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 or opcode == 2 then
            local arg1 = get_param(mem, pos, 1)
            local arg2 = get_param(mem, pos, 2)
            local res = get_arg(mem, pos, 3)
            if opcode == 1 then
                mem[res] = arg1 + arg2
            else
                mem[res] = arg1 * arg2
            end

            pos = pos + 4
        elseif opcode == 3 then
            local arg = get_arg(mem, pos, 1)
            io.stdout:write("> ")
            local val = io.stdin:read("n")
            io.stdout:write(val)
            io.stdout:write("\n")
            mem[arg] = val

            pos = pos + 2
        elseif opcode == 4 then
            io.stdout:write(get_param(mem, pos, 1))

            pos = pos + 2
        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()

memory_p1 = {table.unpack(memory)}
memory_p1[2] = 12
memory_p1[3] = 2
interpret(memory_p1)
print("Part 1:", memory_p1[1])

-- Brute force
for noun = 0, 99 do
    for verb = 0, 99 do
        memory_p2 = {table.unpack(memory)}
        memory_p2[2] = noun
        memory_p2[3] = verb
        interpret(memory_p2)
        if memory_p2[1] == 19690720 then
            print("Part 2:", 100 * noun + verb)
            os.exit(true)
        end
    end
end