From 56e890cec0a28c0a485212ccebfaf774235a79a2 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Wed, 3 Jan 2024 23:35:54 +0100
Subject: prepare for more puzzles ... :)

---
 2023/src/day13.cpp | 115 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100644 2023/src/day13.cpp

(limited to '2023/src/day13.cpp')

diff --git a/2023/src/day13.cpp b/2023/src/day13.cpp
new file mode 100644
index 0000000..549457b
--- /dev/null
+++ b/2023/src/day13.cpp
@@ -0,0 +1,115 @@
+#include <algorithm>
+#include <fstream>
+#include <iostream>
+#include <iterator>
+#include <sstream>
+#include <string>
+#include <vector>
+using namespace std;
+
+string
+read_file(string_view filename)
+{
+	fstream input{ filename };
+	return { istreambuf_iterator<char>{ input }, istreambuf_iterator<char>{} };
+}
+
+vector<string>
+split(string_view line, string_view delimiter)
+{
+	size_t pos_start = 0;
+	size_t pos_end   = 0;
+
+	vector<string> res;
+
+	while ( (pos_end = line.find(delimiter, pos_start)) != std::string::npos ) {
+		auto token = line.substr(pos_start, pos_end - pos_start);
+		pos_start  = pos_end + delimiter.length();
+
+		res.emplace_back(token);
+	}
+
+	res.emplace_back(line.substr(pos_start));
+	return res;
+}
+
+vector<string>
+transpose(const vector<string>& lines)
+{
+	vector<string> result(lines[0].size());
+
+	for ( const auto& line: lines ) {
+		for ( size_t i = 0; i < line.size(); ++i ) {
+			result[i] += line[i];
+		}
+	}
+	return result;
+}
+
+long
+find_mirror(const vector<string>& input)
+{
+	for ( size_t idx = 1; idx < input.size(); ++idx ) {
+		bool equal = true;
+		for ( size_t cnt = 0; cnt != min(idx, input.size() - idx); ++cnt ) {
+			if ( !(input[idx + cnt] == input[idx - 1 - cnt]) ) {
+				equal = false;
+				break;
+			}
+		}
+		if ( equal ) {
+			return long(idx);
+		}
+	}
+	return 0;
+}
+
+long
+count_differences(string_view str1, string_view str2)
+{
+	long diffs = 0;
+	for ( size_t idx = 0; idx != str1.size(); ++idx ) {
+		diffs += long(str1[idx] != str2[idx]);
+	}
+	return diffs;
+}
+
+long
+find_mirror_part2(const vector<string>& input)
+{
+	for ( size_t idx = 1; idx < input.size(); ++idx ) {
+		long errs = 0;
+		for ( size_t i = 0; i != min(idx, input.size() - idx); ++i ) {
+			errs += count_differences(input[idx + i], input[idx - 1 - i]);
+		}
+		if ( errs == 1 ) {
+			return long(idx);
+		}
+	}
+	return 0;
+}
+
+void
+solve(const function<long(const vector<string>&)>& find_mirror)
+{
+	static const long multiplier = 100;
+
+	auto contents = read_file("data/day13.txt");
+	auto parts    = split(contents, "\n\n");
+
+	long sum = 0;
+	for ( const auto& part: parts ) {
+		auto lines = split(part, "\n");
+
+		sum += find_mirror(transpose(lines));
+		sum += find_mirror(lines) * multiplier;
+	}
+	cout << sum << endl;
+}
+
+int
+main()
+{
+	solve(find_mirror);
+	solve(find_mirror_part2);
+}
-- 
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