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/trunk/barber.vhd
0,0 → 1,74
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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.
----------------------------------------------------------------------------------
 
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
use IEEE.std_logic_unsigned.all;
 
entity barbers is
Port ( 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(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(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(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 some_barbers;

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