From 1d47028997c893535a2fd9064812a4a7cddaf5f6 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Fri, 17 Oct 2025 16:37:41 +0200
Subject: day 14, aoc 2016

---
 2016/src/day14.cpp | 121 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 makefile           |   3 ++
 2 files changed, 124 insertions(+)
 create mode 100644 2016/src/day14.cpp

diff --git a/2016/src/day14.cpp b/2016/src/day14.cpp
new file mode 100644
index 0000000..a9be56c
--- /dev/null
+++ b/2016/src/day14.cpp
@@ -0,0 +1,121 @@
+// Standard C++
+#include <array>
+#include <iostream>
+#include <map>
+#include <optional>
+#include <string>
+
+// Standard C
+#include <cstring>
+
+// System
+#include <md5.h>
+
+using namespace std;
+
+namespace {
+
+optional<char>
+contains_repeating_chars(string_view str)
+{
+	for ( size_t i = 2; i < str.size(); ++i ) {
+		if ( str[i] == str[i - 1] && str[i] == str[i - 2] ) {
+			return str[i];
+		}
+	}
+
+	return {};
+}
+
+string
+md5(const string& input)
+{
+	array<char, MD5_DIGEST_STRING_LENGTH> digest{};
+	MD5Data(input.data(), input.size(), digest.data());
+	return digest.data();
+}
+
+int
+solve(const function<string(int)>& get_hash)
+{
+	int keys = 0;
+
+	for ( int index = 0;; ++index ) {
+		auto hash = get_hash(index);
+
+		auto repeat = contains_repeating_chars(hash);
+
+		if ( !repeat.has_value() ) {
+			continue;
+		}
+
+		const string pattern(5, repeat.value());
+
+		for ( auto index2 = index + 1; index2 != index + 1000; ++index2 ) {
+			hash = get_hash(index2);
+			if ( hash.find(pattern) != string::npos ) {
+				++keys;
+				break;
+			}
+		}
+
+		if ( keys == 64 ) {
+			return index;
+		}
+	}
+}
+
+void
+part1(const string& input)
+{
+	map<int, string> cache;
+
+	auto get_hash = [&](int index) {
+		if ( cache.contains(index) ) {
+			return cache.at(index);
+		}
+
+		auto hash = md5(input + to_string(index));
+
+		cache.emplace(index, hash);
+
+		return hash;
+	};
+
+	cout << solve(get_hash) << '\n';
+}
+
+void
+part2(const string& input)
+{
+	map<int, string> cache;
+
+	auto get_hash = [&](int index) {
+		if ( cache.contains(index) ) {
+			return cache.at(index);
+		}
+
+		auto hash = input + to_string(index);
+
+		for ( int i = 0; i != 2017; ++i ) {
+			hash = md5(hash);
+		}
+
+		cache.emplace(index, hash);
+
+		return hash;
+	};
+
+	cout << solve(get_hash) << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	// part1("abc");
+	// part2("abc");
+	part1("qzyelonm");
+	part2("qzyelonm");
+}
diff --git a/makefile b/makefile
index 6efd32d..ffbddf4 100644
--- a/makefile
+++ b/makefile
@@ -21,6 +21,9 @@ all: $(patsubst 2015/src/%.cpp,2015/bin/%,$(wildcard 2015/src/*.cpp)) \
 2016/bin/%: 2016/src/%.cpp | 2016/bin
 	c++ $(CPPFLAGS) $^ -o $@
 
+2016/bin/day14: 2016/src/day14.cpp | 2016/bin
+	c++ $(CPPFLAGS) $^ -lmd -o $@
+
 2016/bin/day05: 2016/src/day05.cpp | 2016/bin
 	c++ $(CPPFLAGS) $^ -lmd -o $@
 
-- 
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