do day 3

input/2025-3.sample.txt

@@ -0,0 +1,4 @@
+987654321111111
+811111111111119
+234234234234278
+818181911112111

src/day3.cpp

@@ -0,0 +1,59 @@
+// :autocmd BufWritePost *.cpp :silent exec "!./build 3"
+#include <cstdint>
+#include <fstream>
+#include <iostream>
+#include <numeric>
+#include <string>
+#include <string_view>
+#include <vector>
+
+using Data = std::string;
+
+int64_t part_1(const std::vector<Data> &);
+int64_t part_2(const std::vector<Data> &);
+
+int main() {
+  std::ifstream file{"input/2025-3.txt"};
+  Data line;
+  std::vector<Data> v;
+  while (std::getline(file, line)) {
+    v.push_back(line);
+  }
+  std::cout << "Part 1: " << part_1(v) << std::endl;
+  std::cout << "Part 2: " << part_2(v) << std::endl;
+}
+
+char get_max_char(std::string_view s) {
+  return std::accumulate(s.begin(), s.end(), '0',
+                         [](const char maximium_char, const char c) {
+                           return std::max(maximium_char, c);
+                         });
+}
+
+int64_t solve(const std::string_view line, size_t cnt_to_activate) {
+  const auto search{line.substr(0, line.size() - cnt_to_activate + 1)};
+  const char max_char{get_max_char(search)}, null_terminator{'\0'};
+  const auto max_char_idx{search.find(&max_char, 0)};
+  const int64_t max_char_as_int = max_char - '0';
+  if (cnt_to_activate == 1) {
+    return max_char_as_int;
+  }
+  const auto recur{solve(line.substr(max_char_idx + 1), cnt_to_activate - 1)};
+  return std::stoll(std::string(&max_char) + std::to_string(recur));
+}
+
+int64_t part_1(const std::vector<Data> &v) {
+  int64_t ans{};
+  for (auto d : v) {
+    ans += solve(d, 2);
+  }
+  return ans;
+}
+
+int64_t part_2(const std::vector<Data> &v) {
+  int64_t ans{};
+  for (auto d : v) {
+    ans += solve(d, 12);
+  }
+  return ans;
+}