3 files changed, 84 insertions, 0 deletions
diff --git a/src/2016/day7/README.md b/src/2016/day7/README.md
index fb460dd..72a6f59 100644
--- a/src/2016/day7/README.md
+++ b/src/2016/day7/README.md
@@ -11,4 +11,15 @@ aaaa[qwer]tyui does not support TLS (aaaa is invalid; the interior characters mu
 ioxxoj[asdfgh]zxcvbn supports TLS (oxxo is outside square brackets, even though it's within a larger string).
 How many IPs in your puzzle input support TLS?
 
+--- Part Two ---
+You would also like to know which IPs support SSL (super-secret listening).
 
+An IP supports SSL if it has an Area-Broadcast Accessor, or ABA, anywhere in the supernet sequences (outside any square bracketed sections), and a corresponding Byte Allocation Block, or BAB, anywhere in the hypernet sequences. An ABA is any three-character sequence which consists of the same character twice with a different character between them, such as xyx or aba. A corresponding BAB is the same characters but in reversed positions: yxy and bab, respectively.
+
+For example:
+
+aba[bab]xyz supports SSL (aba outside square brackets with corresponding bab within square brackets).
+xyx[xyx]xyx does not support SSL (xyx, but no corresponding yxy).
+aaa[kek]eke supports SSL (eke in supernet with corresponding kek in hypernet; the aaa sequence is not related, because the interior character must be different).
+zazbz[bzb]cdb supports SSL (zaz has no corresponding aza, but zbz has a corresponding bzb, even though zaz and zbz overlap).
+How many IPs in your puzzle input support SSL?
diff --git a/src/2016/day7/aoc.cpp b/src/2016/day7/aoc.cpp
index be28c50..b319a1a 100644
--- a/src/2016/day7/aoc.cpp
+++ b/src/2016/day7/aoc.cpp
@@ -1,4 +1,75 @@
 #include "aoc.h"
+#include <vector>
 
 namespace aoc2016 {
+
+bool is_abba(const char* p) { return *p == *(p + 3) && *(p + 1) == *(p + 2) && *p != *(p + 1); }
+bool is_aba(const char* p) { return *p == *(p + 2) && *p != *(p + 1); }
+bool is_reverse(const char* p1, const char* p2) { return *p1 == *(p2 + 1) && *(p1 + 1) == *p2; }
+
+std::vector<line_view> aba(const std::vector<line_view>& ls) {
+  std::vector<line_view> vs;
+  for (auto& l : ls) {
+    const char* p = l.line;
+    while (p < l.line + l.length - 2) {
+      if (is_aba(p)) {
+        vs.emplace_back(p, 3);
+      }
+      p++;
+    }
+  }
+  return vs;
+}
+
+bool match_aba(const std::vector<line_view>& ls1, const std::vector<line_view>& ls2) {
+  for (auto& l1 : ls1) {
+    for (auto& l2 : ls2) {
+      if (is_reverse(l1.line, l2.line)) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
+bool has_abba(const std::vector<line_view>& ls) {
+  for (auto& l : ls) {
+    const char* p = l.line;
+    while (p < l.line + l.length - 3) {
+      if (is_abba(p)) {
+        return true;
+      }
+      p++;
+    }
+  }
+  return false;
 }
+
+std::pair<int, int> day7(line_view file) {
+  int t0{0}, t1{0};
+  per_line(file, [&t0, &t1](line_view lv) {
+    std::vector<line_view> g1;
+    std::vector<line_view> g2;
+    const char* p1 = lv.line;
+    const char* p = p1;
+    while (p < lv.line + lv.length) {
+      if (*p == '[') {
+        g1.emplace_back(p1, p);
+        p1 = p + 1;
+      }
+      if (*p == ']') {
+        g2.emplace_back(p1, p);
+        p1 = p + 1;
+      }
+      p++;
+    }
+    g1.emplace_back(p1, p);
+
+    t0 += int(has_abba(g1) && !has_abba(g2));
+    t1 += int(match_aba(aba(g1), aba(g2)));
+    return true;
+  });
+  return {t0, t1};
+}
+
+} // namespace aoc2016
diff --git a/src/2016/day7/aoc.h b/src/2016/day7/aoc.h
index c73e513..ca88226 100644
--- a/src/2016/day7/aoc.h
+++ b/src/2016/day7/aoc.h
@@ -2,4 +2,6 @@
 #include "common.h"
 
 namespace aoc2016 {
+
+std::pair<int, int> day7(line_view);
 }