Day 9

src/days/09/Day09.cpp

@@ -1,9 +1,47 @@
 #include "Day09.h"
 
 Result Day09::Task1() {
-	return Day::Task1();
+	int64 sum = 0;
+	for (auto &history : parseHistories()){
+		vector<int64> tails = {history.back()};
+		vector<int64> diff = history;
+
+		while (std::any_of(diff.begin(), diff.end(), [](int64 v){return v != 0;})){
+			for (size_t i = 0; i < diff.size() - 1; i++)
+				diff[i] = diff[i + 1] - diff[i];
+			diff.pop_back();
+			tails.push_back(diff.back());
+		}
+
+		sum += std::reduce(tails.begin(), tails.end(), 0, [](int64 a, int64 b){ return a + b; });
+	}
+	return to_string(sum);
 }
 
 Result Day09::Task2() {
-	return Day::Task2();
+	int64 sum = 0;
+	for (auto &history : parseHistories()){
+		vector<int64> heads = {history.front()};
+		vector<int64> diff = history;
+
+		while (std::any_of(diff.begin(), diff.end(), [](int64 v){return v != 0;})){
+			for (size_t i = 0; i < diff.size() - 1; i++)
+				diff[i] = diff[i + 1] - diff[i];
+			diff.pop_back();
+			heads.push_back(diff.front());
+		}
+
+		for (size_t i = 0; i < heads.size(); i++)
+			sum += heads[i] * (i % 2 == 0 ? 1 : -1);
+	}
+	return to_string(sum);
+}
+
+vector<vector<int64>> Day09::parseHistories() const {
+	vector<vector<int64>> histories;
+
+	for (const string &line : input)
+		histories.push_back(parseInts(line));
+
+	return histories;
 }

src/days/09/Day09.h

@@ -7,4 +7,6 @@ protected:
 	Result Task1() override;
 
 	Result Task2() override;
+
+	vector<vector<int64>> parseHistories() const;
 };