1 files changed, 146 insertions, 0 deletions

diff --git a/2017/src/day14.cpp b/2017/src/day14.cpp
new file mode 100644
index 0000000..d226d46
--- /dev/null
+++ b/2017/src/day14.cpp
@@ -0,0 +1,146 @@
+#include <array>
+#include <iostream>
+#include <numeric>
+#include <queue>
+#include <set>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+vector<unsigned>
+gen_data(size_t len)
+{
+        vector<unsigned> data(len);
+        iota(data.begin(), data.end(), 0UL);
+        return data;
+}
+
+void
+reverse(vector<unsigned>& data, size_t start, size_t len)
+{
+        auto end = start + len;
+
+        while ( start < end ) {
+                swap(data.at(start++ % data.size()), data.at(--end % data.size()));
+        }
+}
+
+void
+hash64(vector<unsigned>& data, const vector<size_t>& lengths)
+{
+        size_t pos  = 0;
+        size_t skip = 0;
+        for ( size_t i = 0; i != 64; ++i ) {
+                for ( auto length: lengths ) {
+                        reverse(data, pos, length);
+                        pos += length + skip;
+                        ++skip;
+                }
+        }
+}
+
+vector<uint8_t>
+knothash(string line)
+{
+        line += char(17);
+        line += char(31);
+        line += char(73);
+        line += char(47);
+        line += char(23);
+
+        vector<size_t> lengths;
+        for ( const auto byte: line ) {
+                lengths.emplace_back(static_cast<unsigned char>(byte));
+        }
+
+        auto data = gen_data(256);
+        hash64(data, lengths);
+
+        vector<uint8_t> result;
+        for ( size_t i = 0; i != 256; i += 16 ) {
+                size_t value = 0;
+                for ( size_t j = 0; j != 16; ++j ) {
+                        value ^= data.at(i + j);
+                }
+                result.emplace_back(value & 0xFF);
+        }
+        return result;
+}
+
+void
+part1(const string& line)
+{
+        int count = 0;
+        for ( int i = 0; i != 128; ++i ) {
+                auto result = knothash(line + "-"s + to_string(i));
+
+                for ( const auto value: result ) {
+                        count += popcount(value);
+                }
+        }
+        cout << "Part1: " << count << '\n';
+}
+
+void
+part2(const string& line)
+{
+        set<tuple<size_t, size_t>> grid;
+
+        for ( size_t y = 0; y != 128; ++y ) {
+                auto result = knothash(line + "-"s + to_string(y));
+
+                for ( size_t x = 0; x != 128; ++x ) {
+                        auto value = result.at(x / 8);
+                        auto mask  = 1u << (7 - (x % 8));
+
+                        if ( (value & mask) != 0 ) {
+                                grid.emplace(x, y);
+                        }
+                }
+        }
+
+        int count = 0;
+
+        for ( size_t x = 0; x != 128; ++x ) {
+                for ( size_t y = 0; y != 128; ++y ) {
+                        if ( !grid.contains({ x, y }) ) {
+                                continue;
+                        }
+
+                        ++count;
+
+                        queue<tuple<size_t, size_t>> queue;
+                        queue.emplace(x, y);
+
+                        while ( !queue.empty() ) {
+                                auto pos = queue.front();
+                                queue.pop();
+
+                                grid.erase(pos);
+
+                                static array<tuple<size_t, size_t>, 4> dirs{ make_tuple(-1, 0), make_tuple(1, 0), make_tuple(0, -1), make_tuple(0, 1) };
+
+                                for ( const auto [dx, dy]: dirs ) {
+                                        const auto new_pos = make_tuple(dx + get<0>(pos), dy + get<1>(pos));
+                                        if ( grid.contains(new_pos) ) {
+                                                queue.emplace(new_pos);
+                                        }
+                                }
+                        }
+                }
+        }
+        cout << "Part2: " << count << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+        const string input = "ffayrhll";
+        part1(input);
+        part2(input);
+}