Add another from 2021.

adventofcode/2021/day/09/smoke_basin.cpp

@@ -0,0 +1,59 @@
+#include <algorithm>
+#include <array>
+#include <iostream>
+#include <queue>
+#include <string>
+#include <unordered_set>
+#include <vector>
+
+struct pair_hash {
+    template <class T1, class T2>
+    std::size_t operator () (const std::pair<T1,T2> &p) const {
+        auto h1 = std::hash<T1>{}(p.first);
+        auto h2 = std::hash<T2>{}(p.second);
+        return h1 ^ h2;
+    }
+};
+
+int main() {
+    int R{0}, C;
+    std::vector<std::string> v;
+    std::string line;
+    while (std::getline(std::cin, line)) {
+        v.push_back(line);
+        C = static_cast<int>(line.length());
+        R++;
+    }
+    int part_one{0};
+    constexpr std::array<std::array<int, 2>, 4> deltas = {-1, 0, 1, 0, 0, -1, 0, 1};
+    std::vector<size_t> basins_sizes;
+    for (int i = 0; i < R; i++) for (int j = 0; j < C; j++) {
+        bool found = false;
+        for (const auto& delta : deltas)
+            if (i + delta[0] >= 0 and i + delta[0] < R and  j + delta[1] >= 0 and j + delta[1] < C)
+                if (v[i + delta[0]][j + delta[1]] - '0' <= v[i][j] - '0') found = true;
+        if (found) continue;
+        part_one += v[i][j] - '0' + 1;
+        std::unordered_set<std::pair<int, int>, pair_hash> Qed;
+        std::queue<std::pair<int, int>> Q;
+        Q.emplace(i, j);
+        Qed.emplace(i, j);
+        while (not Q.empty()) {
+            const auto [r, c] = Q.front();
+            Q.pop();
+            for (const auto& delta : deltas)
+                if (r + delta[0] >= 0 and r + delta[0] < R and  c + delta[1] >= 0 and c + delta[1] < C)
+                    if (v[r + delta[0]][c + delta[1]] != '9' and
+                        not Qed.contains(std::make_pair(r + delta[0], c + delta[1]))) {
+                        Q.emplace(r + delta[0], c + delta[1]);
+                        Qed.emplace(r + delta[0], c + delta[1]);
+                    }
+        }
+        basins_sizes.push_back(Qed.size());
+    }
+    std::cout << part_one << '\n';
+    std::ranges::sort(basins_sizes);
+    std::cout << basins_sizes.at(basins_sizes.size() - 1) *
+                 basins_sizes.at(basins_sizes.size() - 2) *
+                 basins_sizes.at(basins_sizes.size() - 3) << '\n'; 
+}