AoC 2019 D2: 1202 Program Alarm

C++ | Problem statement | Source code | Tags: IntcodeBrute force

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This begins a very interesting series of Intcode problems. This is just like normal Assembly, with the only quirk being that the instruction memory can be modified at runtime. I iteratively modified my setup, and I will describe the version produced by each day.

Part 1

In Day 2, I didn't realize that I would be using this Intcode VM for the rest of the year, so the VM still lives in the solution file. It's a single function called int run_prog(vector<int> codes, int noun, int verb), which is just:

for (int i = 0; i + 3 < codes.size() && codes[i] != 99; i += 4) {
    int opcode = codes[i];
    int op1 = codes[i + 1];
    int op2 = codes[i + 2];
    int dest = codes[i + 3];
    if (opcode == 1) {
        codes[dest] = codes[op1] + codes[op2];
    } else if (opcode == 2) {
        codes[dest] = codes[op1] * codes[op2];
    } else {
        throw "Invalid opcode";
    }
}
cpp

Part 2

I originally thought I had to analyze the program to figure out what noun and verb would produce the desired output, but because the program is self-modifying, this is actually tricky. I missed the line where it says:

Each of the two input values will be between 0 and 99, inclusive.

Once I see that, it's easy enough to just brute force all 10000 combinations of noun and verb.

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