From ae1e96f10b399266825e275ca4a8100ceede9cad Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Thu, 23 Oct 2025 19:28:20 +0200
Subject: days 19-25, aoc 2016

---
 2016/src/day21.cpp | 222 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 222 insertions(+)
 create mode 100644 2016/src/day21.cpp

(limited to '2016/src/day21.cpp')

diff --git a/2016/src/day21.cpp b/2016/src/day21.cpp
new file mode 100644
index 0000000..7e8fed7
--- /dev/null
+++ b/2016/src/day21.cpp
@@ -0,0 +1,222 @@
+#include <algorithm>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <ranges>
+#include <sstream>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+void
+swap_at_pos(string& str, size_t x, size_t y)
+{
+	if ( str.empty() ) {
+		return;
+	}
+
+	swap(str.at(x), str.at(y));
+}
+
+void
+swap_letter(string& str, char x, char y)
+{
+	if ( str.empty() ) {
+		return;
+	}
+
+	for ( auto& chr: str ) {
+		if ( chr == x ) {
+			chr = y;
+		}
+		else if ( chr == y ) {
+			chr = x;
+		}
+	}
+}
+
+void
+rotate_left(string& str, size_t n)
+{
+	if ( str.empty() ) {
+		return;
+	}
+
+	ranges::rotate(str, str.begin() + static_cast<int>(n % str.size()));
+}
+
+void
+rotate_right(string& str, size_t n)
+{
+	if ( str.empty() ) {
+		return;
+	}
+
+	rotate_left(str, str.size() - n);
+}
+
+void
+rotate(string& str, char chr)
+{
+	if ( str.empty() ) {
+		return;
+	}
+
+	const auto idx = str.find(chr);
+	if ( idx == string::npos ) {
+		return;
+	}
+
+	const auto steps = 1 + idx + (idx >= 4 ? 1 : 0);
+	rotate_right(str, steps % str.size());
+}
+
+void
+reverse(string& str, size_t x, size_t y)
+{
+	if ( str.empty() ) {
+		return;
+	}
+
+	if ( x > y ) {
+		swap(x, y);
+	}
+
+	reverse(str.begin() + static_cast<string::difference_type>(x),
+	        str.begin() + static_cast<string::difference_type>(y) + 1);
+}
+
+void
+move(string& str, size_t x, size_t y)
+{
+	if ( str.empty() ) {
+		return;
+	}
+
+	const auto chr = str.at(x);
+
+	str.erase(x, 1);
+	str.insert(y, 1, chr);
+}
+
+vector<string>
+readLines(const filesystem::path& filename)
+{
+	ifstream       file{ filename };
+	vector<string> lines;
+
+	for ( string line; getline(file, line); ) {
+		lines.emplace_back(line);
+	}
+
+	return lines;
+}
+
+vector<string>
+split(const string& line, char sep = ' ')
+{
+	vector<string> words;
+	stringstream   input{ line };
+
+	for ( string word; getline(input, word, sep); ) {
+		words.emplace_back(word);
+	}
+
+	return words;
+}
+
+void
+part1(const vector<string>& lines, string str)
+{
+	for ( const auto& line: lines ) {
+		auto words = split(line);
+
+		if ( line.starts_with("swap position") ) {
+			swap_at_pos(str, stoull(words.at(2)), stoull(words.at(5)));
+		}
+		else if ( line.starts_with("swap letter") ) {
+			swap_letter(str, words.at(2).at(0), words.at(5).at(0));
+		}
+		else if ( line.starts_with("rotate left") ) {
+			rotate_left(str, stoull(words.at(2)));
+		}
+		else if ( line.starts_with("rotate right") ) {
+			rotate_right(str, stoull(words.at(2)));
+		}
+		else if ( line.starts_with("rotate based") ) {
+			rotate(str, words.at(6).at(0));
+		}
+		else if ( line.starts_with("reverse") ) {
+			reverse(str, stoull(words.at(2)), stoull(words.at(4)));
+		}
+		else if ( line.starts_with("move") ) {
+			move(str, stoull(words.at(2)), stoull(words.at(5)));
+		}
+		else {
+			cerr << "unknown command: " << line << '\n';
+			return;
+		}
+	}
+	cout << str << '\n';
+}
+
+void
+part2(const vector<string>& lines, string str)
+{
+	for ( const auto& line: ranges::reverse_view(lines) ) {
+		auto words = split(line);
+
+		if ( line.starts_with("swap position") ) {
+			swap_at_pos(str, stoull(words.at(2)), stoull(words.at(5)));
+		}
+		else if ( line.starts_with("swap letter") ) {
+			swap_letter(str, words.at(2).at(0), words.at(5).at(0));
+		}
+		else if ( line.starts_with("rotate left") ) {
+			rotate_right(str, stoull(words.at(2)));
+		}
+		else if ( line.starts_with("rotate right") ) {
+			rotate_left(str, stoull(words.at(2)));
+		}
+		else if ( line.starts_with("rotate based") ) {
+			auto chr = words.at(6).at(0);
+			for ( size_t i = 0; i != str.size(); ++i ) {
+				string tmp = str;
+				rotate_left(tmp, i);
+
+				auto idx   = tmp.find(chr);
+				auto steps = (1 + idx + (idx >= 4 ? 1 : 0));
+
+				if ( ((steps - i + str.size()) % str.size()) == 0 ) {
+					str = tmp;
+					break;
+				}
+			}
+		}
+		else if ( line.starts_with("reverse") ) {
+			reverse(str, stoull(words.at(2)), stoull(words.at(4)));
+		}
+		else if ( line.starts_with("move") ) {
+			move(str, stoull(words.at(5)), stoull(words.at(2)));
+		}
+		else {
+			cerr << "unknown command: " << line << '\n';
+			return;
+		}
+	}
+	cout << str << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	auto lines = readLines("data/day21.txt");
+
+	part1(lines, "abcdefgh");
+	part2(lines, "fbgdceah");
+}
-- 
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