from myhdl import *

ACTIVE = 0
DirType = enum('RIGHT', 'LEFT')

def jc2_alt(goLeft, goRight, stop, clk, q):
    
    """ A bi-directional 4-bit Johnson counter with stop control.

    I/O pins:
    --------
    clk      : input free-running slow clock 
    goLeft   : input signal to shift left (active-low switch)
    goRight  : input signal to shift right (active-low switch)
    stop     : input signal to stop counting (active-low switch)
    q        : 4-bit counter output (active-low LEDs; q[0] is right-most)

    Operation:
    ---------
    The counter is triggered on the rising edge of the clock (clk). 
    A low pulse on the goLeft input will cause the counter to start 
    shifting left from its current state. A low pulse on the goRight
    input will cause the counter to start shifting right from its 
    current state. A low pulse on the stop input will cause the 
    counter to hold its current state until goLeft or goRight is pulsed.

    After power-up, the counter is stopped with all outputs low (LEDs lit).

    """

    @instance
    def logic():
        dir = DirType.LEFT
        run = False
        while True:
            yield clk.posedge
            # direction
            if goRight == ACTIVE:
                dir = DirType.RIGHT
                run = True
            elif goLeft == ACTIVE:
                dir = DirType.LEFT
                run = True
            # stop
            if stop == ACTIVE:
                run = False
            # counter action
            if run:
                if dir == DirType.LEFT:
                    q.next[4:1] = q[3:]
                    q.next[0] = not q[3]
                else:
                    q.next[3:] = q[4:1]
                    q.next[3] = not q[0]

    return logic
