Guesses

{$mode objfpc}
program guesses;
uses crt, sysutils;

type TSetChar = set of char;


function GetKey(const LIST_OF_KEYS : TSetChar; const PROMPT : string = '') : char;
begin
   repeat
      Write(PROMPT);
      GetKey := crt.ReadKey;
   until GetKey in LIST_OF_KEYS;
   WriteLn(GetKey);
end;


// We cannot use DIV or MOD. The student doesn't know bitwise operations (SHR and SHL).
function GetMid(lower, upper : longint) : longint;
var diff : longint;
    rest : longint;
begin
   diff := upper - lower;
   rest := diff;
   while rest > 1 do begin
      Inc(lower);
      Dec(rest, 2);
   end;
   GetMid := lower;
end;


function Guess(lower, upper : longint; var tries : byte) : longint;
var mid : longint;
begin
   Inc(tries);
   if lower = upper then
      guess := lower
   else begin
      mid := GetMid(lower, upper);
      case GetKey(['+','-','='], 'N=' + IntToStr(mid) + ' --> greater = "+", lesser = "-", equal = "=" ?') of
         '+' : Guess := Guess(mid+1, upper, tries);
         '-' : Guess := Guess(lower, mid-1, tries);
         '=' : Guess := mid;
      end;
   end;
end;


(* BLOCO PRINCIPAL *)
const LOWERLIM = 1;
      UPPERLIM = 1000;

var tries : byte = 0;
    g : longint;

begin
   WriteLn('Think in a number between ', LOWERLIM, ' and ', UPPERLIM, ', including those two.');
   g := Guess(LOWERLIM, UPPERLIM, tries);
   WriteLn('Your number = ', g, ' (in ', tries, ' tries)');
end.