Finally, the final day! From what I recall, the last problem is fairly easy compared to the last week or so of problems. The sun unfortunately went down just as a I finished. But I should be able to still finish with plenty of time left over today.

Oh this is annoying. I mean from a design perspective. It's just.... a really weird numbering system.

So first, parsing the numbers into decimal:

while (!input.eof())
{
    uint64_t n = 0;
    getline(input, line);

    for (char c: line)
    {
        n *= 5;
        switch(c)
        {
            case '0': break;
            case '1': n += 1; break;
            case '2': n += 2; break;
            case '-': n -= 1; break;
            case '=': n -= 2; break;
        }
    }

    total += n;
}

Fairly simple. Now, to reverse the process...

string s;
while (total > 0)
{
    switch (((total+2)%5)-2)
    {
        case -2: s.insert(s.begin(), '='); total += 2; break;
        case -1: s.insert(s.begin(), '-'); total += 1; break;
        case  0: s.insert(s.begin(), '0');             break;
        case  1: s.insert(s.begin(), '1'); total -= 1; break;
        case  2: s.insert(s.begin(), '2'); total -= 2; break;
    }
    total /= 5;
}

And that's the end! That was an enjoyable year, even if I stressed myself out by falling a week behind and wanting to cathc up. But it's done, and I feel accomplished!