1 files changed, 115 insertions, 0 deletions

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);
+}