[swift-evolution] About the PermutationGenerator
Susan Cheng
susan.doggie at gmail.com
Thu Dec 31 23:30:07 CST 2015
I didn't explain correctly.
let's take this:
let c = Multipass(Fib(a: 1, b: -1, limit: 10))
this sequences should have results with [1, -1, 0, -1, -1, ...]
So is c.startIndex.successor() equal to
c.startIndex.successor().successor().successor()??
Dave Abrahams <dabrahams at apple.com> 於 2016年1月1日星期五 寫道：
>
> On Dec 31, 2015, at 8:22 PM, 鄭齊峯 via swift-evolution <
> swift-evolution at swift.org
> <javascript:_e(%7B%7D,'cvml','swift-evolution at swift.org');>> wrote:
>
> if you try my modification, it will crash.
>
>
> Only because your Sequence genereates an arithmetic underflow. I don’t
> understand what point you’re trying to make
>
>
> 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))
>
> A SequenceType becomes a CollectionType only work by adding a Index. that
> the most significantly different of sequence and collection.
>
>
> I think I understand that pretty well, having designed both of those
> abstractions, but still don’t get your point. Sorry, I’m trying here, but
> I can’t figure out what you’re trying to tell me.
>
>
> /// 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> : CollectionType {
> public var startIndex: MultipassIndex<Base> {
> var g = _base.enumerate().generate()
> if let (idx, val) = g.next() {
> return MultipassIndex(index: idx, buffer: val, generator: g)
> }
> return MultipassIndex(index: nil, buffer: nil, generator: g)
> }
>
> public var endIndex: MultipassIndex<Base> {
> return MultipassIndex(index: nil, buffer: nil, generator: _base.
> enumerate().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> : ForwardIndexType {
> public func successor() -> MultipassIndex {
> var r = self
> if let (idx, val) = r.generator.next() {
> r.index = idx
> r.buffer = val
> } else {
> r.index = nil
> r.buffer = nil
> }
> return r
> }
> var index: Int?
> var buffer: T.Generator.Element?
> var generator: EnumerateSequence<T>.Generator
> }
>
> public func == <T>(x: MultipassIndex<T>, y: MultipassIndex<T>) -> Bool {
> return x.index == y.index
> }
>
> //===--- 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 = 100
>
> func generate() -> FibGenerator {
> return Generator(a: 0, b: 1, limit: limit)
> }
> }
>
> It's true that ForwardIndexType is also run-through the base one by one
> but it also correct that getting specific value with specific index.
>
> c[c.startIndex.successor().successor().successor()] // it's a defined
> behave
>
>
> Again, not sure what you’re trying to say here.
>
>
>
> Dave Abrahams <dabrahams at apple.com
> <javascript:_e(%7B%7D,'cvml','dabrahams at apple.com');>> 於 2016年1月1日
> 下午12:00 寫道：
>
>
> On Dec 31, 2015, at 7:46 PM, Susan Cheng <susan.doggie at gmail.com
> <javascript:_e(%7B%7D,'cvml','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
> <javascript:_e(%7B%7D,'cvml','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>.
>> 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> ;-). 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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>
>
> -Dave
>
>
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