program Project2;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var
  i, number, rest : integer;
  hex : string;
  decision : char;

//This function is simple one. It calculates n-th power of p.
function power (p : integer; n : integer) : integer;
begin
   result := 1; //set result to 1 because...
   for i := 1 to n do //...if n is 0 then next instruction is never terminated
   result := result * p  //for n times multiply p
end;

function dec2hex (n : integer) : string;
begin
   while (number > 0) do
   begin
    rest := number mod 16; //dzielenie całkowitoliczbowe (result is a whole number)
    number := number div 16; //reszta z dzielenia
    if rest > 10 then Insert(Char((rest-10)+65), result, 0) //becuase result is string and we
    //kind of calculate the number from its and, so we use Insert() to insert calculated digits
    //in front of the whole number
    //Char(x) returns an ASCII charachter with index of x
    //This if is to check if we deal with rest bigger than 10. If yes, we need to change it
    //accordingly to A, B, C and so on. First I subtract 10 to receive index of letter and then
    //I add index of capital A (which is 65) to move to the right index of wanted letter
    else Insert(IntToStr(rest), result, 0);
    //It's simplier if rest is simply number lesser than 10. We just convert it to String (same
    //could be achieved with Char(rest+48), where 48 is ASCII index of 0) and insert it in front of
    //result string.
   end;
end;

//That function is most complicated. It took me almost two hours to find that little problem with
//strings and there strange indexing of letters -_-.
//Algorithm is quite simple. We have to read input string from the end (less meaningfull digit)
//convert char type to integer and then add proper power of 16 multiplied by that number.
function hex2dec (s : string) : integer;
begin
    for i := 1 to Length(s) do  //we'll be going from i=1 to the length of s string
    begin
      rest := integer(s[Length(s)-i+1]); //I used "rest" variable to save memory, it's name is irrelevant to function
      //integer() converts anything that is in bracket to integer, in polish it's rzutowanie, I don't know english equivalent
      //s[x] is simply x-th letter of s string, where 1 is first and length(s) is last
      //we'll be going from length(s) to 1, so we are subtracting i with each going through loop to decrement
      //+1 is what I've been looking for, it is to prevent to reach s[0] which gives crazy values
      if rest < 65 then rest := rest - 48 //same thing as in dec2hex, if rest wasn't a letter
      //it's index in ASCII will be greater than 65. If so, we need to subtract 55, otherwise only 48.
      else rest := rest - 55;
      result := result + rest*power(16, i-1);
    end;
end;

begin
  hex := '';
  number := 0;
  writeln('1: Dec -> Hex');
  writeln('2: Hex -> Dec');
  write('Choose one of the following options: ');
  readln(decision); //this is to know if we're converting dec -> hex or hex -> dec
  writeln;
  case decision of  //this is to break our program to cases: if user choosed '1' then this, or if user
  //choosed '2' then that
  '1': begin
    write('Please write the number you want to convert: ');
    readln(number); //reads the vaule of number from keyboard
    writeln('This number in hexadecimal system is: ' + dec2hex(number));
    end;
  '2': begin
    write('Please write the number you want to convert: ');
    readln(hex);
    writeln('This number in decimal system is: ' + IntToStr(hex2dec(hex))) //IntToStr is a function
    //that converts Integer type to String, since result of hex2dec() is int, and writeln only accepts
    //strings as arguments
  end;
  else
    writeln('Wrong input.');
  end;
  writeln;
  writeln('Press ENTER to end program');
  readln;
end.