Add commonPrefix(with:), commonSuffix(with:)

[timvermeulen]

Find the common prefix/suffix of two sequences or collections.

• commonPrefix(with:)
• commonSuffix(with:)
• endOfCommonPrefix(with:): get a pair of indices that mark the end of the common prefix of two collections
• startOfCommonSuffix(with:): get a pair of indices that mark the start of the common suffix of two collections

extension Sequence {
  public func commonPrefix<Other: Sequence>(
    with other: Other,
    by areEquivalent: (Element, Other.Element) throws -> Bool
  ) rethrows -> [Element]
}

extension Sequence where Element: Equatable {
  public func commonPrefix<Other: Sequence>(
    with other: Other
  ) -> CommonPrefix<Self, Other> where Other.Element == Element
}

extension LazySequenceProtocol {
  public func commonPrefix<Other: Sequence>(
    with other: Other,
    by areEquivalent: @escaping (Element, Other.Element) -> Bool
  ) -> CommonPrefix<Self, Other>
}

extension Collection {
  public func commonPrefix<Other: Sequence>(
    with other: Other,
    by areEquivalent: (Element, Other.Element) throws -> Bool
  ) rethrows -> SubSequence
}

extension Collection where Element: Equatable {
  public func commonPrefix<Other: Sequence>(
    with other: Other
  ) -> SubSequence where Other.Element == Element
}

extension LazyCollectionProtocol where Element: Equatable {
  public func commonPrefix<Other: Sequence>(
    with other: Other
  ) -> CommonPrefix<Self, Other> where Other.Element == Element
}

extension BidirectionalCollection {
  public func commonSuffix<Other: BidirectionalCollection>(
    with other: Other,
    by areEquivalent: (Element, Other.Element) throws -> Bool
  ) rethrows -> SubSequence
}

extension BidirectionalCollection where Element: Equatable {
  public func commonSuffix<Other: BidirectionalCollection>(
    with other: Other
  ) -> SubSequence where Other.Element == Element
}

extension Collection {
  public func endOfCommonPrefix<Other: Collection>(
    with other: Other,
    by areEquivalent: (Element, Other.Element) throws -> Bool
  ) rethrows -> (Index, Other.Index)
}

extension Collection where Element: Equatable {
  public func endOfCommonPrefix<Other: Collection>(
    with other: Other
  ) -> (Index, Other.Index) where Other.Element == Element
}

extension BidirectionalCollection {
  public func startOfCommonSuffix<Other: BidirectionalCollection>(
    with other: Other,
    by areEquivalent: (Element, Other.Element) throws -> Bool
  ) rethrows -> (Index, Other.Index)
}

extension BidirectionalCollection where Element: Equatable {
  public func startOfCommonSuffix<Other: BidirectionalCollection>(
    with other: Other
  ) -> (Index, Other.Index) where Other.Element == Element
}

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[kylemacomber]

I think it's more natural to not pluralize of start and end, i.e. just have endOfCommonPrefix, startOfCommonSuffix

[timvermeulen]

I think it's more natural to not pluralize of start and end, i.e. just have endOfCommonPrefix, startOfCommonSuffix

Fair point. I think I went with this because we already have endOfPrefix(while:) and startOfSuffix(while:) which both return a single index, but that's probably fine.

[CTMacUser]

I think the iterator needs to add a done flag to its state. For instance, if we do "1234A56" vs. "1234B56", and we iterate past the first nil, we'll get a "5", instead of nil forevermore.

[timvermeulen]

Thanks, you're absolutely right!

[CTMacUser]

Shouldn't these be using Elements and elements instead of Self and self?

[timvermeulen]

That's a way to do it, but this is intentional. If you wrap elements instead of self then the type no longer contains any sign of laziness, so you'll need to wrap it in LazySequence<...> again to add the laziness back. But by wrapping self, all we need is for CommonPrefix to conditionally conform to LazySequenceProtocol when the (first) base collection does, in order to propagate the laziness.

CommonPrefix also can't unconditionally conform to LazySequenceProtocol because it's also returned by the Sequence overloads, so in a non-lazy context.

[CTMacUser]

Same as in the LazySequenceProtocol section: shouldn't these be using Elements and elements instead of Self and self?

[CTMacUser]

Don't endsOfCommonPrefix(with:) and startssOfCommonSuffix(with:) overlap with diverges(from:) and converges(with:) from #37 ?