BloodAltar_bot_v4

input_slot = 1      -- location in inventory that the turtle uses to grab an item to place
additional_input_slot = 5 -- location in inventory of the second input to be used when the first is empty
checking_slot = 15  -- locaiton in inventory that the turtle sucks an item to check if it is complete
target_slot = 16    -- location in inventory that the turtle compares an item against to see if it is complete
output_slot = 2     -- location in inventory that the final output is placed
error_slot = 3      -- location in inventory that the final output is placed if output_slot is full of something else

function IsDone()
    local done = false
    if turtle.getItemCount(target_slot) > 0 then -- only compares to target_slot if something in target_slot
        turtle.select(checking_slot)
        turtle.suck()
        if turtle.compareTo(target_slot) then -- compare the new item to the target item
            -- items match, we're done
            turtle.transferTo(output_slot) -- move to final location
            done = true
        else
            -- not done, put it back in and continue
            turtle.drop()
        end
    else
        turtle.select(output_slot)
        turtle.suck()
        done = true
    end
    return done
end

-- main program
local current_input_slot = input_slot
turtle.select(current_input_slot)
if turtle.getItemCount(current_input_slot) > 0 then
    turtle.drop(1)
    while true do
        local lamp = rs.getAnalogInput("front")
        if lamp == 15 then -- only check when done is triggered
            if IsDone() then
                turtle.select(current_input_slot)
                if turtle.getItemCount() == 0 then -- if input_slot is empty, break the loop and end program.
                    current_input_slot = additional_input_slot
                    turtle.select(current_input_slot)
                    if turtle.getItemCount() == 0 then
                        break
                    end
                end
                -- if slot isn't empty, insert the next one
                turtle.drop(1)
            end
        end
        os.pullEvent("redstone")
    end
end
write("Input slot is empty.\n")