function check_mem(max, offs)
    if max < offs then
        print(string.format("Error: Memory offset out of bounds: %d", offs))
        os.exit(false)
    end
end

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

        if mem[pos] == 1 or mem[pos] == 2 then
            if #mem < pos + 3 then
                print(string.format("Error: Opcode too short at end of memory: %d", pos))
                os.exit(false)
            end
            arg1 = mem[pos + 1] + 1
            arg2 = mem[pos + 2] + 1
            res = mem[pos + 3] + 1
            check_mem(#mem, arg1)
            check_mem(#mem, arg2)
            check_mem(#mem, res)
            if mem[pos] == 1 then
                mem[res] = mem[arg1] + mem[arg2]
            else
                mem[res] = mem[arg1] * mem[arg2]
            end

            pos = pos + 4
        elseif mem[pos] == 99 then
            return
        else
            print(string.format("Error: Invalid opcode %d at %d", mem[pos], 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