From fcfdcddd33e858f8385f6b876af3c9f73d96665c Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Wed, 22 Dec 2021 16:36:30 +0000
Subject: [PATCH] Add 2017 Day 25 puzzles

---
 2017/puzzle-25-01.cc | 110 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)
 create mode 100644 2017/puzzle-25-01.cc

diff --git a/2017/puzzle-25-01.cc b/2017/puzzle-25-01.cc
new file mode 100644
index 0000000..da532d2
--- /dev/null
+++ b/2017/puzzle-25-01.cc
@@ -0,0 +1,110 @@
+#include <array>
+#include <iostream>
+#include <map>
+#include <regex>
+#include <string>
+#include <unordered_set>
+
+using Int = long;
+using StateName = char;
+
+struct SubInstruction
+{
+  bool write_;
+  Int move_delta_;
+  StateName next_state_;
+};
+
+struct Instruction
+{
+  [[nodiscard]] auto write(bool value) const noexcept -> bool
+  {
+    return subinstructions_[value ? 1 : 0].write_;
+  }
+
+  [[nodiscard]] auto move_delta(bool value) const noexcept -> Int
+  {
+    return subinstructions_[value ? 1 : 0].move_delta_;
+  }
+
+  [[nodiscard]] auto next_state(bool value) const noexcept -> StateName
+  {
+    return subinstructions_[value ? 1 : 0].next_state_;
+  }
+
+  std::array<SubInstruction, 2> subinstructions_;
+};
+
+auto main() -> int
+{
+  std::map<StateName, Instruction> state_machine;
+  std::regex begin_state_re{"Begin in state (.)."};
+  std::regex step_count_re{"Perform a diagnostic checksum after (\\d+) steps."};
+  std::regex in_state_re{"In state (.):"};
+  std::regex if_current_value_re{"  If the current value is (.):"};
+  std::regex write_re{"    - Write the value (.)."};
+  std::regex move_re{"    - Move one slot to the (left|right)."};
+  std::regex next_state_re{"    - Continue with state (.)."};
+
+  std::string line;
+  StateName current_state{'\0'};
+  unsigned long step_count{0};
+  unsigned current_value{2};
+
+  auto it{state_machine.end()};
+  while (std::getline(std::cin, line) && line != "END") {
+    std::smatch m;
+    if (std::regex_search(line, m, begin_state_re)) {
+      assert(current_state == '\0');
+      current_state = m.str(1)[0];
+    }
+    else if (std::regex_search(line, m, step_count_re)) {
+      assert(step_count == 0);
+      step_count = std::stoul(m.str(1));
+    }
+    else if (std::regex_search(line, m, in_state_re)) {
+      auto [it2, success] = state_machine.insert({m.str(1)[0], {}});
+      assert(success);
+      it = it2;
+      current_value = 2;
+    }
+    else if (std::regex_search(line, m, if_current_value_re)) {
+      assert(it != state_machine.end());
+      current_value = m.str(1)[0] - '0';
+      assert(current_value == 0 || current_value == 1);
+    }
+    else if (std::regex_search(line, m, write_re)) {
+      assert(it != state_machine.end());
+      assert(current_value != 2);
+      it->second.subinstructions_[current_value].write_ = m.str(1) == "1";
+    }
+    else if (std::regex_search(line, m, move_re)) {
+      assert(it != state_machine.end());
+      assert(current_value != 2);
+      it->second.subinstructions_[current_value].move_delta_ = m.str(1) == "left" ? -1 : 1;
+    }
+    else if (std::regex_search(line, m, next_state_re)) {
+      assert(it != state_machine.end());
+      assert(current_value != 2);
+      it->second.subinstructions_[current_value].next_state_ = m.str(1)[0];
+    }
+  }
+
+  std::unordered_set<Int> marked;
+  Int pos{0};
+  for (unsigned long i{0}; i < step_count; ++i) {
+    auto it{state_machine.find(current_state)};
+    assert(it != state_machine.end());
+    auto const& subinstruction = it->second.subinstructions_[marked.contains(pos) ? 1 : 0];
+    if (subinstruction.write_) {
+      marked.insert(pos);
+    }
+    else {
+      marked.erase(pos);
+    }
+    pos += subinstruction.move_delta_;
+    current_state = subinstruction.next_state_;
+  }
+
+  std::cout << "Number marked: " << marked.size() << '\n';
+}