1. 21 st International Unicode Conference Dublin, Ireland, May 2002 1 Optimizing the Usage of Normalization Vladimir Weinstein vweinste@us.ibm.com Globalization Center of Competency, San Jose, CA

2. 21 st International Unicode Conference Dublin, Ireland, May 2002 2 Introduction 1.Unicode standard has multiple ways to encode equivalent strings résumére sumére sume NFD:  NFC:  résume  2.Accents that don’t interact are put into a unique order  

3. 21 st International Unicode Conference Dublin, Ireland, May 2002 3 Introduction (contd.) Normalization provides a way to transform a string to an unique form (NFD, NFC) Strings that can be transformed to the same form are called canonically equivalent Time-critical applications need to minimize the number of passes over the text ICU gives a number of tools to deal with this problem We will use collation (language-sensitive string comparison) as an example

4. 21 st International Unicode Conference Dublin, Ireland, May 2002 4 Avoiding Normalization Force users to provide already normalized data The performance problem does not go away When the strings are processed many times, it could be beneficial to normalize them beforehand Forcing users to provide a specific form can be unpopular

5. 21 st International Unicode Conference Dublin, Ireland, May 2002 5 Check for Normalized Text Most strings are already in normalized form Quick Check is significantly faster than the full normalization Needs canonical class data and additional data for checking the relation between a code point and a normalization form Algorithm in UAX #15 Annex 8 (http://www.unicode.org/unicode/reports/tr15/ #Annex8)http://www.unicode.org/unicode/reports/tr15/ #Annex8

6. 21 st International Unicode Conference Dublin, Ireland, May 2002 6 Normalize Incrementally Instead of normalizing the whole string at once, normalize one piece at a time This technique is usually combined with an incremental Quick Check Useful for procedures with early exit, such as string comparing or scanning Normalizes up to the next safe point

7. 21 st International Unicode Conference Dublin, Ireland, May 2002 7 Incremental Normalization: Example re sume résumé re sumerésumé Initial string Normalize just the parts that fail quick check Non incremental normalization Quick check Incremental normalization If normalized regularly, the whole string is processed by normalization

8. 21 st International Unicode Conference Dublin, Ireland, May 2002 8 Optimized Concatenation Simple concatenation of two normalized strings can yield a string that is not normalized One option is to normalize the result Unnecessarily duplicates normalization

9. 21 st International Unicode Conference Dublin, Ireland, May 2002 9 Optimized Concatenation: Example Find boundariesConcatenate then normalize Concatenate and normalize up to the boundaries re sumé + re sumé résumé rsumé + e r e résumé It is enough to normalize the boundary parts Incremental normalization is used Much faster than redoing the whole resulting string

10. 21 st International Unicode Conference Dublin, Ireland, May 2002 10 Accepting the FCD Form Fast Composed or Decomposed form is a partially normalized form Not unique More lenient than NFD or NFC form It requires that the procedure has support for all the canonically equivalent strings on input It is possible to quick check the FCD format

11. 21 st International Unicode Conference Dublin, Ireland, May 2002 11 FCD Form: Examples SEQUENCEFCDNFCNFD A-ringYY AngstromY A + ringYY A + graveYY A-ring + graveY A + cedilla + ringYY A + ring + cedilla A-ring + cedillaY

12. 21 st International Unicode Conference Dublin, Ireland, May 2002 12 Canonical Closure Preprocessing data to support the FCD form Ensures that if data is assigned to a sequence (or a code point) it will also be assigned to all canonically equivalent FCD sequences Å = X A+ = X Å = X, => A-ring (U+00C5) Angstrom sign (U+212B) A + combining ring above (U+0041 U+030A)

13. 21 st International Unicode Conference Dublin, Ireland, May 2002 13 Collation Locale specific sorting of strings Relation between code points and collation elements Context sensitive: –Contractions: H < Z, but CZ < CH –Expansions: OE < Œ < OF –Both: カー キイ See “Collation in ICU” by Mark Davis

14. 21 st International Unicode Conference Dublin, Ireland, May 2002 14 Collation Implementation in ICU Two modes of operation: –Normalization OFF: expects the users to pass in FCD strings –Normalization ON: accepts any strings Some locales require normalization to be turned on Canonical closure done for contractions and regular mappings Two important services –Sort key generation –String compare function More about ICU at the end of presentation

15. 21 st International Unicode Conference Dublin, Ireland, May 2002 15 FCD Support in Collation Much higher performance Values assigned to a code point or a contraction are equal to those for its FCD canonically equivalent sequences This process is time consuming, but it is done at build time May increase data set

16. 21 st International Unicode Conference Dublin, Ireland, May 2002 16 Sort Key Generation Whole strings are processed Sort keys tend to get reused, so the emphasis is on producing as short sort keys as possible Two modes of operation –Normalization ON: strings are quick checked and normalization is performed, if required –Normalization OFF: depends on strings being in FCD form. The performance increases by 20% to 50%

17. 21 st International Unicode Conference Dublin, Ireland, May 2002 17 String Compare Very time critical Result is usually determined before fully processing both strings First step is binary comparison for equality When it fails, comparison continues from a safe spot A Å No need to backup, normal situation c h c z Must backup to the start of contraction Must backup to the normalization safe spot

18. 21 st International Unicode Conference Dublin, Ireland, May 2002 18 String Compare Continued Normalization ON: incremental FCD check and incremental FCD normalization if required Normalization OFF: assumes that the source strings are FCD Most locales don’t require normalization on and thus are 20% faster by using FCD

19. 21 st International Unicode Conference Dublin, Ireland, May 2002 19 International Components for Unicode International Components for Unicode(ICU) is a library that provides robust and full-featured Unicode support The ICU normalization engine supports the optimizations mentioned here Library services accept FCD strings as input Wide variety of supported platforms Open source (X license – non-viral) C/C++ and JAVA versions http://oss.software.ibm.com/icu/

20. 21 st International Unicode Conference Dublin, Ireland, May 2002 20 Conclusion The presented techniques allow much faster string processing In case of collation, sort key generation gets up to 50% faster than if normalizing beforehand String compare function becomes up to 3 times faster! May increase data size Canonical closure preprocessing takes more time to build, but pays off at runtime

21. 21 st International Unicode Conference Dublin, Ireland, May 2002 21 Q & A

22. 21 st International Unicode Conference Dublin, Ireland, May 2002 22 Summary Introduction Avoiding normalization Check for normalized text Normalize incrementally Concatenation of normalized strings Accepting the FCD form Implementation of collation in ICU