[swift-dev] Quick pitch: Change Linux’s string comparison to match Darwin’s

[Michael Ilseman]

Unfortunately after some investigations and discussion, the situation seems to be more murky. This approach would break transitivity of String comparison on Linux, at least with any implementation of UCA using the normal collation weights. A < B, B < C should imply A < C. But, if both A and B are known-ASCII while C is UTF16, transitivity can be broken for any character that UCA yields a different sort order for (e.g. “#” vs “&”). On Darwin, the comparison implementation happens to preserve transitivity as the platform (in effect) relatively weights ASCII by code unit values.

While I would like to get some performance improvements in time for Linux, I don’t think this approach is viable for Swift 4.0. Unless anyone has any ideas about another minimally invasive approach, my recommendation is to do the long-term solution (lexicographical order of normalized code units) immediately after Swift 4.0.


> On Jul 25, 2017, at 2:01 PM, Michael Ilseman via swift-dev <swift-dev at swift.org> wrote:
> 
> 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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