5 files changed, 1023 insertions, 4 deletions
diff --git a/src/2016/day19/aoc.cpp b/src/2016/day19/aoc.cpp
index a4b9d32..8d08886 100644
--- a/src/2016/day19/aoc.cpp
+++ b/src/2016/day19/aoc.cpp
@@ -23,6 +23,27 @@ elf* del_elf(elf* e1) {
   return e;
 }
 
+elf* cross(elf* e, int n) {
+  n = n / 2 - 1;
+  while (n > 0) {
+    e = e->next;
+    n--;
+  }
+  return e;
+}
+
+elf* del_elf(elf* e1, size_t n) {
+  elf* e = e1;
+  while (e->next != e) {
+    elf* ex = cross(e, n);
+    elf* en = ex->next->next;
+    ex->next = en;
+    e = e->next;
+    n -= 1;
+  }
+  return e;
+}
+
 elf* make_elf(int n) {
   elf* e1 = new elf;
   e1->i = 1;
@@ -53,13 +74,33 @@ size_t last_elf(size_t n) {
   return a;
 }
 
+size_t final_elf(size_t n) {
+  if (n == 1) return 1;
+  size_t x{0};
+  size_t i{1};
+  while (x + 1 < n) {
+    x += i + i;
+    i *= 3;
+  }
+
+  x -= (i / 3) * 2;
+  x += 1;
+  size_t m = x + i / 3;
+  size_t a{1};
+  while (x != n) {
+    a += x < m ? 1 : 2;
+    x += 1;
+  }
+  return a - 1;
+}
+
 std::pair<int64_t, int64_t> day19(line_view) {
   // for (int i = 1; i < 100; i++) {
   //   elf* e = make_elf(i);
-  //   elf* x = del_elf(e);
-  //   printf("%d\n", x->i);
+  //   elf* x = del_elf(e, i);
+  //   printf("%d: %d %zu\n", i, x->i, final_elf(i));
   // }
-  return {last_elf(3005290), 0};
+  return {last_elf(3005290), final_elf(3005290)};
 }
 
 } // namespace aoc2016
diff --git a/src/2016/day20/README.md b/src/2016/day20/README.md
index e69de29..3ba39ac 100644
--- a/src/2016/day20/README.md
+++ b/src/2016/day20/README.md
@@ -0,0 +1,16 @@
+--- Day 20: Firewall Rules ---
+
+You'd like to set up a small hidden computer here so you can use it to get back into the network later. However, the corporate firewall only allows communication with certain external IP addresses.
+
+You've retrieved the list of blocked IPs from the firewall, but the list seems to be messy and poorly maintained, and it's not clear which IPs are allowed. Also, rather than being written in dot-decimal notation, they are written as plain 32-bit integers, which can have any value from 0 through 4294967295, inclusive.
+
+For example, suppose only the values 0 through 9 were valid, and that you retrieved the following blacklist:
+
+5-8
+0-2
+4-7
+
+The blacklist specifies ranges of IPs (inclusive of both the start and end value) that are not allowed. Then, the only IPs that this firewall allows are 3 and 9, since those are the only numbers not in any range.
+
+Given the list of blocked IPs you retrieved from the firewall (your puzzle input), what is the lowest-valued IP that is not blocked?
+
diff --git a/src/2016/day20/input b/src/2016/day20/input
index e69de29..273be89 100644
--- a/src/2016/day20/input
+++ b/src/2016/day20/input
@@ -0,0 +1,959 @@
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diff --git a/src/2016/day20/input0 b/src/2016/day20/input0
index e69de29..b5cde74 100644
--- a/src/2016/day20/input0
+++ b/src/2016/day20/input0
@@ -0,0 +1,3 @@
+5-8
+0-2
+4-7
diff --git a/test/test_2016.cpp b/test/test_2016.cpp
index ada2ecb..14e2188 100644
--- a/test/test_2016.cpp
+++ b/test/test_2016.cpp
@@ -174,7 +174,7 @@ TEST_CASE("An Elephant Named Joseph", "[2016]") {
   line_view lv = load_file("../src/2016/day19/input");
   auto p = aoc2016::day19(lv);
   REQUIRE(1816277 == p.first);
-  REQUIRE(0 == p.second);
+  REQUIRE(1410967 == p.second);
 }