-------------------------------------------------------------------------------
-- parityenable.vhd - Entity and architecture
-------------------------------------------------------------------------------
--
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------------------------------------------------------------------------------
-- Filename:        parity.vhd
--
-- Description:     Generate parity optimally for all target architectures
--
-- VHDL-Standard:   VHDL'93
-------------------------------------------------------------------------------
-- Structure:
--                  parity.vhd
--                    xor18.vhd
--                    parity_recursive_LUT6.vhd
--
-------------------------------------------------------------------------------
-- Author:          stefana
-------------------------------------------------------------------------------
-- Naming Conventions:
--      active low signals:                     "*_n"
--      clock signals:                          "clk", "clk_div#", "clk_#x"
--      reset signals:                          "rst", "rst_n"
--      generics:                               "C_*"
--      user defined types:                     "*_TYPE"
--      state machine next state:               "*_ns"
--      state machine current state:            "*_cs"
--      combinatorial signals:                  "*_com"
--      pipelined or register delay signals:    "*_d#"
--      counter signals:                        "*cnt*"
--      clock enable signals:                   "*_ce"
--      internal version of output port         "*_i"
--      device pins:                            "*_pin"
--      ports:                                  - Names begin with Uppercase
--      processes:                              "*_PROCESS"
--      component instantiations:               "<ENTITY_>I_<#|FUNC>
-------------------------------------------------------------------------------

library ieee;
use ieee.std_logic_1164.all;

entity ParityEnable is
  generic (
    C_USE_LUT6 :     boolean := true;
    C_SIZE     :     integer := 4
    );
  port (
    InA        : in  std_logic_vector(0 to C_SIZE - 1);
    Enable     : in  std_logic;
    Res        : out std_logic
    );
end entity ParityEnable;

library unisim;
use unisim.vcomponents.all;

architecture IMP of ParityEnable is

  -- Non-recursive loop implementation
  function ParityGen (InA : std_logic_vector) return std_logic is
    variable result : std_logic;
  begin
    result := '0';
    for I in InA'range loop
      result := result xor InA(I);
    end loop;
    return result;
  end function ParityGen;

begin  -- architecture IMP

  Using_LUT6 : if (C_USE_LUT6) generate

    --------------------------------------------------------------------------------------------------
    -- Single LUT6
    --------------------------------------------------------------------------------------------------
    Single_LUT6 : if C_SIZE > 1 and C_SIZE <= 5 generate
      signal inA5 : std_logic_vector(0 to 4);
    begin

      Assign_InA : process (InA) is
      begin
        inA5                      <= (others => '0');
        inA5(0 to InA'length - 1) <= InA;
      end process Assign_InA;

      XOR6_LUT : LUT6
        generic map(
          INIT => X"9669699600000000")
        port map(
          O  => Res,
          I0 => InA5(4),
          I1 => inA5(3),
          I2 => inA5(2),
          I3 => inA5(1),
          I4 => inA5(0),
          I5 => Enable);
    end generate Single_LUT6;

  end generate Using_LUT6;

  -- Fall-back implementation without LUT6
  Not_Using_LUT6 : if not C_USE_LUT6 or C_SIZE > 8 generate
  begin
    Res <= Enable and ParityGen(InA);
  end generate Not_Using_LUT6;

end architecture IMP;