Algorithmic Constraint Checking

Roger Costello

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Example: Checking Election Results

• Consider this XML instance document:

1. <xml version="1.0"?>
2. <ElectionResultsByPercentage>
3. <Candidate name="John">61</Candidate>
4. <Candidate name="Sara">24</Candidate>
5. <Candidate name="Bill">15</Candidate>
6. </ElectionResultsByPercentage>

• Vote Count: For the instance document to be valid, the sum of the Candidate values must be 100.

Different Type of Checking

• Note that the checking needed for the Vote Count is a fundamentally different type of checking than we did in the tutorial on co-constraint checking or the tutorial on cardinality checking. There we "compared data values", and "checked existence of", respectively. Here the data is validated by applying an algorithm.

Rule

• Here is a Schematron rule which implements the Vote Count algorithm:

1. <sch:rule context="ElectionResultsByPercentage">
2. <sch:assert test="sum(Candidate) = 100">
3. The sum of the election results must be 100
4. </sch:assert>
5. </sch:rule>

• Read as: Within the context of the <ElectionResultsByPercentage> element (the root element), I assert that the sum of all the <Candidate> values must be 100.

Pattern

• The rule on the previous slide is the only rule needed to implement the Vote Count, so let's embed it within a pattern element:

1. <sch:pattern name="Vote Count">
2. <sch:p>The election results must add up to 100%.</sch:p>
3. <sch:rule context="ElectionResultsByPercentage">...</sch:rule>
4. </sch:pattern>

Schematron Schema

1. <xml version="1.0"?>
2. <sch:schema xmlns:sch="http://www.ascc.net/xml/schematron">
3. <sch:pattern name="Vote Count">
4. <sch:p>The election results must add up to 100%.</sch:p>
5. <sch:rule context="ElectionResultsByPercentage">
6. <sch:assert test="sum(Candidate) = 100">
7. The sum of the election results must be 100
8. </sch:assert>
9. </sch:rule>
10. </sch:pattern>
11. </sch:schema>

• View check-election-results.sch (example03 folder).

Validate

• Validate this (valid) XML instance document against the Schematron schema: valid-document.xml (example03 folder)
• Validate this (invalid) XML instance document against the Schematron schema: invalid-document.xml (example03 folder)

Example: Checksum Validation

• Consider this XML instance document:

1. <xml version="1.0"?>
2. <Election>
3. <ResultsByPercentage>
4. <Candidate name="John">61</Candidate>
5. <Candidate name="Sara">24</Candidate>
6. <Candidate name="Bill">15</Candidate>
7. </ResultsByPercentage>
8. <DocumentNumber>4389023232</DocumentNumber>
9. </Election>

• Vote Count & Checksum: For the instance document to be valid, the sum of the Candidate values must be 100, and the DocumentNumber's checksum digit must equal the sum of the preceding nine digits multiplied by their position, mod 9.

Checksum

• Here's how to validate the checksum digit:

1. <sch:pattern name="Checksum">
2. <sch:rule context="DocumentNumber">
3. <sch:assert test="(
4. (substring(.,1,1) * 1) +
5. (substring(.,2,1) * 2) +
6. (substring(.,3,1) * 3) +
7. (substring(.,4,1) * 4) +
8. (substring(.,5,1) * 5) +
9. (substring(.,6,1) * 6) +
10. (substring(.,7,1) * 7) +
11. (substring(.,8,1) * 8) +
12. (substring(.,9,1) * 9)
13. ) mod 9 = substring(.,10,1)">
14. The checksum (i.e. the tenth digit) must equal the sum of the preceding nine digits multiplied by their position, mod 9.
15. </sch:assert>
16. </sch:rule>
17. </sch:pattern>

Validate

• View check-election-results.sch (example03-a folder).
• Validate this (valid) XML instance document against the Schematron schema: valid-document.xml (example03-a folder)
• Validate this (invalid) XML instance document against the Schematron schema: invalid-document.xml (example03-a folder)

Lab 3

• Currently check-election-results.sch (lab03 folder) assumes that all election results are integers, with no roundoff errors, and must add up to exactly 100.
• Revise it so that the election results can be decimal values, and the sum of the results are acceptable if it is 100 ±0.9
• Validate valid-document.xml (lab03 folder) against your Schematron schema.
• Validate invalid-document.xml (lab03 folder) against your Schematron schema.
• Use both Oxygen XML and Topologi to do the validation.