1 files changed, 75 insertions, 0 deletions

diff --git a/2023/src/day09.cpp b/2023/src/day09.cpp
new file mode 100644
index 0000000..4b30e6e
--- /dev/null
+++ b/2023/src/day09.cpp
@@ -0,0 +1,75 @@
+#include <fstream>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <vector>
+using namespace std;
+
+vector<string>
+read_file(string_view filename)
+{
+        fstream        input{ filename };
+        vector<string> data;
+
+        for ( string line; getline(input, line); ) {
+                data.emplace_back(line);
+        }
+
+        return data;
+}
+
+template<typename T = long>
+vector<T>
+read_ints(const string& line)
+{
+        stringstream iss{ line };
+        return vector<T>{ istream_iterator<T>{ iss }, istream_iterator<T>{} };
+}
+
+template<typename T>
+T
+solve_rec(const vector<T>& values)
+{
+        vector<T> differences;
+
+        for ( size_t idx = 1; idx < values.size(); ++idx ) {
+                differences.emplace_back(values[idx] - values[idx - 1]);
+        }
+        auto all_zeros = all_of(differences.begin(), differences.end(), [](T diff) { return diff == 0; });
+        if ( !all_zeros ) {
+                return solve_rec(differences) + values.back();
+        }
+        else {
+                return values.back();
+        }
+}
+
+void
+part1(const vector<string>& lines)
+{
+        long sum = 0;
+        for ( const auto& line: lines ) {
+                sum += solve_rec(read_ints(line));
+        }
+        cout << sum << endl;
+}
+
+void
+part2(const vector<string>& lines)
+{
+        long sum = 0;
+        for ( const auto& line: lines ) {
+                auto values = read_ints(line);
+                reverse(values.begin(), values.end());
+                sum += solve_rec(values);
+        }
+        cout << sum << endl;
+}
+
+int
+main()
+{
+        auto lines = read_file("data/day09.txt");
+        part1(lines);
+        part2(lines);
+}