3 files changed, 55 insertions, 0 deletions
diff --git a/src/2021/day6/README.md b/src/2021/day6/README.md
index 019eee4..f61cc2c 100644
--- a/src/2021/day6/README.md
+++ b/src/2021/day6/README.md
@@ -48,4 +48,10 @@ In this example, after 18 days, there are a total of 26 fish. After 80 days, the
 
 Find a way to simulate lanternfish. How many lanternfish would there be after 80 days?
 
+--- Part Two ---
 
+Suppose the lanternfish live forever and have unlimited food and space. Would they take over the entire ocean?
+
+After 256 days in the example above, there would be a total of 26984457539 lanternfish!
+
+How many lanternfish would there be after 256 days?
diff --git a/src/2021/day6/aoc.cpp b/src/2021/day6/aoc.cpp
index 7a3c895..98ece02 100644
--- a/src/2021/day6/aoc.cpp
+++ b/src/2021/day6/aoc.cpp
@@ -1,5 +1,52 @@
 #include "aoc.h"
+#include <algorithm>
+#include <vector>
 
 namespace aoc2021 {
 
+static void run(int day, std::vector<int>& v) {
+  // std::for_each(v.begin(), v.end(), [](int i) { printf("%d ", i); });
+  // printf(": %zu\n", v.size());
+  if (day > 0) {
+    int count{0};
+    for (auto& i : v) {
+      if (i > 0) {
+        i--;
+      } else {
+        i = 6;
+        count += 1;
+      }
+    }
+    while (count > 0) {
+      v.push_back(8);
+      count--;
+    }
+    run(day - 1, v);
+  }
 }
+
+size_t day6(int i, int days) {
+  std::vector<int> v = {i};
+  run(days, v);
+  return v.size();
+}
+
+size_t day6(line_view file, int days) {
+  size_t ns[7] = {0};
+  for (int i : {0, 1, 2, 3, 4, 5, 6}) {
+    ns[i] = day6(i, days);
+    printf("--> %d : %zu\n", i, ns[i]);
+  }
+
+  size_t total{0};
+  const char* p = file.line;
+  while (p < file.line + file.length) {
+    if (*p >= '0' && *p <= '9') {
+      total += ns[*p - '0'];
+    }
+    p++;
+  }
+  return total;
+}
+
+} // namespace aoc2021
diff --git a/src/2021/day6/aoc.h b/src/2021/day6/aoc.h
index cd205cd..6c02437 100644
--- a/src/2021/day6/aoc.h
+++ b/src/2021/day6/aoc.h
@@ -3,4 +3,6 @@
 
 namespace aoc2021 {
 
+size_t day6(int, int);
+size_t day6(line_view, int);
 }