StringCollection

[CTMacUser]

Since String objects use dynamic memory, any collection of them that also uses dynamic memory will use up several patches of dynamic allocation. I wondered if we can reduce that memory allocation to one buffer. This is funnily similar to how String objects could be made up of UTF-8 bundles of data.

UTF-8 lay outs its data such that we can naturally find borders between code points. String objects can't have that, so I had to use marker values to indicate where a string's embedded data starts and ends, bloating the total size of the data stored.

Since a StringCollection stores elements with varying lengths, it cannot support RandomAccessCollection nor MutableCollection. Like String, StringCollection only supports up to BidirectionalCollection and RangeReplaceableCollection.

StringCollection uses a copy-on-write policy on its mutating operations. If a collection the only user of a string data block, the block will be manipulated directly, otherwise a computed clone is made.

StringCollection follows many other system operations (Comparable, CustomStringConvertible, Decodable, Encodeable, Equatable, ExpressibleByArrayLiteral, and Hashable).

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[MahdiBM]

I think it's worth linking to the Swift forums thread: https://forums.swift.org/t/a-memory-saving-type-for-collections-of-strings