Patent Application
20080059876
The present invention provides improved techniques for real-time spelling correction of a term against a dictionary of valid words (including all word forms). The dictionary can be multi-lingual, i.e., it can be composed of multiple single language dictionaries. It can also be comprised of such things as sequences of nucleotides in biology, or any collection of valid “words” consisting of letters from a pre-established “alphabet.” While the dictionary size and alphabet size are presumed to be large, their actual size is unimportant, and average/maximum word length is assumed to be relatively small, i.e., many orders of magnitude smaller than the dictionary size.
For a dictionary size, D, alphabet size, A, and a maximum word length, W, the disclosed algorithm corrects distance one misspellings in O(W) time and distance two misspellings in O(W2) time. The required storage is O(D), or in the case W varies with D, equal to O(D*W), for distance one misspellings and O(D*W2) for distance two misspellings. In general, it is assumed that W is more or less constant and does not grow with D so that O(D) equals O(D*W) or O(D*W2).
According to a further aspect of the invention a soft algorithm is disclosed that uses a “soft” definition of distance two misspellings, where distance two spelling correction can be performed in O(W) time and O(D)=O(D*W) storage. “Soft” distance two means that only the following distance two errors are considered: double transposition, transposition-deletion, transposition-insertion, deletion-transposition, deletion-insertion, insertion-transposition, and insertion-deletion.
In the following discussion, the verb “to hash” or any of its grammatical variants refer to the act of placing something in a hash table. For example, the phrase “hashing all dictionary words” means placing all dictionary words in a hash table. Uses of hashtables and performance guarantees for simple hash table operations such as insertion and lookup are described in any standard reference on algorithms. See, for example, C. Cormen et al., Introduction to Algorithms, MIT Press (2001).
The method involves hashing all dictionary words, in a known manner, and all “replacements” of dictionary words, in accordance with the present invention. Replacements are hashed, using, for example, an asterisk ‘*’ as a wild card, as follows. If the dictionary word is COAT, then the following variants are hashed: *OAT, C*AT, CO*T and COA*. In general, if a word is of length W, then W such word variants are hashed. The (key, value) pairs are (*OAT, COAT), (C*AT, COAT), (CO*T, COAT), and (COA*, COAT). Separate hash tables are kept for the words (i.e., the dictionary) and for the replacement variants. These hash tables are assumed to be pre-created prior to when the distance one spelling corrector starts up (Step 110).
In response to obtaining the input candidate word (Step 120), say in this case the term is WXYZ, one first checks the word against the direct dictionary hash (Step 130). One then gets to the decision point 140. If a match is found in the dictionary hash, then the word is spelled correctly, and the program terminates indicating the correct spelling, as in Step 150. If, however, no match is found, a misspelling is assumed and one checks all distance one variants against the appropriate distance one hash tables, accumulating suggested spelling corrections using the process 200, discussed further below in conjunction with
This algorithm affords no false positives. In other words, the algorithm never suggests a spelling correction that is more than distance one from the original word. On the other hand, if one were to just hash the dictionary together with all ordered subsequences of dictionary words of length W−1 as in Greene et al., “Multi-Index Hashing for Information Retrieval,” 35th Annual Symposium on Foundations of Computer Science, 722-731 (1994), and do a corresponding lookup, one would run into false positives. For example, for both the dictionary words COAT and OATH the ordered subsequence OAT would be hashed, and both would be a suggested distance one correction in response to the query “DOAT,” despite the fact that OATH is not distance one from DOAT.
Transposition (t) hash: OCAT, CAOT, COTA
Deletion (d) hash: OAT, CAT, COT, COA
Transposition-replacement (tr) hash: *CAT, O*AT, OC*T, OCA*, *AOT, C*OT, CA*T, CAO*, *OTA, C*TA, CO*A, COT*
Special deletion-transposition hash: ACT, CTO
Double deletion hash: CO, CA, CT, OA, OT, AT
Deletion-replacement hash: *AT, O*T, OA*, C*T, CA*, *OT, CO*, *OA, C*A
Insertion-replacement hash: **OAT, *C*AT, *CO*T, *COA*, C**AT, C*O*T, C*OA*, *O*AT, CO**T, CO*A*, *OA*T, C*A*T, COA**, *OAT*, C*AT*, CO*T*
These hash tables require, in total O(D*W2) storage.
Referring again to
The following table lists the misspelling type, the action, and the hash table checked for each of the 16 possible distance two misspellings. Note that the possible outcomes of two successive misspellings xy, where x,y are elements of {t,d,i,r} are the same as the successive misspellings of yx, except in the single case where td # dt, since for example, on the one hand, starting with the word COAT one can reach CTO via a transposition followed by a deletion, but not via a deletion followed by a transposition, and on the other hand, starting from COAT one can reach OAT via a transposition followed by a deletion but not vice versa. Note that there is also an asymmetry in it and ti, where, for example (again from the word COAT) the CO*AT variant is not obtainable from ti and the ti variant CA*OT is not obtainable from it. However, the first of these variants is caught in a distance one simple insertion check, so can be disregarded, and the second variant is caught just like all other it or ti variants by the d Test Action against the t hash table. Only in the two cases, of td and dt are two separate actions followed by hash table checks required. The dt hash is a special hash since it does not need to store all deletions followed by transpositions, since most of these will be caught by the t test action against the d hash table. The exceptional cases are those where one first deletes a character and then transposes the characters that were originally around the deleted character. Only these O(W) deletion-transpositions need to be stored in the dt hash table.
Returning to
It is noted that except for distance two misspellings that involve double insertions, double deletions, or replacements, all actions can be done in O(W) time with O(D*W)=O(D) storage. However, replacements are less usual than the other single operations, and may be considered to be a deletion followed by an insertion. Also, double insertions and double deletions are relatively rare types of misspellings. Hence, if distance two misspellings are re-defined to exclude these possibilities (i.e., don't test these cases), a correction algorithm is provided that runs in O(W) time with O(D*W)=O(D) storage. This is referred to as a “soft” distance two correction.
System and Article of Manufacture Details
As is known in the art, the methods and apparatus discussed herein may be distributed as an article of manufacture that itself comprises a computer readable medium having computer readable code means embodied thereon. The computer readable program code means is operable, in conjunction with a computer system, to carry out all or some of the steps to perform the methods or create the apparatuses discussed herein. The computer readable medium may be a recordable medium (e.g., floppy disks, hard drives, compact disks, or memory cards) or may be a transmission medium (e.g., a network comprising fiber-optics, the world-wide web, cables, or a wireless channel using time-division multiple access, code-division multiple access, or other radio-frequency channel). Any medium known or developed that can store information suitable for use with a computer system may be used. The computer-readable code means is any mechanism for allowing a computer to read instructions and data, such as magnetic variations on a magnetic media or height variations on the surface of a compact disk.
The computer systems and servers described herein each contain a memory that will configure associated processors to implement the methods, steps, and functions disclosed herein. The memories could be distributed or local and the processors could be distributed or singular. The memories could be implemented as an electrical, magnetic or optical memory, or any combination of these or other types of storage devices. Moreover, the term “memory” should be construed broadly enough to encompass any information able to be read from or written to an address in the addressable space accessed by an associated processor. With this definition, information on a network is still within a memory because the associated processor can retrieve the information from the network.
It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.
