function get_arg(mem, pos, arg)
    local param_mode = mem[pos] // (10 ^ (arg + 1)) % 10
    if #mem < pos + arg then
        print(string.format("Error: Opcode too short at end of memory: %d", pos))
        os.exit(false)
    end
    local arg = mem[pos + arg]
    if param_mode == 0 then
        arg = arg + 1
        if #mem < arg then
            print(string.format("Error: Memory offset out of bounds: %d", arg))
            os.exit(false)
        end
    end
    return arg
end

function get_param(mem, pos, arg)
    local param_mode = mem[pos] // (10 ^ (arg + 1)) % 10
    local arg = get_arg(mem, pos, arg)
    if param_mode == 0 then
        return mem[arg]
    end
    return arg
end

function interpret(mem)
    local pos = 1
    while true do
        if #mem < pos then
            print(string.format("Instruction pointer ran off of memory"))
            os.exit(false)
        end

        local opcode = mem[pos] % 100

        if opcode == 1 then
            mem[get_arg(mem, pos, 3)] = get_param(mem, pos, 1) + get_param(mem, pos, 2)
            pos = pos + 4
        elseif opcode == 2 then
            mem[get_arg(mem, pos, 3)] = get_param(mem, pos, 1) * get_param(mem, pos, 2)
            pos = pos + 4
        elseif opcode == 3 then
            io.stdout:write("> ")
            mem[get_arg(mem, pos, 1)] = io.stdin:read("n")
            pos = pos + 2
        elseif opcode == 4 then
            io.stdout:write(get_param(mem, pos, 1))
            io.stdout:write("\n")
            pos = pos + 2
        elseif opcode == 5 then
            if get_param(mem, pos, 1) ~= 0 then
                pos = get_param(mem, pos, 2) + 1
            else
                pos = pos + 3
            end
        elseif opcode == 6 then
            if get_param(mem, pos, 1) == 0 then
                pos = get_param(mem, pos, 2) + 1
            else
                pos = pos + 3
            end
        elseif opcode == 7 then
            mem[get_arg(mem, pos, 3)] = get_param(mem, pos, 1) < get_param(mem, pos, 2) and 1 or 0
            pos = pos + 4
        elseif opcode == 8 then
            mem[get_arg(mem, pos, 3)] = get_param(mem, pos, 1) == get_param(mem, pos, 2) and 1 or 0
            pos = pos + 4
        elseif opcode == 99 then
            return
        else
            print(string.format("Error: Invalid opcode %d at %d", opcode, pos - 1))
            os.exit(false)
        end
    end
end

file = io.open(arg[1])
memory = {}
for num in string.gmatch(file:read(), "(-?%d+),?") do
    table.insert(memory, tonumber(num))
end
file:close()

interpret(memory)