Converting Decimal to Hexadecimal number using MODULO DIVISION method

Question

I am converting Decimal number

42541956123769884636017138956568135808

to hexadecimal number

20014860486000000000000000008880

My converted value is 20014860486000000000000000019980

My Postgresql function for conversion is

CREATE OR REPLACE FUNCTION ipv6_dec_hex(ip_number numeric)
  RETURNS character varying AS
$BODY$
DECLARE

    ip  int :=null;
    ip1 character varying := '';
    ip2 character varying;

BEGIN
        while(ip_number != 0)
        LOOP
            ip = trunc(mod(ip_number,16));
            ip_number = trunc(ip_number/16);
            ip1:=ip1||to_hex(ip);
            raise notice 'ip is %',ip;
            raise notice 'ip_number is %',ip_number;

        END LOOP;
        ip2:=reverse(ip1);

        return ip2;
END;
   $BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

I am using modulo-remainder method. so I am taking modulus for the given number with 16 and divide given decimal number by 16 for consecutive hexadecimal number conversions. While taking modulus in first iteration I am getting

0 as remainder as last digit of hexadecimal number 20014860486000000000000000008880

and next dividend is 2658872257735617789751071184785508488

In second iteration taking modulus of 2658872257735617789751071184785508488 getting

8 as remainder second from last digit of hexadecimal number 20014860486000000000000000008880

and next dividend is 166179516108476111859441949049094281

In third iteration taking modulus of 166179516108476111859441949049094281 getting

9 as remainder but 8 is the exact hexadecimal digit to come in third from last digit of hexadecimal number 20014860486000000000000000008880

If the above dividend value is 166179516108476111859441949049094280 instead of 166179516108476111859441949049094281 then i will get the remainder value as 8 . How could i get the exact hexadecimal value 20014860486000000000000000008880 instead of 20014860486000000000000000019980

Answer 1

If you say this you'll get floating-point division, and that will give a loss of precision.

ip_number = trunc(ip_number/16);

Say this instead

ip_number = div(ip_number,16);

docs here

full source after edit:

CREATE OR REPLACE FUNCTION ipv6_dec_hex(ip_number numeric)
  RETURNS character varying AS
$BODY$
DECLARE

    ip  int :=null;
    ip1 character varying := '';
    ip2 character varying;

BEGIN
        while(ip_number != 0)
        LOOP
            ip = mod(ip_number,16);
            ip_number = div(ip_number,16);
            ip1:=ip1||to_hex(ip);
            raise notice 'ip is %',ip;
            raise notice 'ip_number is %',ip_number;

        END LOOP;
        ip2:=reverse(ip1);

        return ip2;
END;
   $BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

select ipv6_dec_hex(42541956123769884636017138956568135808)
   ='20014860486000000000000000008880';