[swift-evolution] About the PermutationGenerator

[Dave Abrahams]

> On Dec 31, 2015, at 7:46 PM, Susan Cheng <susan.doggie at gmail.com> wrote:
> 
> 
> How GeneratorType confirm to Equatable??

I don’t understand the question.  In the code I posted there’s a working example of how a GeneratorType model can conform to Equatable..

> 
> struct Fib : SequenceType {
>     
>     var a: Int
>     var b: Int
>     
>     var limit: Int
>     
>     func generate() -> FibGenerator {
>         return Generator(a: a, b: b, limit: limit)
>     }
> }
> 
> let c = Multipass(Fib(a: 1, b: -1, limit: 10))
> 
> -Susan
> 
> 
> 2016-01-01 11:17 GMT+08:00 Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>>:
> FWIW, Indexable is an implementation artifact that will go away when Swift’s generics system is improved.
> 
> But if your real objection is that you have to come up with an Index and a subscripting operator, I can understand that.  Part of the reason for this is our reluctance to create any distinct protocols with identical syntactic requirements <http://news.gmane.org/find-root.php?message_id=2A3E0C76-1C88-4752-8A70-AA64BB14223A@apple.com <http://news.gmane.org/find-root.php?message_id=2A3E0C76-1C88-4752-8A70-AA64BB14223A@apple.com>>.  To justify having a separate multi-pass sequence protocol, there would have to be a significant/important class of multi-pass sequences for which CollectionType was unimplementable without serious costs.
> 
> In principle there’s a way to ease the pain of creating CollectionType conformances for multipass SequenceTypes…if only it didn’t crash the compiler <https://bugs.swift.org/browse/SR-427 <https://bugs.swift.org/browse/SR-427>> ;-).  Here’s a variation that uses a generic adapter instead of a protocol conformance declaration:
> 
> /// A `CollectionType` containing the same elements as `Base`, without storing them.
> ///
> /// - Requires: `Base` supports multiple passes (traversing it does not
> ///   consume the sequence), and `Base.Generator` has value semantics
> public struct Multipass<Base: SequenceType where Base.Generator: Equatable> : CollectionType {
>   public var startIndex: MultipassIndex<Base> {
>     var g = _base.generate()
>     return MultipassIndex(buffer: g.next(), generator: g)
>   }
> 
>   public var endIndex: MultipassIndex<Base> {
>     return MultipassIndex(buffer: nil, generator: _base.generate())
>   }
> 
>   public subscript(position: MultipassIndex<Base>) -> Base.Generator.Element {
>     return position.buffer!
>   }
> 
>   public init(_ base: Base) {
>     _base = base
>   }
> 
>   var _base: Base
> }
> 
> // Note: Requires T.Generator has value semantics
> public struct MultipassIndex<T: SequenceType where T.Generator: Equatable> : ForwardIndexType {
>   public func successor() -> MultipassIndex {
>     var r = self
>     r.buffer = r.generator.next()
>     return r
>   }
>   var buffer: T.Generator.Element?
>   var generator: T.Generator
> }
> 
> public func == <T>(x: MultipassIndex<T>, y: MultipassIndex<T>) -> Bool {
>   return x.buffer == nil && y.buffer == nil || x.generator == y.generator
> }
> 
> //===--- An example fibonacci sequence ------------------------------------===//
> struct FibGenerator : GeneratorType {
>   mutating func next() -> Int? {
>     let c = a + b
>     a = b
>     b = c
>     return a < limit ? a : nil
>   }
>   var a, b, limit: Int
> }
> 
> 
> struct Fib : SequenceType {
>   var limit = 1000
> 
>   func generate() -> FibGenerator {
>     return Generator(a: 0, b: 1, limit: limit)
>   }
> }
> 
> //===--- Adapt Fib for use with Multipass ---------------------------------===//
> extension FibGenerator : Equatable {}
> func == (x: Fib.Generator, y: Fib.Generator) -> Bool {
>   return x.a == y.a
> }
> 
> //===--- Demonstration ----------------------------------------------------===//
> let c = Multipass(Fib())
> print(c.first)
> print(c.count)
> print(c.lazy.map { $0 + 1 })
> 

-Dave

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