Hi Euan I think it’s great that you’re trying this. I hope you know what you’re getting yourself into :)
I’m no Unicode expert but I want to add two points to your list (although you’ve probably already thought of them): - Normalisation and conversion (http://unicode.org/faq/normalization.html). Unicode / ICU provide libraries (libuconv / libiconv) that handle this stuff. Specifically normalisation conversions aren’t trivial and I think it wouldn’t make much sense to reimplement those algorithms. I do think however, that having them available is important (where I work we’re currently writing a VM plugin for access to libiconv through primitives so that we can clean out combining characters through normalisation. And we’ll obviously get nice sorting properties and speeds for free) - Sorting and comparison. Basically the same point as above. libuconv / libiconv provide algorithms for this. Do we need our own implementation? Cheers, Max > On 04 Dec 2015, at 12:42, EuanM <euan...@gmail.com> wrote: > > I'm currently groping my way to seeing how feature-complete our > Unicode support is. I am doing this to establish what still needs to > be done to provide full Unicode support. > > This seems to me to be an area where it would be best to write it > once, and then have the same codebase incorporated into the Smalltalks > that most share a common ancestry. > > I am keen to get: equality-testing for strings; sortability for > strings which have ligatures and diacritic characters; and correct > round-tripping of data. > > Call to action: > ========== > > If you have comments on these proposals - such as "but we already have > that facility" or "the reason we do not have these facilities is > because they are dog-slow" - please let me know them. > > If you would like to help out, please let me know. > > If you have Unicode experience and expertise, and would like to be, or > would be willing to be, in the 'council of experts' for this project, > please let me know. > > If you have comments or ideas on anything mentioned in this email > > In the first instance, the initiative's website will be: > http://smalltalk.uk.to/unicode.html > > I have created a SqueakSource.com project called UnicodeSupport > > I want to avoid re-inventing any facilities which already exist. > Except where they prevent us reaching the goals of: > - sortable UTF8 strings > - sortable UTF16 strings > - equivalence testing of 2 UTF8 strings > - equivalence testing of 2 UTF16 strings > - round-tripping UTF8 strings through Smalltalk > - roundtripping UTF16 strings through Smalltalk. > As I understand it, we have limited Unicode support atm. > > Current state of play > =============== > ByteString gets converted to WideString when need is automagically detected. > > Is there anything else that currently exists? > > Definition of Terms > ============== > A quick definition of terms before I go any further: > > Standard terms from the Unicode standard > =============================== > a compatibility character : an additional encoding of a *normal* > character, for compatibility and round-trip conversion purposes. For > instance, a 1-byte encoding of a Latin character with a diacritic. > > Made-up terms > ============ > a convenience codepoint : a single codepoint which represents an item > that is also encoded as a string of codepoints. > > (I tend to use the terms compatibility character and compatibility > codepoint interchangably. The standard only refers to them as > compatibility characters. However, the standard is determined to > emphasise that characters are abstract and that codepoints are > concrete. So I think it is often more useful and productive to think > of compatibility or convenience codepoints). > > a composed character : a character made up of several codepoints > > Unicode encoding explained > ===================== > A convenience codepoint can therefore be thought of as a code point > used for a character which also has a composed form. > > The way Unicode works is that sometimes you can encode a character in > one byte, sometimes not. Sometimes you can encode it in two bytes, > sometimes not. > > You can therefore have a long stream of ASCII which is single-byte > Unicode. If there is an occasional Cyrillic or Greek character in the > stream, it would be represented either by a compatibility character or > by a multi-byte combination. > > Using compatibility characters can prevent proper sorting and > equivalence testing. > > Using "pure" Unicode, ie. "normal encodings", can cause compatibility > and round-tripping probelms. Although avoiding them can *also* cause > compatibility issues and round-tripping problems. > > Currently my thinking is: > > a Utf8String class > an Ordered collection, with 1 byte characters as the modal element, > but short arrays of wider strings where necessary > a Utf16String class > an Ordered collection, with 2 byte characters as the modal element, > but short arrays of wider strings > beginning with a 2-byte endianness indicator. > > Utf8Strings sometimes need to be sortable, and sometimes need to be > compatible. > > So my thinking is that Utf8String will contain convenience codepoints, > for round-tripping. And where there are multiple convenience > codepoints for a character, that it standardises on one. > > And that there is a Utf8SortableString which uses *only* normal characters. > > We then need methods to convert between the two. > > aUtf8String asUtf8SortableString > > and > > aUtf8SortableString asUtf8String > > > Sort orders are culture and context dependent - Sweden and Germany > have different sort orders for the same diacritic-ed characters. Some > countries have one order in general usage, and another for specific > usages, such as phone directories (e.g. UK and France) > > Similarly for Utf16 : Utf16String and Utf16SortableString and > conversion methods > > A list of sorted words would be a SortedCollection, and there could be > pre-prepared sortBlocks for them, e.g. frPhoneBookOrder, deOrder, > seOrder, ukOrder, etc > > along the lines of > aListOfWords := SortedCollection sortBlock: deOrder > > If a word is either a Utf8SortableString, or a well-formed Utf8String, > then we can perform equivalence testing on them trivially. > > To make sure a Utf8String is well formed, we would need to have a way > of cleaning up any convenience codepoints which were valid, but which > were for a character which has multiple equally-valid alternative > convenience codepoints, and for which the string currently had the > "wrong" convenience codepoint. (i.e for any character with valid > alternative convenience codepoints, we would choose one to be in the > well-formed Utf8String, and we would need a method for cleaning the > alternative convenience codepoints out of the string, and replacing > them with the chosen approved convenience codepoint. > > aUtf8String cleanUtf8String > > With WideString, a lot of the issues disappear - except > round-tripping(although I'm sure I have seen something recently about > 4-byte strings that also have an additional bit. Which would make > some Unicode characters 5-bytes long.) > > > (I'm starting to zone out now - if I've overlooked anything - obvious, > subtle, or somewhere in between, please let me know) > > Cheers, > Euan >