function CompFile(const f1,f2: string): Boolean;
var
ms1,ms2: TMemoryStream;
i,p: Integer;
b1,b2: Byte;
begin
Result := False;
if not (FileExists(f1) and FileExists(f2)) then Exit;

ms1 := TMemoryStream.Create;
ms2 := TMemoryStream.Create;

ms1.LoadFromFile(f1);
ms2.LoadFromFile(f2);

if ms1.Size <> ms2.Size then
begin
ms1.Free;
ms2.Free;
Exit;
end;

Result := True;
Randomize;
for i := 0 to 9 do
begin
p := Random(ms1.Size);
ms1.Position := p;
ms2.Position := p;
ms1.ReadBuffer(b1,1);
ms2.ReadBuffer(b2,1);
if b1 <> b2 then
begin
Result := False;
Break;
end;
end;

ms1.Free;
ms2.Free;
end;
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根据 "峪飞鹰" 的指点, 略作修改; 不过暂不用 CompareMem 对比所有内存了, 10 次抽样可能会更快一点.
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function CompFile(const f1,f2: string): Boolean;
var
fs1,fs2: TFileStream;
ms: TMemoryStream;
i,p: Integer;
b1,b2: Byte;
begin
Result := False;
if not (FileExists(f1) and FileExists(f2)) then Exit;
fs1 := TFileStream.Create(f1, fmOpenRead);
fs2 := TFileStream.Create(f2, fmOpenRead);

if fs1.Size <> fs2.Size then
begin
fs1.Free;
fs2.Free;
Exit;
end;

Result := True;
Randomize;
for i := 0 to 9 do
begin
p := Random(fs1.Size);
fs1.Position := p;
fs2.Position := p;
fs1.ReadBuffer(b1,1);
fs2.ReadBuffer(b2,1);
if b1 <> b2 then
begin
Result := False;
Break;
end;
end;

fs1.Free;
fs2.Free;
end;