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Non-empty collection in Swift
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protocol NonemptyCollection: Collection { | |
var first: Iterator.Element { get } | |
} | |
enum NonemptyIndex<Base: Collection>: Comparable { | |
case head | |
case tail(Base.Index) | |
static func ==(lhs: NonemptyIndex, rhs: NonemptyIndex) -> Bool { | |
switch (lhs,rhs) { | |
case (.head,.head): | |
return true | |
case let (.tail(i),.tail(j)): | |
return i == j | |
default: | |
return false | |
} | |
} | |
static func <(lhs: NonemptyIndex, rhs: NonemptyIndex) -> Bool { | |
switch (lhs,rhs) { | |
case (.head,.head): | |
return true | |
case let (.tail(i),.tail(j)): | |
return i < j | |
default: | |
return false | |
} | |
} | |
} | |
struct NonemptyCollectionWrapper<Base: Collection>: NonemptyCollection { | |
let head: Base.Iterator.Element | |
let tail: Base | |
typealias Index = NonemptyIndex<Base> | |
var startIndex: Index { return .head } | |
var endIndex: Index { return .tail(tail.endIndex) } | |
subscript(idx: Index) -> Base.Iterator.Element { | |
switch idx { | |
case .head: return head | |
case let .tail(tailIdx): | |
return tail[tailIdx] | |
} | |
} | |
func index(after: Index) -> Index { | |
switch after { | |
case .head: return .tail(tail.startIndex) | |
case let .tail(tailIdx): | |
return .tail(tail.index(after: tailIdx)) | |
} | |
} | |
var first: Base.Iterator.Element { return head } | |
} | |
extension NonemptyCollectionWrapper where Base: BidirectionalCollection { | |
var last: Base.Iterator.Element { return tail.last ?? head } | |
} | |
let list = NonemptyCollectionWrapper(head: 1, tail: [2,3]) | |
let x: Int = list.first | |
let y: Int = list.last | |
func f<C: NonemptyCollection>(_ c: C) where C.Iterator.Element == Int { | |
let i: Int = c.first | |
print(i) | |
} | |
f(list) | |
let a = Array(list) | |
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