Fastest way to search a string in a file

The function below returns position of substring in file,

or -1 if such substring can not be found.

Better: use the Boyer-Moore algorithm. There's a Pascal implementation at http://www.dcc.uchile.cl/~rbaeza/handbook/algs/7/713b.srch.p.html.

function PosInFile(Str,FileName:string):integer;

var

Buffer:array[0..1023]of char;

BufPtr,BufEnd:integer;

F:File;

Index:integer;

Increment:integer;

c:char;

function NextChar:char;

begin

if BufPtr>=BufEnd then

begin

BlockRead(F,Buffer,1024,BufEnd);

BufPtr := 0;

Form1.ProgressBar1.Position := FilePos(F);

Application.ProcessMessages;

end;

Result := Buffer[BufPtr];

Inc(BufPtr);

end;

begin

Result := -1;

AssignFile(F,FileName);

Reset(F,1);

Form1.ProgressBar1.Max := FileSize(F);

BufPtr:=0;

BufEnd:=0;

Index := 0;

Increment := 1;

repeat

c:=NextChar;

if c=Str[Increment] then

Inc(Increment)

else

begin

Inc(Index,Increment);

Increment := 1;

end;

if Increment=(Length(Str)+1) then

begin

Result := Index;

Break;

end;

until BufEnd = 0;

CloseFile(F);

Form1.ProgressBar1.Position := 0;

end;