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/day23.cpp | 158 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 158 insertions(+)
 create mode 100644 2017/src/day23.cpp

(limited to '2017/src/day23.cpp')

diff --git a/2017/src/day23.cpp b/2017/src/day23.cpp
new file mode 100644
index 0000000..1f4b960
--- /dev/null
+++ b/2017/src/day23.cpp
@@ -0,0 +1,158 @@
+#include <charconv>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <sstream>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+template<typename T>
+struct CPU {
+	[[nodiscard]]
+	T get(const string& ref)
+	{
+		T value{};
+		auto [ptr, ec] = from_chars(ref.data(), ref.data() + ref.size(), value);
+
+		if ( ec == errc() ) {
+			return value;
+		}
+
+		return memory[ref];
+	}
+
+	void set(const string& ref, T value)
+	{
+		memory[ref] = value;
+	}
+
+	void sub(const string& ref, T value)
+	{
+		memory[ref] -= value;
+	}
+
+	void mul(const string& ref, T value)
+	{
+		memory[ref] *= value;
+	}
+
+	T execute(const vector<vector<string>>& code)
+	{
+		int mul_executed = 0;
+
+		for ( size_t ip = 0; ip < code.size(); ++ip ) {
+			const auto& line   = code.at(ip);
+			const auto& opcode = line.at(0);
+
+			if ( opcode == "set" ) {
+				set(line.at(1), get(line.at(2)));
+			}
+			else if ( opcode == "sub" ) {
+				sub(line.at(1), get(line.at(2)));
+			}
+			else if ( opcode == "mul" ) {
+				++mul_executed;
+				mul(line.at(1), get(line.at(2)));
+			}
+			else if ( opcode == "jnz" ) {
+				auto value = get(line.at(1));
+				if ( value != 0 ) {
+					auto offset = get(line.at(2));
+					ip += static_cast<size_t>(offset - 1);
+				}
+			}
+			else {
+				cerr << "unknown opcode: " << opcode << '\n';
+				return -1;
+			}
+		}
+		return mul_executed;
+	}
+
+	map<string, T> memory;
+};
+
+vector<string>
+split(const string& line)
+{
+	stringstream   strm{ line };
+	vector<string> data;
+
+	for ( string line; strm >> line; ) {
+		data.emplace_back(line);
+	}
+
+	return data;
+}
+
+vector<vector<string>>
+read_file(const filesystem::path& filename)
+{
+	ifstream               file{ filename };
+	vector<vector<string>> data;
+
+	for ( string line; getline(file, line); ) {
+		const auto parts = split(line);
+		data.emplace_back(parts);
+	}
+
+	return data;
+}
+
+void
+part1(const vector<vector<string>>& code)
+{
+	CPU<long> cpu{};
+
+	cout << "Part1: " << cpu.execute(code) << '\n';
+}
+
+void
+part2(const vector<vector<string>>& code)
+{
+	CPU<long> cpu{};
+	cpu.memory["a"] = 1;
+
+	// execute only initialization code
+	cpu.execute({ code.begin(), code.begin() + 8 });
+
+	auto b_start = cpu.memory["b"];
+	auto c_end   = cpu.memory["c"];
+
+	auto is_prime = [](long n) {
+		if ( n < 2 ) {
+			return false;
+		}
+		for ( long i = 2; i * i <= n; ++i ) {
+			if ( n % i == 0 ) {
+				return false;
+			}
+		}
+		return true;
+	};
+
+	const long step     = 17;
+	long       memory_h = 0;
+	for ( long b = b_start; b <= c_end; b += step ) {
+		if ( !is_prime(b) ) {
+			++memory_h;
+		}
+	}
+
+	cout << "Part2: " << memory_h << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	auto instr = read_file("data/day23.txt");
+	part1(instr);
+	part2(instr);
+}
-- 
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