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package uk.ac.starlink.table;
import java.util.function.LongSupplier;
import uk.ac.starlink.util.Splittable;
/**
* RowSequence subinterface that is also suitable for parallel processing.
*
* @author Mark Taylor
* @since 4 Aug 2020
*/
public interface RowSplittable extends RowSequence, Splittable {
/**
* Returns a supplier for the index of the row currently being processed,
* if possible.
* The row index is a global value; when accessing the first row
* of a top-level RowSplittable, the value returned by this supplier
* should be the first index in the table.
* But for objects resulting from splitting a top-level instance
* into sub-instances, the return value should reflect the index
* in the original sequence of rows; each index should be returned
* from only one of the splittables resulting from splitting a
* top-level instance.
*
* Typically, the row index will start at 0 and increment for each row,
* but the numbering may be different depending on the requirements
* of the implementation or usage.
*
*
Before the {@link RowSequence#next} method has been called,
* the return value will be one less than the first row index.
* After RowSequence.next has returned false,
* the value is undefined.
*
*
Depending on the implementation, it may not be possible to
* determine the row index (for instance if the sequence is
* split into sub-splittables of unknown size).
* In such cases, this method returns null.
* The null-ness of the return value must be the same for all
* instances of the splittable hierarchy for a given sequence,
* so don't return a non-null value for the first splittable
* and then null values for some or all of its children.
*
* @return supplier for the globally-referenced row index value
* of the current row of this sequence, or null
*/
LongSupplier rowIndex();
}