aoc(2318): Lavaduct Lagoon

1 files changed, 63 insertions, 0 deletions

diff --git a/2023/18/src/part2.cpp b/2023/18/src/part2.cpp
new file mode 100644
index 0000000..e9b6c02
--- /dev/null
+++ b/2023/18/src/part2.cpp
@@ -0,0 +1,63 @@
+#include <iostream>
+#include <fstream>
+#include <sstream>
+#include <vector>
+#include <tuple>
+
+#include "util.h"
+
+enum Dir { R = 0, D, L, U };
+using Cmd = std::tuple<Dir,long>;
+using Pos = std::pair<long,long>;
+using Polygon = std::vector<Pos>;
+
+std::vector<Cmd> parse(const char* file)
+{
+    std::vector<Cmd> cmds;
+    if (std::ifstream input{ file }; input.is_open())
+    {
+        Dir dir; int steps; std::string color;
+        std::string line;
+        while (not std::getline(input,line).eof())
+        {
+            std::stringstream in{ line };
+            in >> util::skip<char> >> util::skip<int> >> color;
+            steps = std::stoi(color.substr(2, 5), nullptr, 16);
+            dir = static_cast<Dir>(std::stoi(color.substr(7, 1)));
+            cmds.push_back({ dir, steps });
+        }
+    }
+    return cmds;
+}
+
+std::pair<long,Polygon> compute_border(Pos pos, const std::vector<Cmd>& cmds)
+{
+    Polygon poly;
+    long border{};
+    auto [x, y] = pos;
+    for(const auto& cmd : cmds)
+    {
+        auto [dir, steps] = cmd;
+        switch (dir)
+        {
+            case R: y += steps; break;
+            case D: x += steps; break;
+            case L: y -= steps; break;
+            case U: x -= steps;
+        }
+        poly.push_back({ x, y });
+        border += steps;
+    }
+    return { border, std::move(poly) };
+}
+
+int main(int argc, char* argv[])
+{
+    auto cmds = parse(argv[1]);
+    auto [border, polygon] = compute_border({ 0, 0 }, cmds);
+    long area = util::shoelace<long>(polygon.cbegin(), polygon.cend());
+
+    std::cout << border + util::pick(area, border) << std::endl;
+    return 0;
+}
+