3 files changed, 77 insertions, 1 deletions
diff --git a/src/2015/day20/aoc.cpp b/src/2015/day20/aoc.cpp
index 1c33c66..2dcceb0 100644
--- a/src/2015/day20/aoc.cpp
+++ b/src/2015/day20/aoc.cpp
@@ -1,5 +1,76 @@
 #include "aoc.h"
+#include <algorithm>
+#include <set>
+#include <vector>
 
 namespace aoc2015 {
 
+std::pair<int, int> by2(int x) {
+  int a = 0;
+  while (true) {
+    int total = 0;
+    for (int i = 0; i <= a; i++) {
+      total += 1 << i;
+    }
+    // printf("%d -> %d\n", a, total * 10);
+    if (total * 10 > x) {
+      return {1 << (a - 1), 1 << a};
+    }
+    a++;
+  }
 }
+
+std::vector<int> primefactor(int x) {
+  std::vector<int> v;
+  while (x % 2 == 0) {
+    v.push_back(2);
+    x /= 2;
+  }
+  for (int i = 3; i * i <= x; i += 2) {
+    while (x % i == 0) {
+      v.push_back(i);
+      x /= i;
+    }
+  }
+  if (x > 2) {
+    v.push_back(x);
+  }
+
+  return v;
+}
+
+// backtrace
+void combo(size_t m, size_t x, std::vector<int>& is, std::set<int>& rs, const std::vector<int>& ps) {
+  if (is.size() >= m) {
+    int x = 1;
+    std::for_each(is.begin(), is.end(), [&x, &ps](int i) { x *= ps[i]; });
+    rs.insert(x);
+  } else {
+    for (size_t i = x; i < ps.size(); i++) {
+      is.push_back(i);
+      combo(m, i + 1, is, rs, ps);
+      is.pop_back();
+    }
+  }
+}
+
+int combo(int i) {
+  std::vector<int> ps = primefactor(i);
+  std::vector<int> is;
+  std::set<int> rs;
+  rs.insert(1);
+  rs.insert(i);
+  for (size_t i = 1; i < ps.size(); i++) {
+    combo(i, 0, is, rs, ps);
+  }
+
+  int total{0};
+  std::for_each(rs.begin(), rs.end(), [&total](int i) { total += i * 10; });
+  return total;
+}
+
+std::pair<int, int> day20(int x) {
+  return {combo(x), 0};
+}
+
+} // namespace aoc2015
diff --git a/src/2015/day20/aoc.h b/src/2015/day20/aoc.h
index c439c03..80c8da6 100644
--- a/src/2015/day20/aoc.h
+++ b/src/2015/day20/aoc.h
@@ -3,4 +3,5 @@
 
 namespace aoc2015 {
 
+std::pair<int, int> day20(int);
 }
diff --git a/test/test_2015.cpp b/test/test_2015.cpp
index 9529c49..5fc6bc9 100644
--- a/test/test_2015.cpp
+++ b/test/test_2015.cpp
@@ -209,4 +209,8 @@ TEST_CASE("Medicine for Rudolph", "[day19]") {
   REQUIRE(195 == p.second);
 }
 
-TEST_CASE("Infinite Elves and Infinite Houses", "[day20]") {}
+TEST_CASE("Infinite Elves and Infinite Houses", "[day20]") {
+  auto p = aoc2015::day20(4);
+  // auto p = aoc2015::day20(36000000);
+  printf("%d %d\n", p.first, p.second);
+}