inno-setup - 使用 Inno Setup 从文件中读取所需位置的字节

Question

我想打开一个 50 MB 的二进制文件，只读取最后 4 个字节并将其转换为字符串以用于某些目的。

我现在发现的唯一方法是使用 LoadStringFromFile，将文件完全加载到内存中，然后复制最后 4 个字节，但是这种方法非常慢，因为二进制文件很重.

在 Inno Setup 脚本中有更好的方法吗？

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更新:这是我根据 Martin Prikryl 的回答编辑的最终工作函数

function readlast4byte() : AnsiString;
var
  Stream: TFileStream;
  Buffer: string;
  Count: Integer;
  Index: Integer;
begin
  Count := 4;

  Stream := TFileStream.Create('C:\test.txt', fmOpenRead);

  try
    Stream.Seek(-Count, soFromEnd);

    SetLength(Buffer, 1);
    SetLength(Result, Count);
    for Index := 1 to Count do
    begin
      Stream.ReadBuffer(Buffer, 1);
      Result[Index] := Chr(Ord(Buffer[1]))  ;
    end;
  finally
    Stream.Free;
  end;
end;

---

更新 2: 这也是 TLama 编写的另一个很好的工作函数，也应该标记为答案:

[Code]
#IFNDEF Unicode
  #DEFINE CharSize 1
#ELSE
  #DEFINE CharSize 2
#ENDIF
 
type
  TSeekOrigin = (
    soBeginning,
    soCurrent,
    soEnd
  );
 
#IFDEF UNICODE
function BufferToAnsi(const Buffer: string): AnsiString;
var
  W: Word;
  I: Integer;
begin
  SetLength(Result, Length(Buffer) * 2);
  for I := 1 to Length(Buffer) do
  begin
    W := Ord(Buffer[I]);
    Result[(I * 2)] := Chr(W shr 8); // high byte
    Result[(I * 2) - 1] := Chr(Byte(W)); // low byte
  end;
end;
#ENDIF
 
function ReadStringFromFile(
  const FileName: string; Origin: TSeekOrigin; Offset, Length: Integer;
  var S: AnsiString): Boolean;
var
  Buffer: string;
  Stream: TFileStream;
begin
  Result := True;
  try
    Stream := TFileStream.Create(FileName, fmOpenRead);
    try
      Stream.Seek(Offset, Ord(Origin));
      SetLength(Buffer, Length div {#CharSize});
      Stream.ReadBuffer(Buffer, Length);
      #IFNDEF UNICODE
        S := Buffer;
      #ELSE
        S := BufferToAnsi(Buffer);
      #ENDIF
    finally
      Stream.Free;
    end;
  except
    Result := False;
  end;
end;

Answer 1

您可以使用TFileStream support class :

TStream = class(TObject)
  function Read(Buffer: String; Count: Longint): Longint;
  function Write(Buffer: String; Count: Longint): Longint;
  function Seek(Offset: Longint; Origin: Word): Longint;
  procedure ReadBuffer(Buffer: String; Count: Longint);
  procedure WriteBuffer(Buffer: String; Count: Longint);
  function CopyFrom(Source: TStream; Count: Longint): Longint;
  property Position: Longint; read write;
  property Size: Longint; read write;
end;

THandleStream = class(TStream)
  constructor Create(AHandle: Integer);
  property Handle: Integer; read;
end;

TFileStream = class(THandleStream)
  constructor Create(Filename: String; Mode: Word);
end;

使用 .Seek(-4, soFromEnd) 属性将读取指针查找到所需位置。

复杂的是 TStream 使用的是字符而不是字节，因此您必须将读取的 Unicode 字符串转换回字节。

当只读取四个字节时，逐字节读取更容易，防止任何多字节转换:

procedure ReadFileEnd;
var
  Stream: TFileStream;
  Buffer: string;
  Count: Integer;
  Index: Integer;
begin
  Count := 4;

  Stream := TFileStream.Create('my_binary_file.dat', fmOpenRead);

  try
    Stream.Seek(-Count, soFromEnd);

    SetLength(Buffer, 1);
    for Index := 1 to Count do
    begin
      Stream.ReadBuffer(Buffer, 1);
      Log(Format('Byte %2.2x: %2.2x', [Index, Ord(Buffer[1])]));
    end;
  finally
    Stream.Free;
  end;
end;

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这是来自 @TLama 的通用替代方案，它可以有效地处理任意大读取:
https://pastebin.com/nzGEdXVj

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顺便说一句，对我来说LoadStringFromFile function似乎足以加载 50 MB 文件的效率。仅需 40 毫秒。