1 files changed, 66 insertions, 0 deletions

diff --git a/2015/src/day11.cpp b/2015/src/day11.cpp
new file mode 100644
index 0000000..06fd05b
--- /dev/null
+++ b/2015/src/day11.cpp
@@ -0,0 +1,66 @@
+#include <iostream>
+#include <regex>
+#include <string>
+
+using namespace std;
+
+void
+increment(string& str)
+{
+        for ( auto it = rbegin(str); it != rend(str); ++it ) {
+                *it += 1;
+                if ( *it > 'z' ) {
+                        *it = 'a';
+                        continue;
+                }
+                return;
+        }
+}
+
+bool
+has_increasing_chars(string_view str)
+{
+        for ( size_t idx = 2; idx != str.length(); ++idx ) {
+                if ( str[idx - 2] + 1 == str[idx - 1] && str[idx - 1] + 1 == str[idx] ) {
+                        return true;
+                }
+        }
+        return false;
+}
+
+bool
+contains_no_invalid_char(const string& str)
+{
+        return str.find_first_of("iol") == string::npos;
+}
+
+bool
+contains_two_pairs(const string& str)
+{
+        static const regex regex("(.)\\1.*(.)\\2");
+
+        return regex_search(str, regex);
+}
+
+void
+part1(string& str)
+{
+        while ( true ) {
+                if ( has_increasing_chars(str) && contains_no_invalid_char(str) && contains_two_pairs(str) ) {
+                        cout << str << endl;
+                        break;
+                }
+                increment(str);
+        }
+}
+
+int
+main()
+{
+        string str = "hepxcrrq";
+
+        part1(str);
+
+        increment(str);
+        part1(str);
+}