lfsr in vhdl

library ieee;
use ieee.std_logic_1164.all;

package lfsr is
    function maxlen_lfsr_advance(reg : std_logic_vector) return std_logic_vector;
end lfsr;

package body lfsr is
    function l_xnor(r : std_logic_vector; i : integer; val : std_logic) return std_logic is
    begin
        if i /= -1 then
            return val xnor r(i - 1);
        end if;
        return val;
    end function;
    function lfs(r : std_logic_vector; a, b, c, d, e, f, g, h, i, j : integer := -1) return std_logic is
    begin
        return l_xnor(r, b, l_xnor(r, c, l_xnor(r, d, l_xnor(r, e, l_xnor(r, f, l_xnor(r, g, l_xnor(r, h, l_xnor(r, i, l_xnor(r, j, r(a - 1))))))))));
    end function;

    function maxlen_lfsr_advance(r : std_logic_vector) return std_logic_vector is
        variable ret : std_logic_vector(r'range);
        variable f : std_logic;
        variable l : integer := r'length;
    begin
        if ret'left > ret'right then
            ret(r'left downto 1) := r(r'left - 1 downto 0);
        else
            ret(1 to r'right) := r(0 to r'right - 1);
        end if;
        if l = 3 then
            f := r(2) xnor r(1);
        elsif l = 4 then
            f := r(3) xnor r(2);
        elsif l = 5 then
            f := lfs(r, 3, 5);
        elsif l = 6 then
            f := lfs(r, 5, 6);
        elsif l = 7 then
            f := lfs(r, 6, 7);
        elsif l = 8 then
            f := lfs(r, 4, 5, 6, 8);
        elsif l = 9 then
            f := lfs(r, 5, 9);
        elsif l = 10 then
            f := lfs(r, 7, 10);
        elsif l = 11 then
            f := lfs(r, 9, 11);
        elsif l = 12 then
            f := lfs(r, 1, 4, 6, 12);
        elsif l = 13 then
            f := lfs(r, 1, 3, 4, 13);
        elsif l = 14 then
            f := lfs(r, 1, 3, 5, 14);
        elsif l = 15 then
            f := lfs(r, 14, 15);
        elsif l = 16 then
            f := lfs(r, 4, 13, 15, 16);
        elsif l = 17 then
            f := lfs(r, 17, 14);
        elsif l = 18 then
            f := lfs(r, 18, 11);
        elsif l = 19 then
            f := lfs(r, 1, 2, 6, 19);
        elsif l = 57 then
            f := r(56) xnor r(49);
        else
            assert false report "no matching shift register configured for length " & integer'image(r'length) severity failure;
        end if;
        ret(0) := f;
        return ret;
    end function;
end lfsr;