Contributor: RUFUS HENDON
{ BOYERMO2.PAS (23 January 1988) (Rufus S. Hendon) }
{ This Unit provides facilities For searching a Text For a target using
the Boyer-Moore search method. The routine is based on Don Strenczewilk's
Implementation of a Variant form of the Boyer-Moore method (his case-
insensitive version B1, available on CompuServe in File BLINE.ARC in
Borland BPROGA Data Library 4, uploaded 21 August 1987). In addition to
repackaging his routine as a Turbo Pascal 4.0 Unit, I have modified it
(1) to provide protection against endless loops that in the original
version can arise due to wrap-around of the index used to scan the Text
when the the length of the Text approaches the maximum (65521 Characters)
allowed by Turbo Pascal 4.0 For Arrays of Type Char and (2) to improve
efficiency slightly by removing three instructions (a PUSH, a MOV, and a
POP) from the comparison loop.
The Text to be searched must be stored in an Array of Type Char or an
equivalent user-defined Type. The lower bound of the Array must be 1.
The target For which the Text is to be searched must be of Type String.
The Program must also provide a Variable For the storage of the shift
table used by the Boyer-Moore method when it searches the Text. This
Variable must provide 256 Bytes of storage; it can, For example, be a
Variable of Type Array[Char] of Byte. The target Variable and the shift-
table Variable must be in the same segment: they must both be global
Variables (located in the data segment) or both local Variables (stored
in the stack segment).
Whenever the Text is to be searched For a new target, the Program must
call MAKE_BOYER_MOORE_TABLE to create the shift table For the target.
Thereafter the Text can be searched For the target by invoking
BOYER_MOORE_SEARCH, specifying as arguments the target and its shift
table as well as the position in the Text where the search is to begin.
if the Program maintains multiple target Variables and a separate shift
table and starting-position Variable For each target, searches for
occurrences of the Various targets can be underway simultaneously.
In a call to BOYER_MOORE_SEARCH, the argument associated With the
parameter START determines the position in the Text With which the search
begins. To search the entire Text, the Function would be invoked With
START = 1. The Function scans the Text beginning from the START position
For the first subString that matches the target specified by the Variable
associated With the parameter TARGET, using the shift table stored in the
Variable associated With the parameter TABLE. if such a subString is
found, the Function returns the position (Array subscript) of the initial
Character of the matching subString; since the Array is required to have
1 as its lower bound, the position returned after a successful search
will always be greater than 0. if the Function fails to find a matching
subString, it returns 0. (if the requirement that the TARGET and TABLE
Variables be in the same segment is violated, the Function also returns
0.)
When it is required that all occurrences in the Text of a given target
be found, BOYER_MOORE_SEARCH would be invoked in a loop, in which the
START argument would initially have the value of 1; thereafter, after
every successful search, the START argument would be reset to the
position returned by the Function plus 1. The loop would terminate when
the Function reported failure. The loop would have a general structure
similar to this:
item := [the target String];
make_Boyer_Moore_table(item,shift_table);
scan_beginning := 1;
search_Text_length := length(search_Text);
Repeat
i := Boyer_Moore_search(search_Text,scan_beginning,search_Text_length,
item,shift_table);
if i > 0 then begin
[do whatever processing is required when the search is successful];
scan_beginning := i+1
end
Until i = 0
Note that if the Text Array can only be referred to by means of a
Pointer, as will be the Case if the Array is allocated in the heap by
means of the NEW Procedure, the Pointer, when used as the first argument
of BOYER_MOORE_SEARCH, must be dereferenced by writing '^' after it. If,
For example, TextPTR is a Pointer to the Text Array, the call to the
search Function in the loop just given would take this form:
i := Boyer_Moore_search(Textptr^,scan_beginning,search_Text_length,
item,shift_table);
}
{============================================================================}
Unit BOYERMO2;
{============================================================================}
Interface
Procedure MAKE_BOYER_MOORE_TABLE(Var target: String; Var table);
{ TARGET is the target String For which a Text is to be searched. The
shift table For the target String is Constructed in TABLE, which must be
a Variable providing 256 Bytes of storage, e.g. a Variable declared as
Array[Char] of Byte. }
Function BOYER_MOORE_SEARCH(Var Text_Array; start, Text_length: Word;
Var target: String; Var table): Word;
{ Text_Array is an Array of Characters in which a Text is stored; the
Text begins in Text_Array[1] and is Text_LENGTH Characters long. TARGET
must either be the same Variable used as parameter TARGET in an earlier
call to MAKE_BOYER_MOORE_TABLE or another Variable With the same value.
TABLE must be the Variable that was used as parameter TABLE in the same
call to MAKE_BOYER_MOORE_TABLE. TARGET and TABLE must be in the same
segment, i.e. they must both be global Variables or both local Variables.
A Boyer-Moore search is performed on the Text in Text_Array, beginning
With the Character in position START and using shift table TABLE, for
the first subString that matches TARGET. if a match is found, the
position of the first Character of the matching subString is returned.
Otherwise 0 is returned. A Function value of 0 is also returned if TABLE
and TARGET are not in the same segment. }
{============================================================================}
Implementation
Const
copy: String = '';
Var
table: Array[Char] of Byte;
{****************************************************************************}
Procedure MAKE_BOYER_MOORE_TABLE(Var target: String; Var table);
{ TARGET is the target String For which a Text is to be searched. The
shift table For the target String is Constructed in TABLE, which must be
a Variable providing 256 Bytes of storage, e.g. a Variable declared as
Array[Char] of Byte. }
begin { MAKE_BOYER_MOORE_TABLE }
Inline
($1E/ { push ds }
$C5/$76/