(2, 3)
m[row: 0, col: 1] = 5
print(m[row: 0, col: 1]) // 5

enum Day: Int, CaseIterable { case mon = 0, tue, wed, thu, fri, sat, sun }
struct Week {
    private var tasks = Array(repeating: "—", count: Day.allCases.count)
    subscript(day: Day) -> String {
        get { tasks[day.rawValue] }
        set { tasks[day.rawValue] = newValue }
    }
}
var w = Week()
w[.wed] = "Ship"
print(w[.wed])