Programmer 7500/MAX_PROGRAMMERS.iso/PASCAL/BOYMO4.ZIP/BOYERMO1.PAS

Pascal/Delphi Source File | 1988-01-23 | 12.9 KB | 224 lines

{ BOYERMO1.PAS ( 22 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 length of the text approaches the maximum (65521 characters)
  allowed by Turbo Pascal 4.0 for an array 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.
     Whenever the text is to be searched for a new target, the program must
  call MAKE_BOYER_MOORE_TABLE to have the shift table required by the Boyer-
  Moore method created and to inform the unit of the target to be used when
  the text is searched.  The function BOYER_MOORE_SEARCH searches the text
  for the target specified in the most recent call to MAKE_BOYER_MOORE_TABLE,
  beginning the search at a position in the text specified by the argument
  associated with the parameter START.  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.  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.
     When it is required that all occurrences of the target in the text 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);
    scan_beginning := 1;
    search_text_length := length(search_text);
    repeat
      i := Boyer_Moore_search(search_text,scan_beginning,search_text_length);
      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);
                                                                             }
{============================================================================}
unit BOYERMO1;
{============================================================================}
interface

procedure MAKE_BOYER_MOORE_TABLE(var target: string);
{ TARGET is the target string for which a text is to be searched.  A shift
  table for the target string is constructed in private storage in the unit
  and a copy of the target string is also stored inside the unit for
  subsequent use by BOYER_MOORE_SEARCH. }

function BOYER_MOORE_SEARCH(var text_array; start, text_length: word):
    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.  A
  Boyer-Moore search is performed on the text in TEXT_ARRAY, beginning with
  the character in position START, for the first substring that matches
  the target string specified in the most recent invocation of
  MAKE_BOYER_MOORE_TABLE.  If a match is found, the position of the first
  character of the matching substring is returned.  Otherwise 0 is
  returned. }
{============================================================================}
implementation

const
  copy: string = '';  { initialized to an empty string to protect against }
                      { the invocation of BOYER_MOORE_SEARCH without a    }
                      { prior call to MAKE_BOYER_MOORE_TABLE              }
var
  table: array[char] of byte;
{****************************************************************************}
procedure MAKE_BOYER_MOORE_TABLE(var target: string);
{ TARGET is the target string for which a text is to be searched.  A shift
  table for the target string is constructed in private storage in the unit
  and a copy of the target string is also stored inside the unit for
  subsequent use by BOYER_MOORE_SEARCH. }
begin { MAKE_BOYER_MOORE_TABLE }
  inline
    ($1E/             {        push ds             }
     $1E/             {        push ds             }
     $07/             {        pop es              }
     $BF/>copy/       {        mov di,offset copy  }
     $C5/$76/<target/ {        lds si,[bp].target  }
     $8B/$DE/         {        mov bx,si           }
     $32/$E4/         {        xor ah,ah           }
     $8A/$04/         {        mov al,[si]         }
     $26/$88/$05/     {        mov es:[di],al      }
     $FC/             {        cld                 }
     $0A/$C0/         {        or al,al            }
     $74/$06/         {        jz tbl              }
     $46/             {        inc si              }
     $47/             {        inc di              }
     $8B/$C8/         {        mov cx,ax           }
     $F3/$A4/         {        rep movsb           }
     $BF/>table/      { tbl:   mov di,offset table }
     $8B/$F3/         {        mov si,bx           }
     $8B/$D7/         {        mov dx,di           }
     $8A/$E0/         {        mov ah,al           }
     $B9/>$0080/      {        mov cx,80h          }
     $F3/$AB/         {        rep stosw           }
     $8B/$F3/         {        mov si,bx           }
     $8B/$FA/         {        mov di,dx           }
     $46/             {        inc si              }
     $98/             {        cbw                 }
     $3C/$01/         {        cmp al,1            }
     $7E/$13/         {        jle done            }
     $48/             {        dec ax              }
     $8A/$CC/         {        mov cl,ah           }
     $8A/$FC/         {        mov bh,ah           }
     $8A/$1C/         {  next: mov bl,[si]         }
     $8B/$D0/         {        mov dx,ax           }
     $2B/$D1/         {        sub dx,cx           }
     $88/$11/         {        mov [bx+di],dl      }
     $46/             {        inc si              }
     $41/             {        inc cx              }
     $3B/$C8/         {        cmp cx,ax           }
     $75/$F2/         {        jne next            }
     $1F)             {  done: pop ds              }
end; { MAKE_BOYER_MOORE_TABLE }
{****************************************************************************}
function BOYER_MOORE_SEARCH(var text_array; start, text_length: word): 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.  A
  Boyer-Moore search is performed on the text in TEXT_ARRAY, beginning with
  the character in position START, for the first substring that matches
  the target string specified in the most recent invocation of
  MAKE_BOYER_MOORE_TABLE.  If a match is found, the position of the first
  character of the matching substring is returned.  Otherwise 0 is
  returned. }
begin { BOYER_MOORE_SEARCH }
  inline
    ($1E/                  {            push ds                 }
     $33/$C0/              {            xor ax,ax               }
     $8B/$D0/              {            mov dx,ax               }
     $BE/>copy/            {            mov si,offset copy      }
     $8A/$14/              {            mov dl,[si]             }
     $80/$FA/$01/          {            cmp dl,1                }
     $7F/$1F/              {            jg boyer                }
     $7C/$6E/              {            jl notfound2            }
     $8A/$44/$01/          {            mov al,[si+1]           }
     $8B/$56/<start/       {            mov dx,[bp+start]       }
     $4A/                  {            dec dx                  }
     $8B/$4E/<text_length/ {            mov cx,[bp+text_length] }
     $2B/$CA/              {            sub cx,dx               }
     $C4/$7E/<text_array/  {            les di,[bp+text_array]  }
     $8B/$DF/              {            mov bx,di               }
     $03/$FA/              {            add di,dx               }
     $FC/                  {            cld                     }
     $F2/$AE/              {            repne scasb             }
     $75/$56/              {            jne notfound2           }
     $97/                  {            xchg ax,di              }
     $2B/$C3/              {            sub ax,bx               }
     $EB/$53/              {            jmp short exit          }
     $FE/$CA/              { boyer:     dec dl                  }
     $03/$F2/              {            add si,dx               }
     $C4/$7E/<text_array/  {            les di,[bp+text_array]  }
     $8B/$CF/              {            mov cx,di               }
     $03/$4E/<text_length/ {            add cx,[bp+text_length] }
     $49/                  {            dec cx                  }
     $4F/                  {            dec di                  }
     $03/$7E/<start/       {            add di,[bp+start]       }
     $03/$FA/              {            add di,dx               }
     $8A/$74/$01/          {            mov dh,[si+1]           }
     $BB/>table/           {            mov bx,offset table     }
     $55/                  {            push bp                 }
     $8B/$E9/              {            mov bp,cx               }
     $8A/$EC/              {            mov ch,ah               }
     $FD/                  {            std                     }
     $EB/$05/              {            jmp short comp          }
     $D7/                  { nexttable: xlat                    }
     $03/$F8/              {            add di,ax               }
     $72/$2A/              {            jc notfound             }
     $3B/$EF/              { comp:      cmp bp,di               }
     $72/$26/              {            jb notfound             }
     $26/$8A/$05/          {            mov al,es:[di]          }
     $3A/$F0/              {            cmp dh,al               }
     $75/$F0/              {            jne nexttable           }
     $4F/                  {            dec di                  }
     $8A/$CA/              {            mov cl,dl               }
     $F3/$A6/              {            repe cmpsb              }
     $74/$0D/              {            je found                }
     $8A/$C2/              {            mov al,dl               }
     $2B/$C1/              {            sub ax,cx               }
     $03/$F8/              {            add di,ax               }
     $47/                  {            inc di                  }
     $03/$F0/              {            add si,ax               }
     $8A/$C6/              {            mov al,dh               }
     $EB/$DC/              {            jmp short nexttable     }
     $5D/                  { found:     pop bp                  }
     $C4/$46/<text_array/  {            les ax,[bp+text_array]  }
     $97/                  {            xchg ax,di              }
     $2B/$C7/              {            sub ax,di               }
     $40/                  {            inc ax                  }
     $40/                  {            inc ax                  }
     $EB/$03/              {            jmp short exit          }
     $5D/                  { notfound:  pop bp                  }
     $32/$C0/              { notfound2: xor al,al               }
     $89/$46/$FE/          { exit:      mov [bp-2],ax           }
     $FC/                  {            cld                     }
     $1F)                  {            pop ds                  }
end; { BOYER_MOORE_SEARCH }
{****************************************************************************}
end.