Author: Jonas Bilinkevicius

I need to locate a pattern in a file (both text and binary) - just like the Pos 
function does with the strings. Preferably, it should deal with FileStream. 
Straightforward solution first seemed kind of expensive - that is to just plainly 
go through the stream comparing patterns on every step.

Answer:

Solve 1:

You can do it that way but it is much faster to load chunks of data into a sizeable 
buffer and do the search in the buffer. Here is an example:
1   
2   function ScanFile(const filename: string; const forString: string; caseSensitive:
3     Boolean): LongInt;
4   { returns position of string in file or -1, if not found }
5   const
6     BufferSize = $8001; { 32K + 1 bytes }
7   var
8     pBuf, pEnd, pScan, pPos: Pchar;
9     filesize: LongInt;
10    bytesRemaining: LongInt;
11    bytesToRead: Word;
12    F: file;
13    SearchFor: Pchar;
14    oldMode: Word;
15  begin
16    Result := -1; { assume failure }
17    if (Length(forString) = 0) or (Length(filename) = 0) then
18      Exit;
19    SearchFor := nil;
20    pBuf := nil;
21    { open file as binary, 1 byte recordsize }
22    AssignFile(F, filename);
23    oldMode := FileMode;
24    FileMode := 0; { read-only access }
25    Reset(F, 1);
26    FileMode := oldMode;
27    try { allocate memory for buffer and pchar search string }
28      SearchFor := StrAlloc(Length(forString) + 1);
29      StrPCopy(SearchFor, forString);
30      if not caseSensitive then { convert to upper case }
31        AnsiUpper(SearchFor);
32      GetMem(pBuf, BufferSize);
33      filesize := System.Filesize(F);
34      bytesRemaining := filesize;
35      pPos := nil;
36      while bytesRemaining > 0 do
37      begin
38        { calc how many bytes to read this round }
39        if bytesRemaining >= BufferSize then
40          bytesToRead := Pred(BufferSize)
41        else
42          bytesToRead := bytesRemaining;
43        { read a buffer full and zero-terminate the buffer }
44        BlockRead(F, pBuf^, bytesToRead, bytesToRead);
45        pEnd := @pBuf[bytesToRead];
46        pEnd^ := #0;
47        { scan the buffer. Problem: buffer may contain #0 chars! So we
48        treat it as a concatenation of zero-terminated strings. }
49        pScan := pBuf;
50        while pScan < pEnd do
51        begin
52          if not caseSensitive then { convert to upper case }
53            AnsiUpper(pScan);
54          pPos := StrPos(pScan, SearchFor); { search for substring }
55          if pPos <> nil then
56          begin { Found it! }
57            Result := FileSize - bytesRemaining + LongInt(pPos) - LongInt(pBuf);
58            Break;
59          end;
60          pScan := StrEnd(pScan);
61          Inc(pScan);
62        end;
63        if pPos <> nil then
64          Break;
65        bytesRemaining := bytesRemaining - bytesToRead;
66        if bytesRemaining > 0 then
67        begin
68          { no luck in this buffers load. We need to handle the case of the 
69  				search string spanning two chunks of file now. We simply go back a bit in 
70  				the file and read from there, thus inspecting some characters twice }
71          Seek(F, FilePos(F) - Length(forString));
72          bytesRemaining := bytesRemaining + Length(forString);
73        end;
74      end;
75    finally
76      CloseFile(F);
77      if SearchFor <> nil then
78        StrDispose(SearchFor);
79      if pBuf <> nil then
80        FreeMem(pBuf, BufferSize);
81    end;
82  end;



Solve 2:

83  procedure TForm1.Button1Click(Sender: TObject);
84  var
85    s: string;
86    hFile: THandle;
87    hFileMapObj: THandle;
88    pSharedBuf: Pointer;
89    Time0: Integer;
90    p: PChar;
91  begin
92    if not OpenDialog1.Execute then
93      Exit;
94    s := InputBox('Find', 'Match', '');
95    Time0 := GetTickCount;
96    hfile := 0;
97    hFileMapObj := 0;
98    pSharedBuf := nil;
99    try
100     hFile := FileOpen(OpenDialog1.FileName, fmOpenRead);
101     Win32Check(hFileMapObj <> INVALID_HANDLE_VALUE);
102     hFileMapObj := CreateFileMapping(hFile, nil, PAGE_READONLY, 0, 0, nil);
103     Win32Check(hFileMapObj <> 0);
104     pSharedBuf := MapViewOfFile(hFileMapObj, FILE_MAP_READ, 0, 0, 0);
105     Win32Check(pSharedBuf <> nil);
106     P := StrPos(PChar(pSharedBuf), PChar(s));
107   finally
108     if pSharedBuf <> nil then
109       UnMapViewOfFile(pSharedBuf);
110     if hFileMapObj <> 0 then
111       CloseHandle(hFileMapObj);
112     if hFile <> 0 then
113       CloseHandle(hFile);
114   end;
115   if P = nil then
116     Caption := Format('Not found, ticks=%d', [GetTickCount - Time0])
117   else
118     Caption := Format('Found it at pos %d, ticks=%d', [Integer(P - 
119 PChar(pSharedBuf)),
120       GetTickCount - Time0]);
121 end;