Delphi写的验证身份证号有效性函数

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function ValidatePID(const APID: string): string;
{内部函数,取身份证号校验位,最后一位,对18位有效}
function GetVerifyBit(sIdentityNum: string): Char;
var
    nNum: Integer;
begin
    Result := #0;
    nNum := StrToInt(sIdentityNum[1]) * 7 +
            StrToInt(sIdentityNum[2]) * 9 +
            StrToInt(sIdentityNum[3]) * 10 +
            StrToInt(sIdentityNum[4]) * 5 +
            StrToInt(sIdentityNum[5]) * 8 +
            StrToInt(sIdentityNum[6]) * 4 +
            StrToInt(sIdentityNum[7]) * 2 +
            StrToInt(sIdentityNum[8]) * 1 +
            StrToInt(sIdentityNum[9]) * 6 +
            StrToInt(sIdentityNum[10]) * 3 +
            StrToInt(sIdentityNum[11]) * 7 +
            StrToInt(sIdentityNum[12]) * 9 +
            StrToInt(sIdentityNum[13]) * 10 +
            StrToInt(sIdentityNum[14]) * 5 +
            StrToInt(sIdentityNum[15]) * 8 +
            StrToInt(sIdentityNum[16]) * 4 +
            StrToInt(sIdentityNum[17]) * 2;
    nNum := nNum mod 11;
    case nNum of
      0: Result := '1';
      1: Result := '0';
      2: Result := 'X';
      3: Result := '9';
      4: Result := '8';
      5: Result := '7';
      6: Result := '6';
      7: Result := '5';
      8: Result := '4';
      9: Result := '3';
      10: Result := '2';
    end;
end;
var
L : Integer;
sCentury : string;
sYear2Bit : string;
sMonth : string;
sDate : string;
iCentury : Integer;
iMonth : Integer;
iDate : Integer;
CRCFact : string;//18位证号的实际值
CRCTh : string; //18位证号的理论值
FebDayAmt: Byte;//2月天数
begin
L := Length(APID);
if (L in [15, 18]) = False then
begin
     Result := Format('身份证号不是15位或18位(%0:s, 实际位数:%1:d)', [APID, L]);
     Exit;
end;
CRCFact := '';
if L = 18 then
begin
     sCentury := Copy(APID, 7, 2);
     iCentury := StrToInt(sCentury);
     if (iCentury in [18..20]) = False then
     begin
        Result := Format('身份证号码无效:18位证号的年份前两位必须在18-20之间(%0:S)', [sCentury]);
        Exit;
     end;
     sYear2Bit := Copy(APID, 9, 2);
     sMonth := Copy(APID, 11, 2);
     sDate := Copy(APID, 13, 2);
     CRCFact := Copy(APID, 18, 1);
end else
begin
     sCentury := '19';
     sYear2Bit := Copy(APID, 7, 2);
     sMonth := Copy(APID, 9, 2);
     sDate := Copy(APID, 11, 2);
end;
iMonth := StrToInt(sMonth);
iDate := StrToInt(sDate);
if (iMonth in [01..12]) = False then
begin
     Result := Format('身份证号码无效:月份必须在01-12之间(%0:s)', [sMonth]);
     Exit;
end;
if (iMonth in [1, 3, 5, 7, 8, 10, 12]) then
begin
     if (iDate in [01..31]) = False then
     begin
        Result := Format('身份证号码无效:日期无效,不能为零或超出当月最大值(%0:s)', [sDate]);
        Exit;
     end;
end;
if (iMonth in [4, 6, 9, 11]) then
begin
     if (iDate in [01..30]) = False then
     begin
        Result := Format('身份证号码无效:日期无效,不能为零或超出当月最大值(%0:s)', [sDate]);
        Exit;
     end;
end;
if IsLeapYear(StrToInt(sCentury + sYear2Bit)) = True then
begin
     FebDayAmt := 29;
end else
begin
     FebDayAmt := 28;
end;
if (iMonth in [2]) then
begin
     if (iDate in [01..FebDayAmt]) = False then
     begin
        Result := Format('身份证号码无效:日期无效,不能为零或超出当月最大值(%0:s)', [sDate]);
        Exit;
     end;
end;
if CRCFact <> '' then
begin
     CRCTh := GetVerifyBit(APID);
     if CRCFact <> CRCTh then
     begin
        Result := Format('身份证号码无效:校验位(第18位)错:(%0:s)', [APID]);
        Exit;
     end;
end;
end;