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121st International Unicode Conference Dublin, Ireland, May 2002
Optimizing the Usage of Normalization
Vladimir Weinstein
Globalization Center of Competency, San Jose, CA
221st International Unicode Conference Dublin, Ireland, May 2002
Introduction
1. Unicode standard has multiple ways to encode equivalent strings
résumé re sumé re sume NFD: NFC: résume
2. Accents that don’t interact are put into a unique order
321st International Unicode Conference Dublin, Ireland, May 2002
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
421st International Unicode Conference Dublin, Ireland, May 2002
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
521st International Unicode Conference Dublin, Ireland, May 2002
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)
621st International Unicode Conference Dublin, Ireland, May 2002
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
721st International Unicode Conference Dublin, Ireland, May 2002
Incremental Normalization: Example
re sume résumé
re sume
résumé
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
821st International Unicode Conference Dublin, Ireland, May 2002
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
921st International Unicode Conference Dublin, Ireland, May 2002
Optimized Concatenation: Example
Find boundaries Concatenate then normalize
Concatenate and normalize up to the boundaries
re sumé+
re sumé
résumé
r sumé+e
r sumée
résumé
• It is enough to normalize the boundary parts• Incremental normalization is used • Much faster than redoing the whole resulting
string
1021st International Unicode Conference Dublin, Ireland, May 2002
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
1121st International Unicode Conference Dublin, Ireland, May 2002
FCD Form: Examples
SEQUENCE FCD NFC NFD
A-ring Y Y
Angstrom Y
A + ring Y Y
A + grave Y Y
A-ring + grave Y
A + cedilla + ring Y Y
A + ring + cedilla
A-ring + cedilla Y
1221st International Unicode Conference Dublin, Ireland, May 2002
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+ = XÅ = X,=>
A-ring (U+00C5)
Angstrom sign (U+212B)
A + combining ring above (U+0041 U+030A)
1321st International Unicode Conference Dublin, Ireland, May 2002
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 < カイ or キー > キイ
See “Collation in ICU” by Mark Davis
1421st International Unicode Conference Dublin, Ireland, May 2002
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
1521st International Unicode Conference Dublin, Ireland, May 2002
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
1621st International Unicode Conference Dublin, Ireland, May 2002
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%
1721st International Unicode Conference Dublin, Ireland, May 2002
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
1821st International Unicode Conference Dublin, Ireland, May 2002
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
1921st International Unicode Conference Dublin, Ireland, May 2002
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/
2021st International Unicode Conference Dublin, Ireland, May 2002
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
2121st International Unicode Conference Dublin, Ireland, May 2002
Q & A
2221st International Unicode Conference Dublin, Ireland, May 2002
Summary
• Introduction
• Avoiding normalization
• Check for normalized text
• Normalize incrementally
• Concatenation of normalized strings
• Accepting the FCD form
• Implementation of collation in ICU