Add 2016 Day 5 puzzles

2016/puzzle-05-01.cc

@@ -0,0 +1,59 @@
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <iostream>
+#include <string>
+
+#include <openssl/evp.h>
+
+using MD5Digest = std::array<unsigned char, EVP_MAX_MD_SIZE>;
+
+auto md5(MD5Digest& digest, std::string const& s) -> unsigned int
+{
+  EVP_MD const* md{EVP_md5()};
+  unsigned int md_len{0};
+
+  EVP_MD_CTX* md_context{EVP_MD_CTX_new()};
+  assert(md_context != nullptr);
+  EVP_DigestInit_ex2(md_context, md, nullptr);
+  EVP_DigestUpdate(md_context, s.data(), s.length());
+  EVP_DigestFinal_ex(md_context, digest.data(), &md_len);
+  return md_len;
+}
+
+auto get_letter(std::string const& s) -> unsigned
+{
+  MD5Digest digest;
+  auto len = md5(digest, s);
+  assert(len >= 3);
+  if (digest[0] == 0 && digest[1] == 0 && (digest[2] & 0xf0) == 0) {
+    return digest[2] & 0xf;
+  }
+  return 16;
+}
+
+auto main() -> int
+{
+  constexpr std::string_view characters{"0123456789abcdef"};
+  constexpr unsigned password_length{8};
+
+  std::string result;
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read from stdin.\n";
+    return 1;
+  }
+
+  std::uint64_t i{0};
+  for (unsigned letter = 0; letter < password_length; ++letter) {
+    unsigned c{0};
+    while ((c = get_letter(line + std::to_string(i))) == 16) {
+      ++i;
+    }
+    result += characters.at(c);
+    ++i;
+  }
+
+  std::cout << "Password: " << result << '\n';
+  return 0;
+}