Add another 2021 day.

adventofcode/2021/day/6/lanternfish.cpp

@@ -0,0 +1,30 @@
+#include <algorithm>
+#include <iostream>
+#include <numeric>
+#include <vector>
+
+int main(int argc, char **argv) {
+    const size_t num_days = [&](){
+        try {
+            const size_t num_days = argc > 1 ? std::stoi(argv[1]) : 18;
+            return num_days;
+        } catch (...) {
+            std::cerr << "Usage: ./lanternfish [num_days]   with num_days a positive integer\n";
+            std::exit(1);
+        }
+    }();
+    std::vector<size_t> fishes(9);
+    std::iota(fishes.begin(), fishes.end(), 0);
+    constexpr size_t reset_days{6};
+    constexpr size_t spawn_days{8};
+    std::array<uint64_t, spawn_days + 1> hishes{};  // histogram of fishes' internal timers
+    for (const int fish : fishes) hishes[fish]++;
+    for (size_t day = 1; day <= num_days; day++) {
+        uint64_t backup = hishes[0];
+        //for (size_t i = 0; i < hishes.size() - 1; i++) hishes[i] = hishes[i + 1];
+        std::rotate(hishes.begin(), hishes.begin() + 1, hishes.end());
+        hishes[reset_days] += backup;
+        hishes[spawn_days]  = backup;
+    }
+    std::cout << std::accumulate(hishes.cbegin(), hishes.cend(), 0ull) << "\n";
+}