From 02087088f95e93b0689fb7fd077f59a87a48ea57 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Thu, 16 Dec 2021 11:50:49 +0000
Subject: [PATCH] Add 2017 day 15 puzzles.

---
 2017/puzzle-15-01.cc | 48 ++++++++++++++++++++++++++++++++++
 2017/puzzle-15-02.cc | 61 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 109 insertions(+)
 create mode 100644 2017/puzzle-15-01.cc
 create mode 100644 2017/puzzle-15-02.cc

diff --git a/2017/puzzle-15-01.cc b/2017/puzzle-15-01.cc
new file mode 100644
index 0000000..cf9a3c2
--- /dev/null
+++ b/2017/puzzle-15-01.cc
@@ -0,0 +1,48 @@
+#include <iomanip>
+#include <iostream>
+#include <regex>
+#include <string>
+
+using UInt = std::uint32_t;
+
+auto lcg(UInt prev, UInt factor) -> UInt
+{
+  auto t{static_cast<std::uint64_t>(prev) * static_cast<std::uint64_t>(factor)};
+  return static_cast<UInt>(t % 2147483647ULL);
+}
+
+auto lcgA(UInt prev) -> UInt { return lcg(prev, 16807); }
+auto lcgB(UInt prev) -> UInt { return lcg(prev, 48271); }
+
+auto main() -> int
+{
+  static std::regex a_re{"Generator A starts with (\\d+)"};
+  static std::regex b_re{"Generator B starts with (\\d+)"};
+  UInt prevA{0};
+  UInt prevB{0};
+  std::string line;
+  while (std::getline(std::cin, line) && !line.empty()) {
+    std::smatch m;
+    if (std::regex_search(line, m, a_re)) {
+      prevA = std::stoul(m.str(1));
+    }
+    else if (std::regex_search(line, m, b_re)) {
+      prevB = std::stoul(m.str(1));
+    }
+    else {
+      std::cerr << "Unable to interpret: " << line << '\n';
+      return 1;
+    }
+  }
+
+  UInt judged{0};
+  for (UInt i = 0; i < 40'000'000; ++i) {
+    prevA = lcgA(prevA);
+    prevB = lcgB(prevB);
+    if (((prevA ^ prevB) & 0xffffULL) == 0) {
+      ++judged;
+    }
+  }
+
+  std::cout << "Matched " << judged << '\n';
+}
diff --git a/2017/puzzle-15-02.cc b/2017/puzzle-15-02.cc
new file mode 100644
index 0000000..42778dd
--- /dev/null
+++ b/2017/puzzle-15-02.cc
@@ -0,0 +1,61 @@
+#include <iomanip>
+#include <iostream>
+#include <regex>
+#include <string>
+
+using UInt = std::uint32_t;
+
+auto lcg(UInt prev, UInt factor) -> UInt
+{
+  auto t{static_cast<std::uint64_t>(prev) * static_cast<std::uint64_t>(factor)};
+  return static_cast<UInt>(t % 2147483647ULL);
+}
+
+auto lcgA(UInt prev) -> UInt
+{
+  do {
+    prev = lcg(prev, 16807);
+  } while (prev % 4 != 0);
+  return prev;
+}
+
+auto lcgB(UInt prev) -> UInt
+{
+  do {
+    prev = lcg(prev, 48271);
+  } while (prev % 8 != 0);
+  return prev;
+}
+
+auto main() -> int
+{
+  static std::regex a_re{"Generator A starts with (\\d+)"};
+  static std::regex b_re{"Generator B starts with (\\d+)"};
+  UInt prevA{0};
+  UInt prevB{0};
+  std::string line;
+  while (std::getline(std::cin, line) && !line.empty()) {
+    std::smatch m;
+    if (std::regex_search(line, m, a_re)) {
+      prevA = std::stoul(m.str(1));
+    }
+    else if (std::regex_search(line, m, b_re)) {
+      prevB = std::stoul(m.str(1));
+    }
+    else {
+      std::cerr << "Unable to interpret: " << line << '\n';
+      return 1;
+    }
+  }
+
+  UInt judged{0};
+  for (UInt i = 0; i < 5'000'000; ++i) {
+    prevA = lcgA(prevA);
+    prevB = lcgB(prevB);
+    if (((prevA ^ prevB) & 0xffffULL) == 0) {
+      ++judged;
+    }
+  }
+
+  std::cout << "Matched " << judged << '\n';
+}