Subversion Repositories russels_paradox

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-- http://en.wikipedia.org/wiki/Barber_paradox =>

-- "The barber is a man in town who shaves all those, and only those,

-- men in town who do not shave themselves."

--The point is that the barber is represented by the logical value '1' in the two

--stage shift register. The '1' shows which set he currently belongs to. In a more

--sophisticated case we may assume there are several barbers in different cities,

--and the sets may include more elements at once.

--The reset state is arbitrarily choosen, and in a complex system might effect

--the long term behaviour: like if the model of the sets definition would somwhow

--happen to be an LFSR that does not run through all the combinations...

--If the barbers are not identical, FIFO-s can be used to hold their identifiers.

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library IEEE;

use IEEE.std_logic_1164.all;

use IEEE.numeric_std.all;

use IEEE.std_logic_unsigned.all;

entity barbers is

    Port ( big_bang : in  STD_LOGIC;

           a_barber_shaves_event : in  STD_LOGIC;

           a_barber_doesnt_shave_event : in  STD_LOGIC; --like for a day...

           number_of_barbers_who_may_shave_themselves : out  STD_LOGIC;

           number_of_barbers_who_may_not_shave_themselves : out  STD_LOGIC);

end barbers;

architecture simulateable of barbers is

begin

   process(big_bang, a_barber_shaves_event, a_barber_doesnt_shave_event)

   begin

      if big_bang = '1' then

         number_of_barbers_who_may_shave_themselves <= x"0";

         number_of_barbers_who_may_not_shave_themselves <= x"f";

      else

         number_of_barbers_who_may_shave_themselves <= number_of_barbers_who_may_shave_themselves + a_barber_doesnt_shave_event - a_barber_shaves_event;

         number_of_barbers_who_may_not_shave_themselves <= number_of_barbers_who_may_not_shave_themselves - a_barber_doesnt_shave_event + a_barber_shaves_event;

      end if;

   end process;

end simulateable;

architecture synthesisable of barbers is

   signal counter1, counter2 : std_logic_vector(3 downto 0);

begin

   process(big_bang, a_barber_shaves_event)

   begin

      if big_bang = '1' then

         counter1 <= x"f";

      elsif rising_edge(a_barber_shaves_event) then

         counter1 <= counter1 + '1';

      end if;

   end process;

   process(big_bang, a_barber_doesnt_shave_event)

   begin

      if big_bang = '1' then

         counter2 <= x"0";

      elsif rising_edge(a_barber_doesnt_shave_event) then

         counter2 <= counter2 + '1';

      end if;

   end process;

number_of_barbers_who_may_shave_themselves <= counter2 - counter1;

number_of_barbers_who_may_not_shave_themselves <= counter1 - counter2;

end synthesisable;