Add trimmingSuffix, trimmingPrefix and mutating variants

[fedeci]

Description

Closes #101
trimming(while:) is split in two methods trimmingPrefix(while:) which is available to all Collections and trimmingSuffix(while:) available only to BidirectionalCollections.
This also adds a mutable variant of each trim method where SubSequence == Self.

Detailed Design

func trimmingSuffix(
  while predicate: (Element) throws -> Bool
) rethrows -> SubSequence {
  let end = try self[startIndex...].startOfSuffix(while: predicate)
  return self[startIndex..<end]
}

func trimmingPrefix(
  while predicate: (Element) throws -> Bool
) rethrows -> SubSequence {
  let start = try endOfPrefix(while: predicate)
  return self[start..<endIndex]
}

func trimming(
  while predicate: (Element) throws -> Bool
) rethrows -> SubSequence {
  return try trimmingPrefix(while: predicate).trimmingSuffix(while: predicate)
}

The same applies to mutable trims.

• String must access mutable trims

Documentation Plan

Docs not yet updated.

Test Plan

Since the two new methods are just a refactoring of trimming all those tests also apply to the new API.

• Mutating variants tests added.

Source Impact

It doesn't break anything.

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

[LucianoPAlmeida]

For what I understood from the issue we should also add an unconstrained version of trim, right @timvermeulen ?

[natecook1000]

Thanks for this, @fedeci! 👏

I have a few notes for you below, and we also need a couple overloads for each of the mutating methods. The current ones are available for RangeReplaceableCollection types; we can also do these operations on self-slicing types (e.g. where Self == Self.SubSequence). One such type is a Range<Int> — when you write (0..<10).dropFirst(3) you end up with the range 3..<10. Unfortunately, there are also self-slicing, range-replaceable types (like Substring and ArraySlice), so we'll also need overloads that have both constraints.

[fedeci]

Do Substring and ArraySlice inherit the mutating methods from BidirectionalCollection where Self: RangeReplaceableCollection, don't they?

[natecook1000]

@fedeci Looking good! A few notes below and then we can merge this addition.

[fedeci]

Are still missing documents in the docs/ folder.

[natecook1000]

@fedeci This looks ready to go! Would you like to add a skeleton guide and mark this as ready for review?

[fedeci]

Sure, just give me a couple of days because I am full of exams in this period.

---

edit: weeks probably. Sorry :(

[natecook1000]

@swift-ci Please test

[fedeci]

Here it is :)

[fedeci]

I added 0.3.0 supposing that this will be released in a next minor version, but I am not sure.

[natecook1000]

@swift-ci Please test

[natecook1000]

🎉