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Merging tables


XML Merge currently supports CALS, HTML, and DITA simple table processing.

The CALS table processing ensures that when syntactically and semantically valid (as per OASIS CALS table model documentation) input tables are provided the result will be a valid CALS table. Similarly, the HTML tables processing ensures that when valid input tables are provided - according to the HTML 4 or HTML 5 documentation - the result will be a valid HTML 4/5 table. Note that both inputs need to follow the same standard (i.e. be HTML 4 or HTML 5).

In the release 11.0.0, XML merge has two table processing algorithms, the original ‘Version 1’ and the latest algorithm ‘Version 2’.

By default, XML merge will run with table processing algorithm version 1 with an option to switch to version 2. Please note that table processing algorithm version 2 will be used by default in the future releases.

The following sections outline the two algorithm along with their API settings.

Table processing algorithm version 1

XML Merge processes tables defined in the XML input in a special way considering its structure while keeping the table comparison result valid. Some type of changes such as table entries overlapping or spanning multiple rows and columns are difficult to represent at fine granularity, whilst ensuring validity. In these cases, the changes are represented at row (i.e. groups of added/deleted rows) or even whole-table granularity. In case of DITA simple tables, the syntactic constraints ensure that cells cannot overlap or span either rows or columns, therefore changes are represented at a fine grain level of detail.

This approach was roughly based on 2 techniques and stemmed partly from the goal that it should be possible to reconstruct both A and B documents from the result:

  1. Keying cells according to the column to which they came from in sequence.

  2. Where there are structure changes, such as a span in one input that’s not there in another, it’s not possible to show both A and B so the idea was to show separate A and B rows or even tables.


In XML Merge, CALS tables and HTML table processing are configured separately. The following section talks about how to turn table processing on and off and set different CALS table processing modes.

The following section describes the table configuration settings for the ConcurrentMerge class. Similar table configuration settings are also available for the SequentialMerge class.

CALS tables

CALS table processing is enabled/disabled using setCalsTableProcessing.

HTML tables

HTML table processing is enabled/disabled using setHtmlTableProcessing.

Invalid table behaviour

In order to ensure that only valid tables are passed to our specialised CALS table processing, each input table is marked either valid or invalid. This parameter declares what type of processing should be used for those tables that are marked as invalid. The 'warning report mode' parameter configures how recoverable errors are reported.

Three options are provided:

  • FAIL: The fail option stops the comparison by throwing an appropriate exception (that includes the errors identified by the validity checker).

  • PROPAGATE_UP: The propagate up option ensures that changes to an invalid table (or more specifically tgroup) are represented at the table level.

  • COMPARE_AS_XML : The compare as XML option essentially compares the tables as if they were well-formed XML.

This can be configured using setInvalidTableBehaviour.

Warning report mode

This mode specifies the way in which invalid table warnings should be reported. Different options such as comments, messages or processing instructions are available to report warnings. This can be configured using setWarningReportMode.

Table validation level

The invalid table behaviour depends on the table validation level. The table validation level can either be STRICT or RELAXED. This can be configured using setValidationLevel.

Table processing algorithm version 2

Similar to table processing algorithm version 1, version 2 processes tables defined in the XML input in a special way, considering its structure whilst keeping the table comparison result valid. This algorithm produces better results for table changes such as changes to the sizes of row or column spans. Changes in this case are represented at a finer grain level rather than duplicating the table row or table itself.

The new approach is based on different techniques:

  1. Comparing the columns of a table first, based on the content that they contain.

  2. Reconstructing a non-ragged table showing column and row adds and deletes but basing the final structure on that of the Ancestor input. So we are losing some information about the structure of newer versions in order to show changes at a finer granularity. So, for example, where the Ancestor had a span over columns and a newer version does not it will not be easy to tell which new content came from which column. The content from any version will not however be lost.

Please note that Sequential merge does not support version 2 yet.


Version 2 can be enabled using ConcurrentMerge.setTableProcessingAlgorithmVersion2 and has the following settings along with the settings from version 1 algorithm.

Ignore Column Order

This configuration allows user to ignore the order of the columns if they have moved their position between versions. The default for a comparison can be set using setIgnoreColumnOrder. This comparison default can be overridden by adding a processing instruction within a table. The processing instruction <?dxml-orderless-columns?> can be used to ignore the order of columns on a specific table while <?dxml-ordered-columns?> can be used for ordered column comparison.

Column Keying mode

The configuration parameter ColumnKeyingMode can be used to key the column in different ways such as using the colspec colname or the implicit column position. The default is set using setCalsColumnKeyingMode. The processing instructions <?dxml-column-keying-mode auto?> <?dxml-column-keying-mode colname?> <?dxml-column-keying-mode position?> can be used within a table element to apply the column keying mode to a specific table. <?dxml-column-keying-mode auto?> is the default which means that the algorithm will choose the best alignment automatically. <?dxml-column-keying-mode colname?> will align columns based on the colspec colname attributes. <?dxml-column-keying-mode position?> will align columns based on their positions.

It may be that you already know which columns from each version you wish to align. In this case you can give explicit keys to columns so that your pre-determined alignment will override the value based alignment which is the default. You do not have to specify keys for every column, un keyed columns will use the default alignment mode in force for that table. The user defined column keys can be applied using the processing instruction <?dxml-column-keys ....> For example placing <?dxml-column-keys "One, Two, Three, Four"?> within the table element will key the first four columns with the keys “One”, “Two”, “Three”, “Four”, and <?dxml-column-keys "ID,,,Email,,DOB"?> will key the first, fourth and sixth columns.

Currently the version 2 algorithm will not align columns which it considers to be inserted, and you can use this technique to align inserted columns when you know that they should align.


  • The version 2 algorithm does not fully support rule configuration. Although all their content may be removed as normal following rule based resolution, the actual rows and column structure will remain but appear empty. It is hoped to remove rows and columns which are effectively removed during resolution in a future release.

  • One key weakness of version 2 is that inserted columns do not align even when they are equal. You can use column keys to mitigae this if you know which columns should align.

  • The version 2 algorithm does not support InvalidTableBehaviour.COMPARE_AS_XML option. This will be added in future release. An exception will be thrown if this setting is used with version 2.

Change representation

The changes to tables are represented differently according to the type of change and table processing algorithm. Following sections highlights few table result to show how different results are produced with different algorithms.

Row span change




Table processing algorithm version 1 result

Table processing algorithm version 2 result

Column Span change




Table processing algorithm version 1 result

Table processing algorithm version 2 result

Row and column span change




Table processing version 1 result

Table processing version 2 result

Column addition




Table processing version 1 result

Table processing version 2 result

See Comparing Document Tables for details on how changes are represented along with links to a set of examples table comparisons. Please note that, even though most of the these table comparisons are two-way comparisons, the same principles apply to three-way or n-way merge operations.

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