On Darwin, known-ASCII strings are sorted according to the lexicographical
ordering of their code units. All non-known-ASCII strings are otherwise ordered
based on the UCA[1]. On Linux, however, even known-ASCII strings are ordered
based on UCA. I propose to unify these by changing Linux’s string sort order to
match Darwin’s in Swift 4.0.
Background
Swift’s default ordering for strings is appropriate for machine consumption
(e.g. implementing sorted collections). It obeys Unicode canonical
equivalence[2], that is strings compare the same modulo normalization. However,
it is not meant to be sufficient for presenting a meaningful ordering to human
consumers, as that requires incorporating reader-specific information (e.g.
[3]).
Known-ASCII strings are a trivial case for the described sort order semantics:
pure ASCII is unaffected by normalization. Thus, lexicographical ordering of
code units is a valid machine ordering for ASCII strings. On Darwin, this is
used to order known-ASCII strings while Linux uses UCA even for known-ASCII
strings.
Long term, the plan is to switch String’s sort order to be the lexicographical
ordering of normalized code units (or perhaps scalar values), as mentioned in
the String Manifesto[4]. This is a more efficient ordering than that provided
by UCA. However, this will not make it in time for Swift 4.0.
Changes
I propose to change Linux’s sort order for known-ASCII strings to be the same
as it is on Darwin. This will be accomplished by dropping the relevant #if
guards in StringCompare.swift. An example implementation can be found at [5].
In addition to unifying sort order semantics across platforms, this will also
deliver significant performance boosts to pure ASCII strings on Linux.
[1] UTS #10: Unicode Collation Algorithm <http://unicode.org/reports/tr10/>
[2] Canonical Equivalence in Applications <http://unicode.org/notes/tn5/>
[3] UCA: Contextual Sensitivity
<http://unicode.org/reports/tr10/#Contextual_Sensitivity>
[4] String Manifesto: Comparing and Hashing Strings
<https://github.com/apple/swift/blob/master/docs/StringManifesto.md#comparing-and-hashing-strings>
[5] Unifying Linux/Darwin ASCII sort order semantics - github
<https://github.com/milseman/swift/commit/5560e13198d5cc284f46bf190f59a2edf7ed747b>
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