Patent Publication Number: US-9846739-B2

Title: Fast database matching

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 11/585,365 now abandoned filed Oct. 23, 2006, the contents of which are hereby incorporated by reference. Furthermore, U.S. application Ser. No. 11/585,365 was filed concurrently with U.S. application Ser. No. 11/585,358 entitled “Fuzzy Database Matching,” the contents of which is hereby incorporated by reference. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     TECHNICAL FIELD 
     The invention relates to the field of database systems. In particular, it relates to a method and system for improving the speed with which a candidate or sample record may reliably be matched against a record previously enrolled within the database. 
     BACKGROUND OF THE INVENTION 
     There is increasing need within a variety of fields to be able to determine very rapidly whether or not a particular sample record already exists within a large database, and if so to identify one or more matches. One particular field is biometrics, in which the requirement is to determine whether or not the individual who has provided a particular biometric sample is already in the database. A further exemplary field is that of digital rights management, where the need is to check whether a particular piece of music, video, image or text matches a corresponding record within a database of copyright works. 
     Databases of the type described can be extremely large, and it may be impractical to attempt a full match analysis between the sample record and every one of the records within the database. In order to reduce the computational workload, a variety of pre-screening processes are in use, but many of these have very restricted fields of application since they often rely upon specific peculiarities of the matching algorithm or of the data that are to be matched. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a method of identifying possible matches between a sample record and a plurality of stored records, the method comprising:
         (a) Defining a plurality of reference positions within a data record, and;   (b) Defining a key pattern related to each said reference position, and;   (c) Combining data from said key pattern into a key value, and;   (d) Associating a list of record identifiers with each said key value for at least some of said reference positions, and;   (e) On enrollment of a data record, providing an enrollment mask associated with said reference positions and adding a record identifier for said data record to said list of record identifiers associated with the key value determined by combining data from said key pattern for at least some of said reference positions where indicated by said associated enrollment mask, and;   (f) On seeking a match for a sample data record, providing sample mask associated with said reference positions and extracting said list of record identifiers associated with the key value determined by combining key patterns for at least some of said reference positions where permitted by said associated sample mask; and,   (g) Determining the number of occurrences of at least one record identifier in the lists associated with said key values for at least some of said reference positions where permitted by said associated reference sample mask; and,   (h) Identifying a given enrolled data record as being a match or possible match with the sample data record if said number of occurrences is at least some required number.       

     The required number for matching may be determined according to any convenient algorithm, such as a threshold dependent upon the application. The threshold may conveniently be a simple numerical count, or could alternatively be some more complex metric depending not only upon the number of matching key values, but also upon the number of times that those key values match the sample record and/or match the corresponding stored record. By means of the enrolment and sample masks, for example, the numerical count may be modified or scaled according to the particular masks associated with enrolled and sample records. 
     Any or all of the reference positions, bit patterns and means of forming a key value or list of key values may be hand-crafted (user-generated) or alternatively could be generated automatically from the stored records. The list of key values could be selective (for example, some of the words to be found within the text of a book), or could be comprehensive (all occurring words are automatically added to the list). The key values may all be of the same type or class, but that is not essential and it is contemplated that a single list may contain features of a variety of types (for example, individual words, phrases, font size and font information, layout information and so on). Instead of being a fragment of the stored record, the key values might alternatively be derived in some other way, for example, by hashing of the record or applying some other type of operation to it or to a part of it. 
     Similarly the enrolment and/or sample masks may be hand-crafted (user generated) or alternatively could be generated automatically from the stored records. 
     Once a list of possible matches between the sample record and the stored records has been generated, further analysis may be carried out on those retrieved records. Typically, although not necessarily, the sample record and the list of possible matching records may then be passed to a more sophisticated or exhaustive matching algorithm to determine which of the possible matches are true matches. 
     Such a method provides very fast candidate-matching at the expense of additional effort and memory utilization when registering a new record within the database. The trade-off is well worth while in a system where a record is enrolled only once and subsequently searched against many sample records. This is true of many, if not most applications. It can be of great advantage to devote more processing cost to enrolling than to searching, and as is not generally appreciated, trade faster matching for larger memory. 
     According to a further aspect of the present invention, there is provided a system for identifying possible matches between a sample record and a plurality of stored records, the system comprising:
         (a) A list of key values which occur at selected reference positions, each key value having associated with it those stored records which display said key value;   (b) A processor for matching key values selected by a key value mask against the sample record; and   (c) A processor for identifying a given stored record as being a possible match with the sample if it is associated with a required number of matching key values.       

     In some embodiments, separate processors may be used for matching key values against sample records, and for identifying stored records as possible matches. These processors may be on separate computers, and may be remote from each other. 
     In one particular embodiment, the main data list including the full collection of stored records may be held separately from the lists of record identifiers. That allows a local processor, for example, a processor embedded within a photocopying machine, to carry out the initial analysis using key values extracted from a sample record such as a photocopied page of text. Once a list of possible matches has been identified, that list can then be passed to a remote server, where a more detailed analysis can be carried out by comparing the sample with the full text of each of the possible matches. 
     This approach has the further advantage that the designer of the system does not need to distribute to a large number of users full copies of the entire corpus of copyright works. Instead, each user simply receives a list of key values, which is enough for the initial analysis to be carried locally. Where one or more possible matches are found, the system may then be automatically report to a central location where further analysis can be carried out against the full documents. 
     One skilled in the art of data base matching will recognize the underlying method described here as a novel variation of what is commonly known as ‘reverse indexing’, in which a key value is used as an entry to a table giving the identities of data records which display that key value. It is, for example, used in context addressable searching. The present invention adds to that technology a set of specified positions from which the key values are formed which may in some circumstances make the matching accuracy better, and the use of a mask to direct the indexing only to positions in the data known to be suitable for matching. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be carried in practice in a number of ways and some specific embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows the database structure according to an embodiment of the invention; 
         FIG. 2  is a histogram exemplifying the matching process; 
         FIG. 3  is another exemplary histogram; 
         FIG. 4  shows some exemplary hardware; 
         FIG. 5  shows records of text in a database of books with associated data according to an exemplary embodiment of the invention; 
         FIG. 6  illustrates the formation of key values from key patterns in a record of text according to an exemplary embodiment of the invention; 
         FIG. 7  illustrates a mask that may be associated with reference positions in a record of data; 
         FIG. 8  shows how lists of Record Identifiers are associated with Key Values at Reference Positions through a Key Mask according to an embodiment of the invention; 
         FIG. 9  describes the process of enrolment of record identifiers from a data record into a database when the particular key value has not previously occurred at a particular reference position; 
         FIG. 10  describes the process of enrolment of record identifiers from a data record into a database when the particular key value has previously occurred at a particular reference position; 
         FIG. 11  illustrates the indexed matching of a sample record; 
         FIG. 12  is a histogram exemplifying the matching for an example embodiment of the invention; 
         FIG. 13  is another exemplary histogram; and 
         FIG. 14  shows some exemplary hardware. 
     
    
    
     In the following detailed description, numerous specific details are set forth to provide a thorough understanding of claimed subject matter. However, it will be understood by those skilled in the art that claimed subject matter may be practiced without these specific details. In other instances, well-known methods, procedures, components and/or circuits have not been described in detail. 
     Some portions of the detailed description which follow are presented in terms of algorithms and/or symbolic representations of operations on data bits and/or binary digital signals stored within a computing system, such as within a computer and/or computing system memory. These algorithmic descriptions and/or representations are the techniques used by those of ordinary skill in the data processing arts to convey the substance of their work to others skilled in the art. An algorithm is here, and generally, considered to be a self-consistent sequence of operations and/or similar processing leading to a desired result. The operations and/or processing may involve physical manipulations of physical quantities. Typically, although not necessarily, these quantities may take the form of electrical and/or magnetic signals capable of being stored, transferred, combined, compared and/or otherwise manipulated. It has proven convenient, at times, principally for reasons of common usage, to refer to these signals as bits, data, values, elements, symbols, characters, terms, numbers, numerals and/or the like. It should be understood, however, that all of these and similar terms are to be associated with appropriate physical quantities and are merely convenient labels. Unless specifically stated otherwise, as apparent from the following discussion, it is appreciated that throughout this specification discussions utilizing terms such as “processing”, “computing”, “calculating”, “determining” and/or the like refer to the actions and/or processes of a computing platform, such as a computer or a similar electronic computing device, that manipulates and/or transforms data represented as physical electronic and/or magnetic quantities and/or other physical quantities within the computing platform&#39;s processors, memories, registers, and/or other information storage, transmission, and/or display devices. 
     For the sake of clarity, the description below will be directed toward an exemplary embodiment in the digital rights management field. In the embodiment to be described, a database contains details of a large number of published books which are currently in copyright. A website has been found onto which has been posted lengthy extracts from a variety of books. The task is to determine which, if any, of those extracts have been taken from books which are recorded within the database. It will of course be understood that this particular example is simply used to illustrate the general principles behind the invention, and that the same techniques will be equally applicable in other fields. The invention in its broadest form is not restricted to any particular class or type of data held within the database, nor to the details of the matching algorithms that are used. Of particular although not exclusive interest is the field of iris matching. 
     DETAILED DESCRIPTION 
     The database structure of an exemplary embodiment is shown schematically in  FIG. 1 . Bibliographic details of the individual books within the database are held within a case list or table  16 , each row  17  of which represents an individual book. Columns  18 ,  20 ,  22  respectively hold a unique reference number, the book title, and the author. Of course, in a practical embodiment, many more details about each individual book would probably be held. 
     The full text of each book is held within a data list or table  10 , each row  11  of which represents an individual book. This table consists of two columns, the first  12  being the unique reference number, mentioned above, and the second  14  holding the complete text of the book in some suitable encoded form. More generally, the column  14  may be considered to hold some generalised representation which uniquely identifies the individual record. 
     To assist in searching the database, a characteristic list or table  24  is created. Each row  26  holds a variety of different characteristics which may be found within the records of column  14  within the data list  10 . These characteristics are selected so as to be reasonably common (but not overwhelmingly so), in at least some of the books. The characteristics may be any easily-measurable attribute of the data, and the type of characteristic chosen will clearly depend upon the application. In some embodiments, as here, the characteristic may be a sub-feature; in others it may be extracted from the data or some part of it by the application of an operation/function such as a hash function. 
     In the embodiment being described the characteristics are individual words, namely “boy”, “grandmother”, “Peter”, “rabbit” and “witch”. Each row in the characteristic table points to a corresponding row  27  within a look-up table  25  which holds a series of pointers which have, here, been designated a, b, c and so on. Each pointer points to a specific memory location which defines the start of an individual case occurrence list  28  which corresponds to the particular linked characteristic within the table  24 . There will accordingly, be multiple case occurrence lists, one for each characteristic within the table  24 . The individual case occurrence lists  28  are populated with the unique reference number of every book in which that particular characteristic can be found. Conveniently, each row  30  in each list or table simply contains the reference of a single book which includes, displays or demonstrates the relevant characteristic, or from which the characteristic can be extracted. 
     Thus, in the example shown, the first case occurrence list contains the data  1 ,  2  and  4 , which implies that the characteristic “boy” appears in or can be extracted from the books “ The Witches”, “The Lion, The Witch and the Wardrobe ” and “ Peter Pan ”. The second list which relates to the characteristic “grandmother” consists of a single row which is populated with the reference number 1, indicating that the word “grandmother” occurs in the book “ The Witches ” only. 
     In another arrangement (not shown) the characteristic table  24  and the lookup table  25  may be merged into a single table having two columns. 
     The way in which the system is maintained and is used for searching will now be described. 
     To add a new characteristic (in this example, a new word) the characteristic is added to the list  24  of registered characteristics, in the appropriate position if that list is ordered. A block of memory is allocated for a new case occurrence list, and the relevant pointer added to the look-up table  25 . Finally, the new case occurrence list is populated with the reference numbers of those cases, (e.g., books) from which the newly-added characteristic can be extracted. 
     When a new case (book) is to be registered, the case list  16  and the data list  10  are updated accordingly, and the new case number is then added to the respective case occurrence list for each extracted characteristic. In some embodiments, the list of characteristics  24  may consist all of those characteristics which are contained within or which can be extracted or derived from the entire corpus of data within the data list  10 ; then, the addition of a new case may automatically trigger the registration of any new characteristics, extracted from the new case, which are not already included within the list  24 . 
     We now turn to the task of matching, or in other words determining whether an unknown data set or sample of text has been taken from one of the books within the database. Rather than matching the sample against the data  14  (the full text of each book), which would be computationally lengthy, characteristics are simply extracted from the sample for comparison with the already-registered characteristics. By referring to the individual case occurrence lists  28 , a count may be kept of the number of times a reference to a particular book occurs within a matched table. 
     In a simplistic embodiment, the matching might be carried out by way of a straightforward row-by-row search through the rows  26  of the characteristic list, but it will often be preferable to avoid this by ensuring that the characteristic list is ordered, and then using some more sophisticated search such as a binary search. Such an approach allows a matching characteristic to be found rapidly, and for a non-match to be identified rapidly in the event that the extracted sample characteristic is not registered within the list. 
       FIG. 2  illustrates an example in which the sample text has matched against the characteristics “witch” and “boy”. The count is shown schematically as histogram, although such a histogram would not necessarily be plotted in a working embodiment. As may be seen, there are two books in the database that have matched characteristics, namely “ The Witches ” and “ The Lion, the Witch and the Wardrobe”. “Peter Rabbit ” has no matches, and “ Peter Pan ” one. 
     Next, a threshold is applied to the count, and any book which scores at least the threshold value is considered to be a candidate match. Here, if the threshold is taken as one, all of the books except  Peter Rabbit  are candidate matches, and if the threshold is taken as two then the candidates are  The Witches  and  The Lion, the Witch and the Wardrobe.    
     A further example is given in  FIG. 3 , which represents another text sample in which matches have been found against the characteristics “witch”, “Peter” and “boy”. If a threshold of two is chosen, all of the books within the database match except for  Peter Rabbit.    
     The value of the threshold may be selected by the user by trial and error, according to the particular application and the extent to which the pre-selection process needs to remove a large number of cases from consideration in order to speed up the overall matching process. Although the use of a simple count and a fixed threshold is a convenient way of dividing possible matches from non-matches, other algorithms could equally well be used. One possible approach, for example, would be to select as a candidate match all of those cases having a characteristic count which is more than a fixed percentage higher than the average characteristic count taken across all cases. 
     Depending upon the size of the sample to be evaluated, it may not be necessary to use the sample in its entirety. For example, if the sample consists of several chapters of a book, it may be enough to carry out the pre-selection based on just one page of text. 
     The selection of characteristics, the matching criteria and the size of sample to be analysed will in most applications be chosen so that there is an acceptably low risk of a false rejection. 
     As described above, a characteristic might be a data fragment such as a word or phrase, or could alternatively represent some other attribute of the data. The characteristic might, for example, be extracted or derived from the data by applying to it or to some part thereof an operation such as a hash function. The output of the operation may then be used to access and/or search the characteristic table  24 . Where the number of possible characteristics is finite and is known in advance, it may be desirable in some applications for all possible characteristics within a defined characteristic space to be pre-registered. Such an arrangement obviates the need, on matching, to search the characteristic list  24 . Instead the sample record is simply processed to extract its characteristics, and the corresponding rows in the table  24  are used as indexes to the case occurrence lists applicable to those particular characteristics. 
     For example, in a biometric application, the characteristic might be a numeric code of a particular length (e.g., 16 bits, allowing 65536 possible characteristic values to occur). In a database there might be millions or billions of records, so that each possible characteristic may occur many times. To match a sample, one simply extracts one or more characteristics from it, for example, by hashing, and uses the characteristic to address the characteristic table and thus go to straight to the relevant lists  28  of stored records. 
     In some applications it may even be possible to dispense with the characteristic list  24  entirely. If the list is ordered and contains all possible characteristic values within a defined characteristic space (for example, the numbers 1 to 65536), maintaining the list as a separate entity is unnecessary since all of its values can be inferred. In such a case, a characteristic n which has been extracted from a sample can be used as an index to go straight to row n of the look-up table  25 , and thus directly to the corresponding case occurrence list  28 . 
     More generally, where the list of possible characteristics is finite and can be defined in advance, those characteristics can be mapped onto a numerical sequence 1 . . . N. Let us assume that applying the same mapping to a characteristic which has been extracted from an unknown sample gives a value of n&lt;=N. If the look-up table  25  is held as a vector L(N), then the location in memory of the relevant case occurrence list  28  for that particular characteristic may be found by looking at the pointer which is held at the position L(n). 
     It will, of course, be understood that the case occurrence lists  28  may in some embodiments be empty. 
     Once a list of candidate matches has been selected, using one of the procedures described above, a more detailed match may then be carried out against each of the possibilities, using any convenient matching algorithm. In the text example described, the sample text may be compared word for word against the full text of each of the possible matches. 
     In one embodiment, the database itself may be held on the same computer or at the same location where the preliminary and/or the final matching takes place. Alternatively, the process may be distributed, with the preliminary matching being carried out according to a characteristic list held at a local computer, and the preliminary matches being passed on to a remote computer for the detailed matching to take place. Such an arrangement allows the primary data list  10  (which includes the full data representing all the cases) to be held at a central location, with a local machine needing to hold just the characteristic list  24  and the individual lists  28 . 
     In another embodiment, shown in  FIG. 4 , the process of the present invention may further be speeded up by using multiple computers or processors operating in parallel. A user computer  32  forwards a matching task to a controller  34  which splits it up and distributes it between a plurality of computers or processors  36 . Each processor  36  may be instructed to handle a particular characteristic or group of characteristics, and is responsible for creating a subset of the case occurrence lists; alternatively, the controller  34  may split up the work in some other way. The processors  36  pass their lists onto a consolidator  38 , which finalizes the selection of candidate matches (for example, using the histogram/count procedures illustrated in  FIGS. 2 and 3 ). The list of possibilities is then forwarded as required, either to a computer or processor  42  which carries out more detailed matching, or as shown by reference numeral  40  back to the user  32  for further analysis. 
     The database structure of this exemplary embodiment is shown schematically in  FIG. 5 . Details of the individual books within the database are held within a case list or table  110 , each row  111  of which represents an individual book. Columns  112 ,  113 ,  114  respectively hold a unique record identifier for each book, the book title, and the author. Of course, in a practical embodiment, many more details about each individual book would probably be held. The text of each book is held in the data records of column  115  in some suitable encoded form. It may be convenient to subdivide the text of a book into pages, for example. More generally, the column  114  may be considered to hold some generalised record of data. 
     In general the representation of data in a database may take many different forms, not limited to the details in  FIG. 5  which are specific to the exemplary embodiment may contain many different types of information encoded in many different forms. What is important to the present invention that records of data, such as represented here by the text of a book  115 , should be associated with a unique record identifier  114 . It will be understood, of course, that the table  110  could be replaced by multiple linked tables in some embodiments. 
     For purposes of describing the exemplary embodiment, the text of the book Peter Rabbit  116  will be used to describe the enrolment of a the data record and also the matching of a sample record against enrolled data records. 
       FIG. 6  illustrates the formation of key values from key patterns in a record of text  200  which is taken from the book whose unique record identifier is  237 , namely the book ‘Peter Rabbit’. Such a record could be a record for enrolment or a sample record for matching. In general, key values may be created from patterns of data at pre-defined or calculated reference positions in a record of data. In the embodiment being described, however, for simplicity the key values are individual words from reference positions in the text in different books. In the book Peter Rabbit, for example, some of these may be “mother”  201 , “accident”  202 , and “parsley”  203 . 
     In the illustration, the reference positions are on different successive pages, however, the invention is not so limited. In the illustration, the specified data positions are in different places on each page of text. It is not necessary that all pages are chosen, nor that the pages are in any particular order; however, the reference positions chosen should be the same whenever key values are formed for a particular data record. These reference positions need not be fixed however provided there is some means of determining them, for example, by processing of the data itself, such that the reference positions are always the same for the same record of data. For simplicity, individual words of text will be used in the illustration; however, the invention is not so limited. The data patterns used to construct a key value could be of any form and could be different for different reference positions and can be constructed by any method provided that they always form the same key value for a particular reference position in a particular record of data. 
     To assist in searching the database, a pattern of data positions  201 ,  202 ,  203 ,  204 ,  205  is created. In the text example, the positions might be on the same or different pages, for example. Related to the specified positions, a pattern of data is created. In the text example this might be individual words or even characters from a pattern related to the chosen page. For simplicity, in the example individual words are chosen. It may, however, give better results in a practical embodiment for the pattern to be a selection of individual characters from particular parts of the page which are therefore unlikely to form a recognizable word. The chosen pattern of data is then combined into a key value, which is the means of referencing the database. In the text example, the key value might be a string of characters made up from the pattern of characters selected for the particular page, and the process of combining them may, for example, be a simple rearrangement, or indeed a mathematical operation which turns the characters into an item of binary data. 
     In  FIG. 6 , at the reference position  8  (a place chosen in this example for page 8) the book Peter Rabbit has the word ‘mother’  201 , which is the key value for reference position  8 . Similarly at reference position  9  the key value is ‘accident’  202 , and jumping forward to reference position  27  the key value is ‘parsley’  203 . Those taking the trouble to verify this example will find that these words actually occur on these pages in the first edition of ‘Peter Rabbit’ by Beatrix Potter. The key values are then an easily-measurable attribute of the data, and the type of key chosen will clearly depend upon the application. In some embodiments, as here, the characteristic may be a sub-feature of the data; in others it may be the result of an operation applied to the data, or some part of it, such as a hash function. 
     It will be evident that the method described in which key values are created from reference positions in a record of data will identify a set of features which are highly specific to a particular record of data. The more reference positions are chosen, the more specific it will be. For example, there will be very few books with ‘mother’, ‘accident’ and ‘parsley’ at specific positions on pages 8, 9 and 27. Perhaps there is only one such book, namely ‘Peter Rabbit’, but among the many millions of books published one cannot be sure. 
     Also associated with the embodiment is a mask which is in one to one correspondence with the data positions, which selects which positions in the data are to be entered in the database. Conventionally, a mask is binary although it could be more general providing that it can be used to select or exclude a particular key value for processing. In the text example, a mask may select only certain pages of a book. By means of the mask, books may have different numbers of pages, for example, with pages that do not exist masked out. Other pages from within the book may be masked out, for example, if they contain little or no text, or are missing or incomplete or in some other way unreliable. There are many ways in which this mask may be used, and the examples given are not intended to be limiting. 
       FIG. 7  illustrates a mask that may be associated with reference positions in a record of data, and is used to select or exclude key values: in the illustrative embodiment using key values from the book ‘Peter Rabbit’. 
     For illustration the positions are shown as a list  301 , although no such physical list of positions is actually necessary provided that the formation and use of key values is always associated implicitly or explicitly with a reference position. 
     The key values associated with the positions are also shown as a list, although no such physical list of key values is necessary provided every key value is associated implicitly or explicitly with a reference position. 
     Similarly, the mask is shown as a list  302 , although no such physical list is necessary provided every key value used in an embodiment has a mask associated with it and is associated implicitly or explicitly with a reference position. Such a mask could be one which selects all positions or no positions either implicitly or explicitly (but those are but special cases of the present invention). 
     In the illustrative example of a database of books,  FIG. 7  shows how the mask is associated with the reference positions in the book ‘Peter Rabbit’ to select only pages on which words of the story actually occur: pages 8, 9, 14, 15, 20, 21, 26, 27, 32, 33 for example, have text, others may be illustrations or may be blank. Such a mask may be used with the present invention either when enrolling or matching records of data. 
     For enrolment, it is clearly only useful to select reference positions where there is useful data. Similarly for matching its only useful to select reference positions where there is useful data. These selected positions may, in general, be different for different instances of the same data for enrolment or matching. Therefore in some embodiments of the present invention, the mask may be enrolled with the data. If that is done, then on matching account can be taken of the number of reference positions that actually contribute to a particular mask by finding the intersection of the enrolled mask and the mask presented with sample data for matching, and using it to influence a decision about whether a sample record may match an enrolled mask. This will be discussed further below. 
     Enrol and sample records may have different masks, and therefore not all enrolled values may be used for matching. 
     In general, the use of the mask prevents spurious key values from participating in matching and may therefore enhance the accuracy of masking, preventing, for example, false matches. 
     In the case-specific alternative of matching the code of a biometric iris, the specified positions may be a subset of portions of the iris known to be reliable, for example, avoiding ill defined boundaries of positions where reflections are known to occur. For example, the reflection of the nose from the surface of the cornea is a feature known to degrade recognition of irides. Such a mask may be fixed for a set or subset of irides enrolled in a database. However, the mask may be used to exclude portions of a particular iris that are nor useful, for example, an eyelid which conceals the iris texture. The eyelid position will in practice be slightly different in every data record of the same eye, so this mask may be different for an enrolled iris and a sample iris from the same eye. For that reason it is valuable to store the mask of every enrolled iris. Finally, different matches will involve different numbers of iris positions, so in particular embodiments the intersection of the masks, i.e., the number of positions where both enrolment and sample masks allow data to be recovered, might be used to enhance the matching score of a relatively highly masked pair of data records, further improving the accuracy of the method. 
     Intelligent use of masks may allow the method to be used in applications where data positions may be variable rather than fixed, for example, in fingerprints where the relationship between features is more important than their exact position. Groups of features can be enrolled as keys in the database at several positions, and then on matching keys from a sample record can be matched against sets of positions. 
       FIG. 8  shows how lists of Record Identifiers are associated with Key Values at Reference Positions through a Key Mask according to an embodiment of the invention.  FIG. 8  illustrates the masked index  400  with multiple reference positions  410 . To construct the database, a separate record identifier list  420  is created for each of the chosen key values  430  at each of the reference positions  410 . A list of key values  440  is presented and selected by a mask  450  either for enrolment or for matching as appropriate. 
     In the case of the database of books, then, there could be a list of key values  430  created for each page. Each row  430  holds a variety of different key values which may be found as in  FIG. 5  within the records of column  115  within the case list  110 . 
     For illustration the reference positions are shown as a list  410 , although no such physical list of positions is actually necessary provided that the formation and use of key values  440  is always associated implicitly or explicitly with a reference position. 
     The key values associated with the positions are shown as a list  430 , although no such physical list of key values is necessary provided that every key value used is associated implicitly or explicitly with a reference position. 
     Similarly, the mask is shown as a list  450 , although no such physical list is necessary provided every key value has a mask associated with it and is associated implicitly or explicitly with a reference position. Such a mask could be one which selects all positions or no positions either implicitly or explicitly (but those are but special cases of the present invention). 
     Finally, in the exemplary embodiment, the lists of identifiers  420  associated with each key value  430  for each selected position  410  within the individual key value lists  430  are populated with the reference number of every book in which that particular key value  460  can be found at that reference position, but only if the reference position is selected by the mask  450 . Conveniently, each row in each record identifier list  420  or table simply contains the reference number of a single book which includes the relevant key at the relevant position, as will be further described below. 
     The way in which the system is maintained and is used for searching will now be described. 
     In the example embodiment, whenever a new book is to be registered or enrolled within the database, its details are added to the case  110  and a check is carried out to see which of the keys values  430  are contained at the particular reference positions  410  within that new book. The book&#39;s record identifier is then added, as appropriate, to the individual record identifier occurrence lists  420 . If desired, one or more new key values may be added to the key value lists  430 , in which case additional record identifier lists  420  are automatically created. 
       FIG. 9  illustrates enrolment of a book in the database when a key value is not previously known at a particular reference position. For enrolment, a record identifier  505 , reference position  506 , key value  507  and key mask  508  are provided in any way which may be convenient to an embodiment. While enrolling book  237 , ‘Peter Rabbit’ as at  116  in  FIG. 5 , at reference position  8  the key value ‘mother’  507  is formed as at  201  in  FIG. 6 . Because reference position  8   506  is selected for enrolment by the key mask  508   f  or ‘Peter Rabbit’ at  304  in  FIG. 3 , the key value list for reference position  8  at  501  in  FIG. 9  is examined and it is seen that no book with the key value ‘mother’  507  has previously been enrolled for this position. As the process of enrolment involves checking the key value list  501  for previous enrolment of a key value at a reference position, it will be clear to one skilled in the art that there may be a speed advantage if the key value list  501  is ordered, although the present invention is not limited to key value lists  501  which are ordered. In the present example, because the key value ‘mother’ does not exist in the key value list  501 , it is necessary to add ‘mother’ to the list  501 , either at the end  502  as illustrated, or if the list is ordered by insertion at the appropriate place, preserving the link of each key value with its record identifier list. In addition a new record identifier list  503  is created and the record identifier of the new data record is stored as its first record identifier  504 , in this case, the identifier ‘ 237 ’ of ‘Peter Rabbit’. 
       FIG. 10  illustrates enrolment of a book in the database when a key value is previously known at a particular reference position. As before for enrolment, a record identifier  605 , reference position  606 , key value  607  and key mask  608  are provided in any way which may be convenient to an embodiment. While enrolling book  237 , ‘Peter Rabbit’ as at  116  in  FIG. 5 , at reference position  9  the key value ‘accident’ is formed as at  202  in  FIG. 2 . Because reference position  9  is selected for enrolment by the key mask for ‘Peter Rabbit’ at  305  in  FIG. 7 , the key value list for reference position  9  at  601  in  FIG. 9  is examined and it is seen that no book with the key value ‘mother’ has previously been enrolled for this position. As the process of enrolment involves checking the key value list  601  for previous enrolment of a key value at a reference position, it will be clear to one skilled in the art that there may be a speed advantage if the key value list  601  is ordered, although the present invention is not limited to key value lists  601  which are ordered. In the present example, because the key value ‘accident’ exists in the key value list  601 , it is not necessary to add ‘accident’ to the key value list. To enrol ‘accident; at reference position  9 , the record identifier of the new data record is added to the record identifier list for ‘accident’  603  at  604 , in this case the identifier ‘ 237 ’ of ‘Peter Rabbit’. In the description which follows below of matching new data records against the database it will be clear to one skilled in the art that there may be a speed advantage if the record identifier list  503 ,  603  is ordered, although the present invention is not limited to record identifier lists which are ordered. In the present example, when adding the record identifier ‘ 637 ’ for ‘Peter Rabbit’ to the record identifier list  603  at  604 , ‘ 637 ’ may be added to the record identifier list  603 , either at the end  604  as illustrated, or if the list is ordered it may be added by insertion at the appropriate place. 
     We now turn to the task of matching, or in other words, determining whether an unknown sample of text has been taken from one of the books within the database. 
     Rather than matching the sample against the data  115  (the full text of each book), which would be computationally lengthy, instead at selected reference positions  201 ,  202 ,  203  selected key values from the text sample  303 ,  404  are matched against the database. By referring to the record identifier lists  420  a count may be kept of the number of times a particular book occurs within the record identifier lists of an enrolled record. A sample mask  450  may be associated with the test sample, to exclude or include particular portions of the sample data. For example, only selected pages may be available. By this means great flexibility in the selection of key values and the reference positions in which they match may be used. 
     Two kinds of matching tasks are common in the fields of use, namely 1:1 matching in which one is required to verify whether a sample record is a match with a particular chosen data record, and 1:N matching in which a sample record is to be matched against a database of N enrolled records with no prior knowledge of the expected answer. The present invention can be used for both purposes, although the illustrative embodiment is concerned with the 1:N case when a sample of text is compared against an entire database of enrolled books to seek a match. A match will occur if a sufficient number of key values at selected reference positions return the same record identifier. It may be an exact match in either 1:1 or 1:N matching if a particular key value occurs at all selected reference positions. It may rarely occur that more than one enrolled record gives an exact match. Otherwise it may be a partial match. Depending on the application further processing may be desirable if there are several candidate record identifiers with a sufficient number of occurrences, or ‘hits’ in either the 1:1 or 1:N cases. 
       FIG. 11  illustrates the indexed matching of a sample record which is the text of case  237 , the book ‘Peter Rabbit’ in a database index such as  400  after a significant number of books have been enrolled. To carry out the match as illustrated by  FIG. 11 , a number of key values k 8    701 , k 9    702 , k 27    703  are provided together with mask values m 8    704 , m 9    705 , m 27    706 , all taken from a sample record. The sample record may be an exact match to a book in the database, as for example, ‘Peter Rabbit’ at these positions. The sample record may be a partial match, as for example, ‘Peter Rabbit’ with pages missing or key values in error. The sample record may be from a different book which happens to have the same key values at some positions. 
     In the illustrative database, the enrolled key values are held in lists  707 ,  708 ,  709  for each reference position which are not ordered. In other embodiments this list may be ordered or may not physically exist. To match, the key values from a reference position of the sample are used to look up in the database the record identifier list  710 ,  711 ,  712  for the particular key value at the particular reference position. In some embodiments where the appropriate record identifier list may be selected by some automatic method, therefore, the key value lists may not physically exist. However, the record identifier lists are physically created and maintained. 
     In  FIG. 11 , for example, at reference position  8  the key value ‘k 8  mother’ is presented  701  and the key mask m 8    704  indicates this to be a selected position. Assuming selection by the mask, a record identifier list  710  is selected which contains all the record identifiers of all data records which contain the selected key value at the selected reference position  701 . All the record identifiers in the selected record identifier list  710  are passed to a means of counting the occurrences or ‘hits’ on particular data records  713 . In the case of 1:1 matching this may consist simply of counting the hits at a particular sample record identifier that has been presented for verification. Operation of the index in this way to provide a 1:1 verification is but one way of using the masked indexed structure of the present invention. In the case of 1:N matching of an unknown sample record in  FIG. 11  counting of hits may at  713  be by a more general method, including but not restricted to the formation of a histogram or bin-count  714  for at least some of the record identifiers in the database. Such a histogram counts the hits  715  for a selection of record identifiers  716  could be created and initialised in advance, for example, or on the fly as a sample match proceeds. 
     Having processed the key value for reference position  8  in the example, the processing can continue to extract and count record identifiers from selected record identifier lists in which a key value is enrolled at selected reference positions. In general, the occurrences of record identifiers are counted at reference positions which match a key value extracted from the reference positions and selected by a mask. 
     In the example embodiment of text from pages of a book, then, if the key values ‘mother’, and ‘accident’ are presented at reference positions  8  and  9 , there are in total two hits from the lists of  FIG. 11  on record identifier  237  which is ‘Peter Rabbit’ but also two hits on record identifier  193  which is ‘The lion, the witch and the wardrobe’. This is illustrated by the histogram of  FIG. 12 . Numerous other record identifiers may occur with one hit such as  101  ‘The Witches’ or zero hits such as  477  ‘Peter Pan’. Therefore at this stage in the matching process there may be two candidates for matching, ‘Peter Rabbit’ and ‘The lion, the witch and the wardrobe’. If only these two key values had been presented, then one has the choice of carrying on with more key values, or perhaps making a lengthy comparison of the two data records indicated. This may not be onerous, for example, if millions of books are enrolled and only these two books are to be compared. However it is likely that more reference positions would be used, as in  FIG. 11 . One may go directly to reference position  27 , for example, and present the key value ‘parsley’ as at  703  in  FIG. 11 , and find a third hit for ‘Peter Rabbit’, but no more hits that increase any hit value in the histogram beyond 2, as in  FIG. 13 . At this point one may decide to cease processing and accept ‘Peter Rabbit’ as a match, or one might continue to present further key values as in  FIG. 11  until the histogram or other counting means produces an answer that the embodiment considers definitive. Of course there may be no such match in which case the sample record remains unknown, or there may be several candidate matches making a further decision process such as exhaustive comparison desirable. 
       FIG. 12  illustrates an example in which the sample text has matched against the key values “mother” and “accident”. The count is shown schematically as histogram, although such a histogram would not necessarily be plotted in a working embodiment. As may be seen, there are two books in the database that have two matched key values, namely “ The Lion, the Witch and the Wardrobe ” and “ Peter Rabbit”. “The Witches ” has one match and “ Peter Pan ” has none. 
     Next, a threshold is applied to the count, and any book which scores at least the threshold value is considered to be a candidate match. Here, if the threshold is taken as one, all of the books except  Peter Pan  are candidate matches, and if the threshold is taken as two then the candidates are  The Lion, the Witch and the Wardrobe and Peter Rabbit.    
     A further example is given in  FIG. 13 , which represents another text sample in which matches have been found against the key values “mother”, “accident” and “parsley”. If a threshold of three is chosen, a match has been found in  Peter Rabbit.    
     The value of the threshold may be selected by the user by trial and error, according to the particular application and the extent to which the pre-selection process needs to remove a large number of cases from consideration in order to speed up the overall matching process. Although the use of a simple count and a fixed threshold is a convenient way of dividing possible matches from non-matches, other algorithms could equally well be used. One possible approach, for example, would be to select as a possible match all of those cases having a record identifier count which is more than a fixed percentage higher than the average (e.g. mean, median or mode) characteristic count taken across all cases. 
     According to another aspect of the invention, it may be advantageous to scale the numbers of hits observed according to the numbers of key values used in matching. If a sample record is presented to a database for matching, then different data records may have been enrolled with different numbers of reference positions selected. The data records may have been of different lengths, for example, in the case of books the number of pages may vary widely, so that it is possible that a short book such as ‘Peter Rabbit’ which has only 17 pages with text may be matched against a much more substantial volume such as ‘The lion, the witch and the wardrobe’ with over 200 pages. Because of the difference in size, in general a longer text may have more hits that a shorter one. The present invention can provide a means of correcting for differences in the number of reference positions selected using the key value mask. On enrolment a key value mask provided for enrolment may be saved for data records. On matching a different key value mask may be presented with the sample data. Record identifiers from the selected record identifier lists will only be recovered if they are selected on both enrolment and matching. Assuming all pages of text from ‘Peter Rabbit’ and ‘The lion the witch and the wardrobe’ are enrolled, then if all key values from ‘Peter Rabbit’ are presented for a match, only 17 hits will occur on the enrolled Peter Rabbit and there is a danger, however small, that some other larger text will produce a similar number of hits. If, however, we know the enrolment and sample masks, it is possible to calculate the ‘intersection’ of the masks, that is, the number of reference positions where both masks select key values for processing by the index. The number of hits can then be scaled in some manner using the statistics of the masks. 
     A practical example of scaling the hits in matching masked data records may be in the field of biometrics, for example, in matching data records which are templates coded from images of human irides. Suppose for this example that the number of reference positions in an enrolled template is always the same, s t . An enrolled template may, for example, be accompanied by a mask of length s t  which indicates that some regions of the iris are not to be processed, for example eyelids, eyelashes and unwanted reflections particularly but not exclusively from sources of illumination. Only key values selected by the mask presented at enrolment may be used to select record identifier lists  503  where record identifiers are entered  504 . The number of positions where the record identifier is entered in a record identifier list will usually be less than the total number of reference positions used, s t , because of the masking. To use this feature the mask presented at enrolment may be saved in the database and associated with the record identifier in some way. Later, on presenting a sample for 1:1 or 1:N matching, only those reference positions selected by the sample mask are used for retrieval of the record identifiers indicated by the key value. The sample mask will, in general, be different from the mask saved at enrolment. Therefore, the number of reference positions from which the matching identifier may produce hits is reduced still further. Thus the number of hits will always be no greater than the number of positions selected by both masks, which we call the intersection s i  often considerably less. One method of scaling may therefore be to scale the number of hits according to 
     
       
         
           
             
               Scaled 
               ⁢ 
               
                   
               
               ⁢ 
               Hits 
             
             = 
             
               Raw 
               ⁢ 
               
                   
               
               ⁢ 
               Hits 
               * 
               
                 
                   s 
                   t 
                 
                 
                   s 
                   i 
                 
               
             
           
         
       
     
     By this scaling, a matching iris which has its number of hits arising from matching, here called the Raw Hits, will in general have its score increased if the combined effect of the sample and enrollment mask reduces the number of available reference positions. This may make the Scaled Hits a more reliable indication of the quality of a match, and may lead to a smaller number of false matches in practice. However, if very few reference positions are available because of very heavy masking leading to a small intersection, it may be better to reject a data record rather than risk a false match which could be the result of a large scaling factor 
                 s   t       s   i       .         
This factor could of course be infinite, although one skilled in the art would be expected to avoid this occurring. In practice a large scaling factor may be a very rare event, but should be borne in mind, for example, in some biometric systems where a false match may be considered far more serious than a false rejection.
 
     Depending upon the size of the sample to be evaluated, it may not be necessary to use the sample in its entirety. For example, if the sample consists of several chapters of a book, it may be enough to carry out the pre-selection based on just one page of text. 
     The selection of key values, the matching criteria and the size of sample to be analysed will in most applications be chosen so that there is an acceptably low risk of a false rejection. 
     As described above, a key value might be a fragment or pattern of data of a stored record, or it might alternatively be derived in some other way from the stored record, for example, by applying some operation such as a hash function. The latter approach may be advantageous in some applications since it can avoid the need to carry out a search when matching the sample record. Instead, the sample record is simply processed (e.g., by hashing) to extract one or more key values from it, these then directly being used as indexes to a list of key values with pointers to all the lists of stored records which contain those particular keys. 
     Where the number of possible key values is finite and is known in advance, it may be desirable in some applications for all possible key values within a defined range to be pre-registered. Such an arrangement obviates the need, on matching, to search the key values lists  430 . Instead the sample record is simply processed to extract its key values, and subject to selection by the mask, the corresponding rows in the lists  430  or  707 , for example, are used as pointers to the record identifier lists applicable to those particular key values. 
     In certain embodiments, explicit or physical key value lists may not be necessary, for example, in a biometric database where key values may be 16 bit numbers, in which case there are 2^16 (65536) possible key values and hence 2^16 (65536) possible record identifier lists for each reference position. It may be convenient for all of these record identifier lists to be created when the database is initialised and before any cases are enrolled. In such a situation great efficiency may be achieved if the lists are held in order and accessed through a list of pointers. If the arrangement of the record identifier lists is organized according to some pattern, it may be possible to access the appropriate record identifier lists by processing the key values and dispense with any form of list or pointer. 
     On the other hand, as in the text example, lists of key values may be stored in the database, and a search may be required to determine if the key value exists and where its associated record identifier list is to be found. Clearly if such a list of key values is ordered there may be strategies for locating the key values and associated lists quickly, for example, by a binary search of the values and an associated list of pointers to the lists of record identifiers. However, the present invention is not limited by any particular method of associating a key value with a record identifier list and those skilled in the art may identify many such methods. 
     In the illustrative embodiment, for example, the key values do not represent every possible word and every possible position, but are stored in lists of key values  430 ,  701 ,  702 ,  703 . It is accordingly necessary to examine the lists when carrying out a match. This might be done by a straightforward search of key values at a reference position to determine if a sample key value exists. A similar search is carried out at enrolment as described above. However when matching, nothing will be added to the database, but instead information will be extracted from the record identifier lists to determine to what extent the sample record matches a data record already enrolled in the data base. 
     For example, in a biometric application, the key value might be a numeric code of a particular length (e.g., 16 bits, allowing 65536 possible characteristic values to occur). In a database there might be millions or billions of records, so that each possible key value may occur many times. Thus, having a plurality of reference positions and using a mask may enhance the performance of the database. To match a sample, one simply extracts one or more key values from it at the reference positions, for example, by hashing, and uses the key values to look up addresses for the relevant lists of record identifiers. 
     In some applications it may even be possible to dispense with the key value lists  430  entirely. If the list is ordered and contains all possible characteristic values within a defined characteristic space (for example, the numbers 1 to 265536), maintaining the list as a separate entity is unnecessary since all of its values can be inferred. In such a case, a key value n which has been extracted from a sample can be used as an index to go straight to row n of a look-up table and thus directly to the corresponding record identifier list. 
     More generally, where the list of possible key values is finite and can be defined in advance, those key values can be mapped onto a numerical sequence 1 . . . N. By applying the same mapping to a key value which has been extracted from an unknown sample gives a value of n&lt;=N. If a look-up table is held as a vector L(N), then the location in memory of the relevant record identifier lists  430  that particular characteristic may be found by looking at the pointer which is held at the position L(n). If the locations of the record identifier lists are arranged in some regular manner, then processing of the key values may lead directly to the appropriate record identifier lists without the need for any lookup table. Other methods of accessing record identifier lists from the key values will occur to those skilled in the art. 
     Once a list of possible candidate matches has been selected, using one of the procedures described above, a more detailed match may then be carried out against each of the possibilities, using any convenient matching algorithm. In the text example described, the sample text may be compared word for word against the full text of each of the possible matches. 
     In one embodiment, the database itself may be held on the same computer or at the same location where the preliminary and/or the final matching takes place. Alternatively, the process may be distributed, with the preliminary matching being carried out according to a characteristic list held at a local computer, and the preliminary matches being passed on to a remote computer for the detailed matching to take place. Such an arrangement allows the primary case list  110  (which includes the full data representing all the cases) to be held at a central location, with a local machine needing to hold just the key value lists  430  (if any) and the individual record identifier lists  420 . 
     In another embodiment, shown in  FIG. 14 , the process of the present invention may further be speeded up by using multiple computers or processors operating in parallel. A user computer  1010  forwards a matching task to a controller  1020  which splits it up and distributes it between a plurality of computers or processors  1030 . Each processor  1030  may be instructed to handle a particular characteristic or group of keys; alternatively, the controller  1020  may split up the work in some other way. The processors  1030  pass their results onto a consolidator  1040 , which finalises the selection of possible matches (for example, using the procedure illustrated in  FIG. 11 . The list of possibilities is then forwarded as required, either to a computer or processor  1050  which carries out the detailed matching or as shown by reference numeral  1060  back to the user  1010  for further analysis. 
     It will, of course, be understood that, although particular embodiments have just been described, the claimed subject matter is not limited in scope to a particular embodiment or implementation. For example, one embodiment may be in hardware, such as implemented to operate on a device or combination of devices, for example, whereas another embodiment may be in software. Likewise, an embodiment may be implemented in firmware, or as any combination of hardware, software, and/or firmware, for example. Likewise, although claimed subject matter is not limited in scope in this respect, one embodiment may comprise one or more articles, such as a storage medium or storage media. This storage media, such as, one or more CD-ROMs and/or disks, for example, may have stored thereon instructions, that when executed by a system, such as a computer system, computing platform, or other system, for example, may result in an embodiment of a method in accordance with claimed subject matter being executed, such as one of the embodiments previously described, for example. As one potential example, a computing platform may include one or more processing units or processors, one or more input/output devices, such as a display, a keyboard and/or a mouse, and/or one or more memories, such as static random access memory, dynamic random access memory, flash memory, and/or a hard drive. 
     In the preceding description, various aspects of claimed subject matter have been described. For purposes of explanation, specific numbers, systems and/or configurations were set forth to provide a thorough understanding of claimed subject matter. However, it should be apparent to one skilled in the art having the benefit of this disclosure that claimed subject matter may be practiced without the specific details. In other instances, well known features were omitted and/or simplified so as not to obscure the claimed subject matter. While certain features have been illustrated and/or described herein, many modifications, substitutions, changes and/or equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and/or changes as fall within the true spirit of claimed subject matter.