3 files changed, 65 insertions, 1 deletions
diff --git a/src/2019/day4/README.md b/src/2019/day4/README.md
index 616ef85..0c8f826 100644
--- a/src/2019/day4/README.md
+++ b/src/2019/day4/README.md
@@ -14,4 +14,12 @@ Other than the range rule, the following are true:
 123789 does not meet these criteria (no double).
 How many different passwords within the range given in your puzzle input meet these criteria?
 
+--- Part Two ---
+An Elf just remembered one more important detail: the two adjacent matching digits are not part of a larger group of matching digits.
 
+Given this additional criterion, but still ignoring the range rule, the following are now true:
+
+112233 meets these criteria because the digits never decrease and all repeated digits are exactly two digits long.
+123444 no longer meets the criteria (the repeated 44 is part of a larger group of 444).
+111122 meets the criteria (even though 1 is repeated more than twice, it still contains a double 22).
+How many different passwords within the range given in your puzzle input meet all of the criteria?
diff --git a/src/2019/day4/aoc.cpp b/src/2019/day4/aoc.cpp
index f0cbc1b..8f15fb5 100644
--- a/src/2019/day4/aoc.cpp
+++ b/src/2019/day4/aoc.cpp
@@ -1,5 +1,61 @@
 #include "aoc.h"
+#include <string.h>
+#include <string>
+#include <vector>
 
 namespace aoc2019 {
 
+bool smaller(const char* s1, const char* s2) { return std::atoi(s1) < std::atoi(s2); }
+
+bool two_identical(const char* s) {
+  const char* n = s + strlen(s);
+  while (s < n - 1) {
+    if (*s == *(s + 1)) {
+      return true;
+    }
+    s++;
+  }
+  return false;
+}
+
+bool two_identical_plus(const char* s) {
+  auto is_right = [s](int i, int j) -> bool {
+    bool b1 = s[i] == s[j];
+    bool b2 = i > 0 ? s[i] != s[i - 1] : true;
+    bool b3 = j < 5 ? s[j] != s[j + 1] : true;
+    return b1 && b2 && b3;
+  };
+  for (int i = 0; i < 5; i++) {
+    if (is_right(i, i + 1)) {
+      return true;
+    }
+  }
+  return false;
 }
+
+typedef bool (*identical_f)(const char*);
+
+void next(size_t i, size_t j, char p[], const char* low, const char* high, int* c1, int* c2) {
+  static char digits[] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'};
+  if (i == strlen(high)) {
+    if (smaller(p, high) && smaller(low, p)) {
+      *c1 += int(two_identical(p));
+      *c2 += int(two_identical_plus(p));
+    }
+  } else {
+    for (size_t x = j; x < ARRAY_SIZE(digits); x++) {
+      p[i] = digits[x];
+      next(i + 1, x, p, low, high, c1, c2);
+    }
+  }
+}
+
+std::pair<int, int> day4(const char* low, const char* high) {
+  char pass[7] = {0};
+  int c1{0};
+  int c2{0};
+  next(0, 0, pass, low, high, &c1, &c2);
+  return {c1, c2};
+}
+
+} // namespace aoc2019
diff --git a/src/2019/day4/aoc.h b/src/2019/day4/aoc.h
index 56ffadb..a877696 100644
--- a/src/2019/day4/aoc.h
+++ b/src/2019/day4/aoc.h
@@ -3,5 +3,5 @@
 
 namespace aoc2019 {
 
-int day4(line_view);
+std::pair<int, int> day4(const char*, const char*);
 }