From bdd35a7bee7f23c912d0c453abc263805d8d8ccf Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Fri, 7 Nov 2025 12:04:46 +0100
Subject: aoc 2017, days 21-25

---
 2017/src/day25.cpp | 128 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 2017/src/day25.cpp

(limited to '2017/src/day25.cpp')

diff --git a/2017/src/day25.cpp b/2017/src/day25.cpp
new file mode 100644
index 0000000..36b7dd0
--- /dev/null
+++ b/2017/src/day25.cpp
@@ -0,0 +1,128 @@
+#include <array>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <iterator>
+#include <map>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+vector<string>
+split(const string& line, const string& delimiter)
+{
+	vector<string> result;
+
+	size_t start = 0;
+	size_t end   = 0;
+
+	while ( (end = line.find(delimiter, start)) != string::npos ) {
+		if ( end != start ) {
+			result.emplace_back(line.substr(start, end - start));
+		}
+
+		start = end + delimiter.length();
+	}
+
+	if ( start != line.size() ) {
+		result.emplace_back(line.substr(start));
+	}
+
+	return result;
+}
+
+vector<vector<vector<string>>>
+read_file(const filesystem::path& filename)
+{
+	ifstream file{ filename };
+	string   content{ istreambuf_iterator<char>{ file }, {} };
+
+	vector<vector<vector<string>>> result;
+
+	auto blocks = split(content, "\n\n");
+	for ( const auto& block: blocks ) {
+		vector<vector<string>> data;
+
+		auto lines = split(block, "\n");
+		for ( auto line: lines ) {
+			line.pop_back();
+			auto words = split(line, " ");
+			data.emplace_back(words);
+		}
+
+		result.emplace_back(data);
+	}
+
+	return result;
+}
+
+using action_type    = tuple<int, int, string>;       // value (0, 1), direction (-1, 1), next_state)
+using condition_type = array<action_type, 2>;         // [0] -> action, [1] -> action
+using rule_type      = map<string, condition_type>;   // "A" -> condition
+using puzzle_type    = tuple<string, int, rule_type>; // (start_state, steps, rules)
+
+puzzle_type
+parse_input(const vector<vector<vector<string>>>& input)
+{
+	rule_type rules{};
+
+	for ( size_t idx = 1; idx < input.size(); ++idx ) {
+		const auto& block = input.at(idx);
+
+		condition_type condition{};
+
+		for ( size_t i = 0; i != 2; ++i ) {
+			const auto base = i * 4;
+
+			auto read_val   = stoul(block.at(base + 1).back());
+			auto write_val  = stoi(block.at(base + 2).back());
+			auto dir        = block.at(base + 3).back() == "left" ? -1 : 1;
+			auto next_state = block.at(base + 4).back();
+
+			condition.at(read_val) = { write_val, dir, next_state };
+		}
+
+		rules[block.at(0).back()] = condition;
+	}
+
+	auto start_state = input[0][0][3];
+	auto steps       = stoi(input[0][1][5]);
+
+	return puzzle_type{ start_state, steps, rules };
+}
+
+void
+part1(const puzzle_type& puzzle)
+{
+	auto [state, steps, rules] = puzzle;
+
+	map<int, int> tape;
+
+	int pos = 0;
+
+	while ( steps-- > 0 ) {
+		auto value                            = tape[pos];
+		const auto& [write, move, next_state] = rules.at(state).at(size_t(value));
+		state                                 = next_state;
+		tape[pos]                             = write;
+		pos += move;
+	}
+
+	int sum = 0;
+	for ( const auto& [key, value]: tape ) {
+		sum += value;
+	}
+	cout << "Part1: " << sum << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	auto input = read_file("data/day25.txt");
+	part1(parse_input(input));
+}
-- 
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