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/day09.cpp | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 2023/src/day09.cpp

(limited to '2023/src/day09.cpp')

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);
+}
-- 
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