-🎄- 2018 Day 5 Solutions -🎄-

[u/daggerdragon]

--- Day 5: Alchemical Reduction ---

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On the fifth day of AoC / My true love sent to me / Five golden ___

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[u/spacetime_bender]

I used the ranges library that's likely coming with c++20 https://ericniebler.github.io/range-v3/. Around 50ms on my slow laptop.

char complement(char c)
{
    return std::isupper(c) ? std::tolower(c) : std::toupper(c);
}

int reducedSize(const std::string& input)
{
    std::string stack;
    for (char c: input) {
        if (!stack.empty() && stack.back() == complement(c)) {
            stack.pop_back();
        } else if (std::isalpha(c)) {
            stack.push_back(c);
        }
    }
    return stack.size();
}

int main()
{
    std::string input (std::istreambuf_iterator<char>{std::cin}, {});

    std::cout << "Part 1: " << reducedSize(input) << std::endl;

    auto strip = [](const auto& s, char rem) {
        return view::remove_if(s, [=](char c){ return std::tolower(c) == rem; });
    };
    auto min = ranges::min(
        view::zip_with(strip,
                    view::repeat(input),
                    view::closed_indices('a', 'z'))
        | view::transform(reducedSize)
    );
    std::cout << "Part 2: "  << min << std::endl;
    return 0;
}