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    { 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/<target/  {       lds si,[bp+target] }
         $89/$F3/          {       mov bx,si          }
         $8A/$04/          {       mov al, [si]       }
         $88/$C4/          {       mov ah,al          }
         $B9/$80/$00/      {       mov cx,$0080       }
         $C4/$7E/<table/   {       les di,[bp+table]  }
         $89/$FA/          {       mov dx,di          }
         $FC/              {       cld                }
         $F2/$AB/          {       rep stosw          }
         $89/$DE/          {       mov si,bx          }
         $89/$D7/          {       mov di,dx          }
         $46/              {       inc si             }
         $98/              {       cbw                }
         $3C/$01/          {       cmp al,1           }
         $7E/$13/          {       jle done           }
         $48/              {       dec ax             }
         $88/$E1/          {       mov cl,ah          }
         $88/$E7/          {       mov bh,ah          }
         $8A/$1C/          { next: mov bl,[si]        }
         $89/$C2/          {       mov dx,ax          }
         $29/$CA/          {       sub dx,cx          }
         $88/$11/          {       mov [bx+di],dl     }
         $46/              {       inc si             }
         $41/              {       inc cx             }
         $39/$C1/          {       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;
        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. }
    begin { BOYER_MOORE_SEARCH }
      Inline
        ($1E/                  {            push ds                 }
         $33/$C0/              {            xor ax,ax               }
         $C5/$5E/<table/       {            lds bx,[bp+table]   } { if TABLE and  }
         $8C/$D9/              {            mov cx,ds           } { TARGET are in }
         $C5/$76/<target/      {            lds si,[bp+target]  } { different     }
         $8C/$DA/              {            mov dx,ds           } { segments, re- }
         $3B/$D1/              {            cmp dx,cx           } { port failure  }
         $75/$76/              {            jne notfound2       } { at once       }
         $8A/$F4/              {            mov dh,ah               }
         $8A/$14/              {            mov dl,[si]             }
         $80/$FA/$01/          {            cmp dl,1                }
         $7F/$1F/              {            jg boyer                }
         $7C/$6B/              {            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/$53/              {            jne notfound2           }
         $97/                  {            xchg ax,di              }
         $2B/$C3/              {            sub ax,bx               }
         $EB/$50/              {            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]           }
         $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.

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