LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.numeric_std.all;
USE work.mips_types.all;

ENTITY e_mips_shifter IS
    PORT (
        i_arith: IN  std_logic;
        i_left:  IN  std_logic;
        i_val:   IN  std_logic_vector(31 DOWNTO 0);
        i_num:   IN  std_logic_vector(31 DOWNTO 0);
        o_val:   OUT std_logic_vector(31 DOWNTO 0)
    );
END ENTITY e_mips_shifter;

ARCHITECTURE a_mips_shifter OF e_mips_shifter IS

    SIGNAL s_fill: std_logic;

    FUNCTION l_shift(b:    std_logic;
                     val:  std_logic_vector(31 DOWNTO 0);
                     n:    natural) RETURN std_logic_vector IS
        VARIABLE v_fill: std_logic_vector(31 DOWNTO 0);
    BEGIN
        v_fill := (OTHERS => '0');
        IF b = '1' THEN
            RETURN val(31 - n DOWNTO 0) & v_fill(n - 1 DOWNTO 0);
        ELSE
            RETURN val;
        END IF;
    END FUNCTION l_shift;

    FUNCTION r_shift(b:    std_logic;
                     val:  std_logic_vector(31 DOWNTO 0);
                     fill: std_logic;
                     n:    natural) RETURN std_logic_vector IS
        VARIABLE v_fill: std_logic_vector(31 DOWNTO 0);
    BEGIN
        v_fill := (OTHERS => fill);
        IF b = '1' THEN
            RETURN v_fill(n - 1 DOWNTO 0) & val(31 DOWNTO n);
        ELSE
            RETURN val;
        END IF;
    END FUNCTION r_shift;

BEGIN

    s_fill <= i_arith AND i_val(31);

    p_shift: PROCESS(i_left, i_val, i_num, s_fill)
        VARIABLE v1:   std_logic_vector(31 DOWNTO 0);
        VARIABLE v2:   std_logic_vector(31 DOWNTO 0);
        VARIABLE v3:   std_logic_vector(31 DOWNTO 0);
        VARIABLE v4:   std_logic_vector(31 DOWNTO 0);
        VARIABLE v5:   std_logic_vector(31 DOWNTO 0);
        CONSTANT zero: std_logic_vector(31 DOWNTO 5) := (OTHERS => '0');
    BEGIN
        IF i_left = '1' THEN
            v1 := l_shift(i_num(0), i_val,  1);
            v2 := l_shift(i_num(1), v1,     2);
            v3 := l_shift(i_num(2), v2,     4);
            v4 := l_shift(i_num(3), v3,     8);
            v5 := l_shift(i_num(4), v4,    16);
        ELSE
            v1 := r_shift(i_num(0), i_val, s_fill,  1);
            v2 := r_shift(i_num(1), v1,    s_fill,  2);
            v3 := r_shift(i_num(2), v2,    s_fill,  4);
            v4 := r_shift(i_num(3), v3,    s_fill,  8);
            v5 := r_shift(i_num(4), v4,    s_fill, 16);
        END IF;
        IF i_num(31 DOWNTO 5) = zero THEN
            o_val <= v5;
        ELSE
            o_val <= (OTHERS => s_fill);
        END IF;
    END PROCESS p_shift;

END ARCHITECTURE a_mips_shifter;