Initial commit

2020/puzzle-15-01.cc

@@ -0,0 +1,58 @@
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <iostream>
+#include <list>
+#include <map>
+#include <regex>
+#include <string>
+
+using NumMap = std::map<int, int>;
+
+int add_to_map(NumMap &nums, int num, int turn) {
+  auto [it, success] = nums.insert({num, turn});
+  if (success) {
+    // std::cout << "Turn " << turn << ": " << num << " (new)\n";
+    return 0;
+  } else {
+    int r = turn - it->second;
+    // std::cout << "Turn " << turn << ": " << num << " (previous seen turn "
+    //          << it->second << " dist = " << r << ")\n";
+    it->second = turn;
+    return r;
+  }
+}
+
+int run(std::string const &s, int num_turns) {
+  NumMap seen;
+  std::size_t pos = 0;
+  std::size_t len;
+  int next_num = 0;
+  int turn = 1;
+  while (pos < s.size()) {
+    int num = std::stoi(s.substr(pos), &len);
+    pos += len;
+    while (pos < s.size() && s[pos] == ',') {
+      ++pos;
+    }
+    next_num = add_to_map(seen, num, turn);
+    ++turn;
+  }
+
+  while (turn < num_turns) {
+    next_num = add_to_map(seen, next_num, turn);
+    ++turn;
+  }
+
+  return next_num;
+}
+
+int main(int argc, char **argv) {
+  std::string line;
+  while (std::getline(std::cin, line)) {
+    int r = run(line, 2020);
+    std::cout << "2020'th number: " << r << "\n";
+  }
+
+  return 0;
+}