1 files changed, 17 insertions, 32 deletions

diff --git a/src/day09.cpp b/src/day09.cpp
index 1afcfe2..1e9bde5 100644
--- a/src/day09.cpp
+++ b/src/day09.cpp
@@ -27,25 +27,21 @@ read_ints(const string& line)
 }
 
 template<typename T>
-void
-solve_rec_part1(vector<T>& values)
+T
+solve_rec_part1(const vector<T>& values)
 {
         vector<T> differences;
 
         for ( size_t idx = 1; idx < values.size(); ++idx ) {
-                auto difference = values[idx] - values[idx - 1];
-                differences.emplace_back(difference);
+                differences.emplace_back(values[idx] - values[idx - 1]);
         }
-        auto all_zeros = all_of(differences.begin(), differences.end(), [](long diff) { return diff == 0; });
-        if ( !all_zeros) {
-                solve_rec_part1(differences);
+        auto all_zeros = all_of(differences.begin(), differences.end(), [](T diff) { return diff == 0; });
+        if ( !all_zeros ) {
+                return solve_rec_part1(differences) + values.back();
         }
         else {
-                differences.emplace_back(0);
+                return values.back();
         }
-        auto last_value = values.back();
-        auto last_difference = differences.back();
-        values.emplace_back(last_value + last_difference);
 }
 
 void
@@ -55,46 +51,35 @@ part1()
 
         long sum = 0;
         for ( const auto& line: lines ) {
-                auto values = read_ints(line);
-                solve_rec_part1(values);
-                sum += values.back();
+                sum += solve_rec_part1(read_ints(line));
         }
         cout << sum << endl;
 }
 
 template<typename T>
-void
-solve_rec_part2(vector<T>& values)
+T
+solve_rec_part2(const vector<T>& values)
 {
         vector<T> differences;
 
         for ( size_t idx = 1; idx < values.size(); ++idx ) {
-                auto difference = values[idx] - values[idx - 1];
-                differences.emplace_back(difference);
+                differences.emplace_back(values[idx] - values[idx - 1]);
         }
-        auto all_zeros = all_of(differences.begin(), differences.end(), [](long diff) { return diff == 0; });
-        if ( !all_zeros) {
-                solve_rec_part2(differences);
+        auto all_zeros = all_of(differences.begin(), differences.end(), [](T diff) { return diff == 0; });
+        if ( !all_zeros ) {
+                return values.front() - solve_rec_part2(differences);
         }
         else {
-                differences.insert(differences.begin(), 0);
+                return values.front();
         }
-
-        auto first_value = values.front();
-        auto first_difference = differences.front();
-        values.insert(values.begin(), first_value - first_difference);
 }
 
 void
 part2()
 {
-        auto lines = read_file("data/day09.txt");
-
         long sum = 0;
-        for ( const auto& line: lines ) {
-                auto values = read_ints(line);
-                solve_rec_part2(values);
-                sum += values.front();
+        for ( const auto& line: read_file("data/day09.txt") ) {
+                sum += solve_rec_part2(read_ints(line));
         }
         cout << sum << endl;
 }