delphi分享Pos函数。(比FastPos还要快)

function QuickPos(const Substr, S: WideString; MatchesIndex: Integer = 1): Integer;
function QuickPosBack(const Substr, S: WideString; MatchesReverseIndex: Integer = 1): Integer;

主要用途是搜索字符串中第n个Substr。
经过测试，这2个函数的速度比直接用Pos+Copy快好几倍(如果字符串够长，可能10几倍)
比Pos+Delete(如JVCL的函数NPos)处理要快(至少2-3倍以上)。

说明1：虽然说速度上的差异是微秒级别的，但是一个快速方便的Pos函数，还是会给编程带来很多方便。
说明2：经过大量测试和FastString作者的承认，FastString的速度还没有DELPHI标准函数快。

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// Compares a substring with a string. *for inline use"
// C: 2004-07-05 | M: 2004-07-05
function _InlineCompareText(const Substr, S: WideString; StartIndex: Integer = 1; LenOfSubstr: Integer = -1; LenOfS: Integer = -1): Boolean;
var
I: Integer;
begin
if LenOfSubstr = -1 then LenOfSubstr := Length(Substr);
if LenOfS = -1 then LenOfS := Length(S);
if LenOfSubstr > LenOfS then
begin
Result := False;
Exit;
end;
for I := 1 to LenOfSubstr do
if Substr[I] <> S[I + StartIndex - 1] then
begin
Result := False;
Exit;
end;
Result := True;
end;

// Returns the 1. index of a substring within a string start at a certain index.
// C: 2004-07-05 | M: 2004-07-05 | P: 1.0+
function _PosForward(const Substr, S: WideString; StartIndex: Integer; LenOfSubstr: Integer = -1; LenOfS: Integer = -1): Integer;
var
I: Integer;
begin
Result := 0;
case LenOfSubstr of
0: Exit;
-1: LenOfSubstr := Length(Substr);
end;
if LenOfS = -1 then LenOfS := Length(S);

for I := StartIndex to LenOfS do
begin
if (S[I] = Substr[1]) and _InlineCompareText(Substr, S, I, LenOfSubstr, LenOfS) then
begin
Result := I;
Exit;
end;
end;
end;

// Returns the 1. index of a substring within a string.
// Note: Searching time will increase when MatchesIndex increased.
// C: 2004-04-09 | M: 2004-07-05 | P: 1.0+
function QuickPos(const Substr, S: WideString; MatchesIndex: Integer = 1): Integer;
var
LenOfS, LenOfSubstr: Integer;
begin
Result := Pos{Pos}(Substr, S);

if (MatchesIndex = 1) or (Result = 0) then Exit;
LenOfS := Length(S);
LenOfSubstr := Length(Substr);

while (MatchesIndex > 1) and (Result > 0) do
begin
Result := _PosForward{Pos}(Substr, S, Result + 1, LenOfSubstr, LenOfS); // Tip!! Do not use func.Copy!!
if Result = 0 then Exit;
Dec(MatchesIndex);
end;
end;

// Returns the last index of a substring within a string.
// Todo: Using asm to rewrite this function. The asm-code looks very like func.Pos!
// C: 2004-04-09 | M: 2004-07-03 | P: n/a
function _PosBack(const Substr, S: WideString; StopIndex: Integer = -1; LenOfSubstr: Integer = -1): Integer;
var
I: Integer;
begin
Result := 0;
case LenOfSubstr of
0: Exit;
-1: LenOfSubstr := Length(Substr);
end;
if StopIndex = -1 then StopIndex := Length(S);

for I := StopIndex - LenOfSubstr + 1 downto 1 do
begin
if (S[I] = Substr[1]) and _InlineCompareText(Substr, S, I, LenOfSubstr) then
begin
Result := I;
Exit;
end;
end;
end;

// Returns the last index of a substring within a string.
// C: 2004-04-09 | M: 2004-07-03 | P: n/a
function QuickPosBack(const Substr, S: WideString; MatchesReverseIndex: Integer = 1): Integer;
var
LenOfSubstr: Integer;
begin
Result := _PosBack{Pos}(Substr, S);

if (MatchesReverseIndex = 1) or (Result = 0) then Exit;
LenOfSubstr := Length(Substr);

while (MatchesReverseIndex > 1) and (Result > 0) do
begin
Result := _PosBack{Pos}(Substr, S, Result + LenOfSubstr - 2, LenOfSubstr);
Dec(MatchesReverseIndex);
end;
end;