Green function: verify ID card number Validity

{Verify the validity of the ID card number. If the returned value is null, it indicates normal. Otherwise, it indicates an error message}
{Author: Moon night kite, edocu@163.com}
Function validatepid (const apid: string): string;
{Internal function, which is the last digit of the ID card number, valid for 18 digits}
Function getverifybit (sidentitynum: string): Char;
VaR
Nnum: integer;
Begin
Result: = #0;
Nnum: = strtoint (sidentitynum [1]) * 7 +
Strtoint (sidentitynum [2]) * 9 +
Strtoint (sidentitynum [3]) x 10 +
Strtoint (sidentitynum [4]) x 5 +
Strtoint (sidentitynum [5]) x 8 +
Strtoint (sidentitynum [6]) x 4 +
Strtoint (sidentitynum [7]) x 2 +
Strtoint (sidentitynum [8]) * 1 +
Strtoint (sidentitynum [9]) x 6 +
Strtoint (sidentitynum [10]) x 3 +
Strtoint (sidentitynum [11]) * 7 +
Strtoint (sidentitynum [12]) * 9 +
Strtoint (sidentitynum [13]) * 10 +
Strtoint (sidentitynum [14]) x 5 +
Strtoint (sidentitynum [15]) x 8 +
Strtoint (sidentitynum [16]) x 4 +
Strtoint (sidentitynum [17]) * 2;
Nnum: = nnum mod 11;
Case nnum
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; // the actual value of the 18-digit certificate number
Crcth: string; // theoretical value of the 18-digit certificate number
Febdayamt: byte; // day of the month
Begin
L: = length (apid );
If (L in [15, 18]) = false then
Begin
Result: = format ('Id card number is not 15 or 18 bits (% 0: S, actual bits: % 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 ('invalid ID number: The first two digits of the 18-digit ID number must be between 18-20 (% 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 ('invalid ID card number: The month must be between 01-12 (% 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 ('invalid ID card number: Invalid date, cannot be zero or exceed the maximum value of the current month (% 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 ('invalid ID card number: Invalid date, cannot be zero or exceed the maximum value of the current month (% 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 ('invalid ID card number: Invalid date, cannot be zero or exceed the maximum value of the current month (% 0: s) ', [sdate]);
Exit;
End;
End;
If crcfact <> ''then
Begin
Crcth: = getverifybit (apid );
If crcfact <> crcth then
Begin
Result: = format ('invalid ID card number: Check bit (18th bits) Error :( % 0: s) ', [apid]);
Exit;
End;
End;
End;