PB4 part2 done

main.cpp

@@ -29,5 +29,12 @@ int main() {
             std::cout << "Problem 4 part 1: " << result << '\n';
         }
     }
+
+    {
+        std::cout << "Begin Problem 4 part 2\n";
+        if (const auto result = pb4::problem_part2("inputs/pb4.txt"); result != 0) {
+            std::cout << "Problem 4 part 2: " << result << '\n';
+        }
+    }
     return EXIT_SUCCESS;
 }

pb_4/problem_4.cpp

@@ -7,6 +7,7 @@
 
 #include <vector>
 #include <set>
+#include <unordered_map>
 #include <string>
 #include <cmath>
 #include <algorithm>
@@ -72,3 +73,53 @@ std::size_t pb4::problem_part1(const std::filesystem::path &problemFile) noexcep
     }
     return std::accumulate(compute_points.cbegin(), compute_points.cend(), 0);
 }
+
+std::size_t pb4::problem_part2(const std::filesystem::path &problemFile) noexcept {
+    auto file_opt = common::read_file(problemFile);
+    if (!file_opt) {
+        return 0;
+    }
+    std::ifstream &file = *file_opt;
+    std::string line;
+    std::unordered_map<int, int> matching_number;
+    int i = 1;
+    while (std::getline(file, line)) {
+        if (!line.empty()) {
+            line = line.substr(line.find_first_of(':') + 1);
+
+            const auto split_char = line.find_first_of('|');
+            const auto left = line.substr(0, split_char);
+            const auto right = line.substr(split_char + 1);
+
+            const auto left_cards = extract_unique_cards(left);
+            const auto right_cards = extract_unique_cards(right);
+            std::vector<int> intersection;
+            intersection.reserve(left.size() + right_cards.size());
+            std::set_intersection(left_cards.cbegin(),
+                    left_cards.cend(),
+                    right_cards.cbegin(),
+                    right_cards.cend(),
+                    std::back_inserter(intersection));
+
+            const int inter_size = static_cast<int>(intersection.size());
+            if (matching_number.contains(i)) {
+                matching_number[i] += 1;
+            } else {
+                matching_number.emplace(i, 1);
+            }
+            for (int j = 1; j <= inter_size; ++j) {
+                const int next_match = i + j;
+                if (!matching_number.contains(next_match)) {
+                    matching_number.emplace(next_match, matching_number.at(i));
+                } else {
+                    matching_number[next_match] += matching_number.at(i);
+                }
+            }
+            ++i;
+        }
+    }
+    return std::accumulate(matching_number.cbegin(),
+            matching_number.cend(),
+            0,
+            [](const auto input, const auto &bPair) -> int { return input + bPair.second; });
+}