Software For Full Text Information Retrieval:


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  1. General description of the program:

    This program is an experiment to evaluate using infix boolean operations as a heuristic to determine the relevance of text files in electronic literature searches. The operators supported are, "&" for logical "and," "|" for logical "or," and "!" for logical "not." Parenthesis are used as grouping operators, and "partial key" searches are fully supported, (meaning that the words can be abbreviated.) For example, the command:

      rel "(((these & those) | (them & us)) ! we)" file1 file2 ...

    would print a list of filenames that contain either the words "these" and "those", or "them" and "us", but doesn't contain the word "we" from the list of filenames, file1, file2, ... The order of the printed file names is in order of relevance, where relevance is determined by the number of incidences of the words "these", "those", "them", and "us", in each file. The general concept is to "narrow down" the number of files to be browsed when doing electronic literature searches for specific words and phrases in a group of files using a command similar to:

      more `rel "(((these & those) | (them & us)) ! we)" file1 file2`

    Although regular expressions were supported in the prototype versions of the program, the capability was removed in the release versions for reasons of syntactical formality, for example, the command:

      rel "((john & conover) & (joh.*over))" files

    has a logical contradiction since the first group specifies all files which contain "john" any place and "conover" anyplace in files, and the second grouping specifies all files that contain "john" followed by "conover". If the last group of operators takes precedence, the first is redundant. Additionally, it is not clear whether wild card expressions should span the scope multiple records in a literature search, (which the first group of operators in this example does,) or exactly what a wild card expression that spans multiple records means, ie., how many records are to be spanned, without writing a string of EOL's in the infix expression. Since the two groups of operators in this example are very close, operationally, (at least for practical purposes,) it was decided that support of regular expressions should be abandoned, and such operations left to the grep(1) suite.

  2. Comparative benchmarks of search algorithm:

    The benchmarks were run on a System V, rel. 4.2 machine, (20Mhz 386 with an 18ms. ESDI drive,) and searched the catman directory, (consisting of 782 catman files, totaling 6.8 MB,) which was searched for either one or two 9 character words that did not exist in any file, ie., there could be no matches found. The comparison was between the standard egrep(1), agrep(1), and rel(1). (Agrep is a very fast regular expression search program, and is available by anonymous ftp from, IP

    1. for complex search patterns (after cd'ing to the cat1 directory:)

      1. the command "egrep 'abcdefwxy|wxyabcdef' *" took 74.93 seconds

      2. the command "agrep 'abcdefwxy,wwxyabcdef' *" took 72.93 seconds

      3. the command "rel 'abcdefwxy|wxyabcdef' *" took 51.95 seconds

    2. for simple search patterns (after cd'ing to the cat1 directory:)

      1. the command "egrep 'abcdefwxy' *" took 73.91 seconds

      2. the command "agrep 'abcdefwxy' *" took 25.87 seconds

      3. the command "rel 'abcdefwxy' *" took 43.68 seconds

    3. For simple search patterns, agrep(1) is significantly faster, and for complex search patterns, rel(1) is slightly faster.

  3. Applicability:

    Applicability of rel varies on complexity of search, size of database, speed of host environment, etc., however, as some general guidelines:

    1. For text files with a total size of less than 5 MB, rel, and standard egrep(1) queries of the text files will probably prove adequate.

    2. For text files with a total size of 5 MB to 50 MB, qt seems adequate for most queries. The significant issue is that, although the retrieval execution times are probably adequate with qt, the database write times are not impressive. Qt is listed in "Related information retrieval software:," below.

    3. For text files with a total size that is larger than 50 MB, or where concurrency is an issue, it would be appropriate to consider one of the other alternatives listed in "Related information retrieval software:," below.

  4. Extensibility:

    The source was written with extensibility as an issue. To alter character transliterations, see uppercase.c for details. For enhancements to phrase searching and hyphenation suggestions, see translit.c.

    It is possible to "weight" the relevance determination of documents that are composed in one of the standardized general markup languages, like TeX/LaTeX, or SGML. The "weight" of the relevance of search matches depends on where the words are found in the structure of the document, for example, if the search was for "numerical" and "methods," \chapter{Numerical Methods} would be weighted "stronger" than if the words were found in \section{Numerical Methods}, which in turn would be weighted "stronger" than if the words were found in a paragraph. This would permit relevance of a document to be determined by how author structured the document. See eval.c for suggestions.

    The list of identifiers in the search argument can be printed to stdio, possibly preceeded by a '+' character and separated by '|' characters to make an egrep(1) compatible search argument, which could, conceivably, be used as the search argument in a browser so that something like:

      "browse `rel arg directory'"

    would automatically search the directory for arg, load the files into the browser, and skip to the first instance of an identifier, with one button scanning to the next instance, and so on. See postfix.c for suggestions.

    The source architecture is very modularized to facilitate adapting the program to different environments and applications, for example, a "mailbot" can be constructed by eliminating searchpath.c, and constructing a list of postfix stacks, with perhaps an email address element added to each postfix stack, in such a manner that the program could be used to scan incoming mail, and if the mail was relevant to any postfix criteria, it would be forwarded to the recipient.

    The program is capable of running as a wide area, distributed, full text information retrieval system. A possible scenario would be to distribute a large database in many systems that are internetworked together, presumably via the Unix inet facility, with each system running a copy of the program. Queries would be submitted to the systems, and the systems would return individual records containing the count of matches to the query, and the file name containing the matches, perhaps with the machine name, in such a manner that the records could be sorted on the "count field," and a network wide "browser" could be used to view the documents, or a script could be made to use the "r suite" to transfer the documents into the local machine. Obviously, the queries would be run in parallel on the machines in the network-concurrency would not be an issue. See the function, main(), below, for suggestions.

  5. References:

    1. "Information Retrieval, Data Structures & Algorithms," William B. Frakes, Ricardo Baeza-Yates, Editors, Prentice Hall, Englewood Cliffs, New Jersey 07632, 1992, ISBN 0-13-463837-9.

      The sources for the many of the algorithms presented in 1) are available by ftp,

    2. "Text Information Retrieval Systems," Charles T. Meadow, Academic Press, Inc, San Diego, 1992, ISBN 0-12-487410-X.

    3. "Full Text Databases," Carol Tenopir, Jung Soon Ro, Greenwood Press, New York, 1990, ISBN 0-313-26303-5.

    4. "Text and Context, Document Processing and Storage," Susan Jones, Springer-Verlag, New York, 1991, ISBN 0-387-19604-8.

    5. ftp

    6. ftp

  6. Related information retrieval software:

    1. Wais, available by ftp,

    2. Lq-text, available by ftp,

    3. Qt, available by ftp,

  7. The general program strategy:

    1. Translate the the infix notation of the first non-switch argument specified on the command line into a postfix notation list.

    2. Compile each token in the postfix notation list, from 1), into a Boyer-Moore-Horspool-Sunday compatible jump table.

    3. Recursively descend into all directories that are listed on the remainder of the command line, searching each file in each directory, using the Boyer-Moore-Horspool-Sunday algorithm, for the counts of incidences of each word in the postfix notation list-at the conclusion of the search of each file, evaluate the postfix notation list to determine the relevance of the file, and if the relevance is greater than zero, add the filename and relevance value to the relevance list.

    4. Quick sort the relevance list from 3), on the relevance values, and print the filename of each element in the relevance list.

  8. Module descriptions:

    1. The module uppercase.c constructs an array of MAX_ALPHABET_SIZE characters, in such a manner that the implicit index of any element contains the toupper() of the offset into the array of the specific index value, (ie., it is a look up table for uppercase characters,) and is called from main() for initialization in rel.c. The arrays use is to make a locale specific, fast, uppercase character translator, and is used in lexicon.c and searchfile.c to translate the first argument of the command line, and file data, respectively, to uppercase characters.

      note: care must be exercised when using this array in systems where the native type of char is signed, for example:
        signed char ch;
        unsigned char cu;
        cu = uppercase[ch];
      will not give the desired results, since ch indexed a negative section of the array, (which does not exist.). Particularly meticulous usage of lint is advisable.

      See uppercase.c and translit.c for suggestions in implementing hyphenation and phrase searching strategies.

    2. The module translit.c translates all of the characters in an array, using the array established in uppercase.c. See translit.c and uppercase.c for suggestions in implementing hyphenation and phrase searching strategies.

    3. The module lexicon.c parses the first argument of the command line into tokens, and is repetitively called by postfix.c for each token in the first argument of the command line. Lexicon.c uses a simple state machine to parse the tokens from the argument.

    4. The module posfix.c translates the first argument of the command line from infix notation to a postfix notation list, and is called from main() in rel.c. Syntax of the infix expression is also verified in this module.

    5. The module bmhsearch.c contains all of the Boyer-Moore-Horspool-Sunday (BMH) string search functions, including the bmhcompile_postfix() function which is called from main() in rel.c, to compile each token in the postfix notation list into a jump table, and the bmhsearch_list () function which is called repetitively to search each file in searchfile.c. See the bmhsearech.c module for a complete description of the assembled data structures.

    6. The module searchpath.c is a POSIX compliant, recursive descent directory and file listing function that is called from main() in rel.c to search files using the module in searchfile.c.

    7. The module searchfile.c is repetitively called from searchpath() in searchpath.c to search each file found in 5), using the BMH string search functions in bmhsearch.c. Searchfile.c uses POSIX compliant functions to open, lock, read, and close each file. The files are read locked for compatability with those systems that write lock files during write operations with utilities, for example, like vi(1). This provides concurrency control in a multi user environment. Searchfile.c uses fcntl(2) to read lock the file, and will wait if blocked by another process (see man fcntl(2).)

    8. The module eval.c contains postfix_eval(), which is called for each file searched in searchfile.c to compute the relevance of the file by evaluating the postfix notation list-the functions that compute the "and," "or," and "not" evaluations are contained in this module. If the value of the relevance computed is greater than zero, an element is allocated, and added to the relevance list. This module also contains a description of how the document's relevance is determined.

    9. The module qsortlist.c is a general function that is used to quick sort a linked list-in this case the relevance list-and is called from main() in rel.c.

    10. The module rel.c contains main(), which is the main dispatch function to all program operations.

    11. The module relclose.c is called to shut down all operations, allocated memory, and close all directories and files that may have been opened by this program. For specifics, see below under "Exception and fault handling," and relclose.c.

    12. The module message.c is a general error message look up table, for printing error message in a systematic manner, for all modules in the program. This module may contain port specific error messages that are unique to a specific operating system. For specifics, see message.c.

    13. The module version.c contains only the version of the program, and serves as a place holder for information from the revision control system for automatic version control.

    14. The module stack.h contains defines for all list operations in all modules. The lists are treated as "stacks," and this module contains the PUSH() and POP() defines for the stack operations. This module is general, and is used on many different types of data structures. For structure element requirements, see stack.h.

    15. The module memalloc.c is used as a general memory allocation routine, and contains functions for allocating memory, and making a list of the allocated the memory areas, such that it may be deallocated when the program exits, perhaps under exception or fault conditions.

      Note that all file and directory operations are POSIX compliant for portability reasons.

  9. Exception and fault handling:

    Since this program is a full text information retrieval system, it is not unreasonable to assume that some of the modules may find application in client/server architectures. This places constraints on how the program handles fault and exception issues. Note that it is not unreasonable to assume that signal interrupt does NOT cause the program to exit in a client/server environment, and, therefore, there can be no reliance on exit() to deallocate memory, close files and directories, etc. Specifically, the program must be capable of vectoring to a routine that deallocates any and all memory that has been allocated, and closes all files and directories that have been opened to prevent "memory leaks" and file table overflows. Since the modules are involved in list operations, in recursive functions, a strategy must be deployed that unconditionally deallocates all allocated memory, closes all files and directories, and resets all variables in the program the to their initial "state."

    The basic strategy to address the issues of exception and fault handling in client/server architectures is to Centralize memory allocation, and file and directory functions in such a manner that shutdown routines can be called from relclose() that will deallocate all memory allocated (memdealloc() in memalloc.c,) and close any files and/or directories (int_searchfile () in searchfile.c, and int_searchpath () in searchpath.c,) that may have been opened. The function, relclose() in relclose.c, is installed as an "interrupt handler," in main(), in rel.c.

  10. Constructional and stylistic issues follow, generally, a compromise agreement with the following references:

    1. "C A Reference Manual", Samuel P. Harbison, Guy L. Steele Jr. Prentice-Hall. 1984

    2. "C A Reference Manual, Second Edition", Samuel P. Harbison, Guy L. Steele Jr. Prentice-Hall, 1987

    3. "C Programming Guidelines", Thomas Plum. Plum Hall, 1984

    4. "C Programming Guidelines, Second Edition", Thomas Plum. Plum Hall, 1989

    5. "Efficient C", Thomas Plum, Jim Brodie. Plum Hall, 1985

    6. "Fundamental Recommendations on C Programming Style", Greg Comeau. Microsoft Systems Journal, vol 5, number 3, May, 1990

    7. "Notes on the Draft C Standard", Thomas Plum. Plum Hall, 1987

    8. "Portable C Software", Mark R. Horton. Printice Hall, 1990

    9. "Programming Language - C", ANSI X3.159-1989. American National Standards Institute, 1989

    10. "Reliable Data Structures", Thomas Plum. Plum Hall, 1985

    11. "The C Programming Language", Brian W. Kernighan and Dennis M. Ritchie. Printice-Hall, 1978

    Since each module is autonomous, (with the exception of service functions) each module has an associated ".h" include file that declares function prototypes of external scoped variables and functions. These files are are made available to other modules by being included in rel.h, which is included in all module's "c" source file. One of the issues is that an include file may not have been read before a variable declared in the include file is used in another include file, (there are several circular dependencies in the include files.) To address this issue, each module's include file sets a variable, the first time it is read by the compiler, and if this variable is set, then any subsequent reads will be skipped. This variable name is generally of the form of the module name, concatenated with "_H".

    Each "c" source file and associated include file has an "rcsid" static character array that contains the revision control system "signatures" for that file. This information is included, for both the "c" source file and its associated include file, in all object modules for audit and maintenence.

    If the stylistics listed below are annoying, the indent program from the gnu foundation, (anonymous ftp to in /pub/gnu,) is available to convert from these stylistics to any desirable.

    Both ANSI X3.159-1989 and Kernighan and Ritchie standard declarations are supported, with a typical construct:

      #ifdef __STDC__
          ANSI declarations.
          K&R declarations.

    Brace/block declarations and constructs use the stylistic, for example:

      for (this < that; this < those; this ++)
          that --;

    as opposed to:

      for (this < that; this < those; this ++) {
          that --;

    Nested if constructs use the stylistic, for example:

      if (this)
          if (that)

    as opposed to:

      if (this)
          if (that)

    The comments in the source code are verbose, and beyond the necessity of commenting the program operation, and the one liberty taken was to write the code on a 132 column display. Many of the comments in the source code occupy the full 132 columns, (but do not break up the code's flow with interline comments,) and are incompatible with text editors like vi(1). If the verbose comments are annoying, see the ./README file for a sed(1) script to remove the comments.

John Conover
January 6, 2004

A license is hereby granted to reproduce this software source code and to create executable versions from this source code for personal, non-commercial use. The copyright notice included with the software must be maintained in all copies produced.


So there.

Copyright © 1994-2011, John Conover, All Rights Reserved.

Comments and/or bug reports should be addressed to:
John Conover
January 6, 2004

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