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- **Fast and compact data structure.** Xcdat employs the *double-array trie* [3] known as the fastest data structure for trie implementation. However, the double-array trie resorts to many pointers and consumes a large amount of memory. To address this, Xcdat applies the *XCDA* method [2] that represents the double-array trie in a compressed format while maintaining the fast searches.
- **Cache efficiency.** Xcdat employs a *minimal-prefix trie* [4] that replaces redundant trie nodes into strings, resulting in reducing random access and improving locality of references.
- **Dictionary encoding.** Xcdat maps `N` distinct keywords into unique IDs from `[0,N-1]`, and supports the two symmetric operations: `lookup` returns the ID corresponding to a given keyword; `decode` returns the keyword associated with a given ID. The mapping is so-called *dictionary encoding* (or *domain encoding*) and is fundamental in many DB applications as described by Martínez-Prieto et al [1] or Müller et al. [5].
- **Prefix search operations.** Xcdat supports prefix search operations realized by trie search algorithms: `prefix_search` returns all the keywords contained as prefixes of a given string; `predictive search` returns all the keywords starting with a given string. These will be useful in many NLP applications such as auto completions [6], stemmed searches [7], or morphological analysis [8].
- **Prefix search operations.** Xcdat supports prefix search operations realized by trie search algorithms: `prefix_search` returns all the keywords contained as prefixes of a given string; `predictive search` returns all the keywords starting with a given string. These will be useful in many NLP applications such as auto completions [6], stemmed searches [7], or input method editors [8].
- **64-bit support.** As mentioned before, since the double array is a pointer-based data structure, most double-array libraries use 32-bit pointers to reduce memory consumption, resulting in limiting the scale of the input dataset. On the other hand, the XCDA method allows Xcdat to represent 64-bit pointers without sacrificing memory efficiency.
- **Binary key support.** In normal mode, Xcdat will use the `\0` character as an end marker for each keyword. However, if the dataset include `\0` characters, it will use bit flags instead of end markers, allowing the dataset to consist of binary keywords.
- **Memory mapping.** Xcdat supports *memory mapping*, allowing data to be deserialized quickly without loading it into memory. Of course, deserialization by the loading is also supported.
@ -288,8 +288,8 @@ Xcdat can be used by including `xcdat.hpp`.
The two dictionary types are difined.
- `xcdat::trie_8_type` is the trie dictionary using standard DACs using 8-bit integers for elements.
- `xcdat::trie_7_type` is the trie dictionary using pointer-based DACs using 7-bit integers for elements.
- `xcdat::trie_8_type` is the trie dictionary using standard DACs [9] using 8-bit integers for elements.
- `xcdat::trie_7_type` is the trie dictionary using pointer-based DACs [2] using 7-bit integers for elements.
### Trie dictionary class
@ -501,9 +501,13 @@ If you use the library in academic settings, please cite the following paper.
## References
1. J. Aoe. An efficient digital search algorithm by using a double-array structure. IEEE Transactions on Software Engineering, 15(9):1066–1077, 1989.
2. N. R. Brisaboa, S. Ladra, and G. Navarro. DACs: Bringing direct access to variable-length codes. Information Processing & Management, 49(1):392–404, 2013.
3. S. Kanda, K. Morita, and M. Fuketa. Compressed double-array tries for string dictionaries supporting fast lookup. Knowledge and Information Systems, 51(3): 1023–1042, 2017.
4. M. A. Martínez-Prieto, N. Brisaboa, R. Cánovas, F. Claude, and G. Navarro. Practical compressed string dictionaries. Information Systems, 56:73–108, 2016
5. S. Yata, M. Oono, K. Morita, M. Fuketa, T. Sumitomo, and J. Aoe. A compact static double-array keeping character codes. Information Processing & Management, 43(1):237–247, 2007.
1. M. A. Martínez-Prieto, N. Brisaboa, R. Cánovas, F. Claude, and G. Navarro. Practical compressed string dictionaries. Information Systems, 56:73–108, 2016
2. S. Kanda, K. Morita, and M. Fuketa. Compressed double-array tries for string dictionaries supporting fast lookup. Knowledge and Information Systems, 51(3): 1023–1042, 2017.
3. J. Aoe. An efficient digital search algorithm by using a double-array structure. IEEE Transactions on Software Engineering, 15(9):1066–1077, 1989.
4. S. Yata, M. Oono, K. Morita, M. Fuketa, T. Sumitomo, and J. Aoe. A compact static double-array keeping character codes. Information Processing & Management, 43(1):237–247, 2007.
5. Müller, Ingo, Cornelius Ratsch, and Franz Faerber. Adaptive string dictionary compression in in-memory column-store database systems. In EDBT, pp. 283–294, 2014.
6. Gog, Simon, Giulio Ermanno Pibiri, and Rossano Venturini. Efficient and effective query auto-completion. In SIGIR, pp. 2271–2280, 2020.
7. Ricardo Baeza-Yates, and Berthier Ribeiro-Neto. Modern Information Retrieval. 2nd ed. Addison Wesley, Boston, MA, USA, 2011.
8. Kudo, Taku, et al. Efficient dictionary and language model compression for input method editors. In WTIM, pp. 19–25, 2011.
9. N. R. Brisaboa, S. Ladra, and G. Navarro. DACs: Bringing direct access to variable-length codes. Information Processing & Management, 49(1):392–404, 2013.