In ICU, setVariableTop() has a documented limitation: It requires that the primary weight has only 1 or 2 bytes. Until a few years ago, this was true for most characters. Since then, Unicode added many more characters and we ran out of space for 2-byte weights, given our constraints. So we use 3-byte primaries for the majority of characters now. See this doc from a few years ago: http://site.icu-project.org/design/collation/uca-weight-allocation
Unfortunately, this makes setVariableTop() not work with most characters<http://bugs.icu-project.org/trac/ticket/8103>. I believe we have not had any real bug reports about this. I think that means that very few people care to change the variable top. As Richard discovered, in UCA+DUCET with 16-bit weights, the same sort of limitation could be applied, requiring that the primary weight not use the Large Weight Values<http://www.unicode.org/draft/reports/tr10/tr10.html#Large_Weight_Values> mechanism (that is, that it fits into a single 16-bit weight). However, it does not make a lot of sense to set the variable top to something above the currency symbols range -- it's basically an option for an "ignore punctuation" mode, and you wouldn't want to ignore nearly every assigned character in Unicode. I am not even sure it makes sense to set it to a currency symbol. In UCA+DUCET, that means that any sensible variable-top value does not use the large-weight-values mechanism anyway. Also, the highest assigned primary in the UCA spec is FFFD, not the last-explicitly-mentioned mapping's primary. Remember that Han and unassigned code points get implicit weights, and we have special weights. In my pending ICU collation rewrite, I do not use the large-weight-values mechanism any more, so the new code would permit a 4-byte variableTop. However, we have agreed to replace the hard-to-use variableTop attribute with an easy-to-use maxVariable attribute, so this whole discussion will become moot at that point: http://unicode.org/cldr/trac/ticket/5016 Best regards, markus -- Google Internationalization Engineering
