krossningsortering

Krossningsortering is a concept in sorting theory described in Nordic computer science literature as a family of multi-key sorting methods that use cross-classification and partitioning to order records with several keys. The term combines the Swedish words for cross and sorting, and the idea is to improve efficiency by coordinating how multiple keys are considered across passes.

The basic idea is to partition the data by one key and then sort within each partition by another key, followed by a merge that preserves the overall multi-key order. This approach aims to improve data locality and cache performance by aligning successive passes with the layout of memory. Intermediate steps typically rely on stable sorts to maintain the relative order of equal elements from previous passes.

There are several variants, including two-key and multi-key versions. For comparison-based implementations, the overall time complexity is usually in the vicinity of O(n log n) for n elements, with space usage depending on how partitions are managed. Non-comparison based variants that use bounded key ranges, such as counting or radix-like passes, can achieve near-linear time for suitable data. In-place or low-overhead partitioning schemes reduce additional memory, at the cost of more complex implementation.

Applications of krossningsortering include database and data-warehousing tasks where records must be ordered by multiple attributes and memory or I/O costs are critical. It is particularly discussed in contexts emphasizing cache-efficient multi-key ordering and external sorting when data does not fit in memory.

See also: Sorting algorithm, multi-key sort, radix sort, stable sort, external sorting.