The art of computer programming vol.1 coroutine example in lua (3 passes)

main.lua

INPUT = "A2B5E3426FG0ZYW3210PQ89R."

-- Registers
CH = nil
NUM = nil
COUNTER = 0

READ_INDEX = 1
WRITE_INDEX = 1

-- Coroutines
READ = coroutine.create(function()

    for READ_INDEX = 1, #INPUT do
        CH = INPUT:sub(READ_INDEX, READ_INDEX)

        if CH ~= ' ' then
            coroutine.yield(FILTER)

            if CH == '.' then
                print() -- line feed
                break 
            end
        end
    end
end)

FILTER = coroutine.create(function() 

    while true do
        NUM = tonumber(CH)

        if NUM ~= nil then
            NUM = NUM + 1
            coroutine.yield(READ)
        else
            NUM = 1
        end
        
        coroutine.yield(WRITE)
    end
end)

WRITE = coroutine.create(function() 

    while true do
        for WRITE_INDEX = 1, NUM do

            if COUNTER > 0 and COUNTER % 3 == 0 then 
                io.write(' ')
            end
            io.write(CH)
            COUNTER = COUNTER + 1
        end

        coroutine.yield(READ)
    end
end)

-- Dispatcher
CUR_ROUTINE = READ
while true do
    local stat, val = coroutine.resume(CUR_ROUTINE)
    CUR_ROUTINE = val

    if CUR_ROUTINE == nil then break end
end