添加自定义比较器以支持数字排序，优化问题ID获取逻辑

src/db/problems.cpp

@@ -2,17 +2,43 @@
 
 #include <print>
 
+#include <leveldb/comparator.h>
 #include <leveldb/db.h>
 #include <leveldb/write_batch.h>
 
 #include <nlohmann/json.hpp>
-#include <string>
+
+class NumericComparator : public leveldb::Comparator
+{
+public:
+    int Compare(const leveldb::Slice& a, const leveldb::Slice& b) const override
+    {
+        auto x = std::stoi(a.ToString()), y = std::stoi(b.ToString());
+
+        if (x < y) {
+            return -1;
+        } else if (x > y) {
+            return 1;
+        } else {
+            return 0;
+        }
+    }
+
+    const char* Name() const override
+    {
+        return "NumericComparator";
+    }
+
+    void FindShortestSeparator(std::string* start, const leveldb::Slice& limit) const override {}
+    void FindShortSuccessor(std::string* key) const override {}
+};
 
 using leveldb_ptr = leveldb::DB*;
 
 auto problems_db    = leveldb_ptr{nullptr};
 auto max_problem_id = 0;
 
+auto comparator = NumericComparator{};
 
 extern "C"
 {
@@ -20,6 +46,8 @@ extern "C"
     {
         auto opts = leveldb::Options{};
 
+        opts.comparator = &comparator;
+
         opts.create_if_missing = true;
 
         auto status = leveldb::DB::Open(opts, "db/problems", &problems_db);
@@ -29,22 +57,20 @@ extern "C"
             return 0;
         }
 
-        auto value = std::string{};
-        status     = problems_db->Get(leveldb::ReadOptions{}, "@", &value);
-        if (status.ok()) {
-            auto json = nlohmann::json::parse(value);
-            json["max_problem_id"].get_to(max_problem_id);
-        } else if (status.IsNotFound()) {
-            auto json = nlohmann::json{
-                {"max_problem_id", 0}
-            };
-            status = problems_db->Put(leveldb::WriteOptions{}, "@", json.dump());
-            return 1;
-        } else {
-            std::println(stderr, "Failed to get max problem id: {}", status.ToString());
+        auto it = problems_db->NewIterator(leveldb::ReadOptions());
+        for (it->SeekToFirst(); it->Valid(); it->Next()) {
+            auto id = std::stoi(it->key().ToString());
+            if (id > max_problem_id) max_problem_id = id;
+        }
+
+        if (!it->status().ok()) {
+            std::println(stderr, "Failed to get max problem id: {}", it->status().ToString());
+            delete it;
             delete problems_db;
             return 0;
         }
+
+        delete it;
         return 1;
     }
 
@@ -58,10 +84,6 @@ extern "C"
         auto batch = leveldb::WriteBatch{};
         ++max_problem_id;
         auto json = nlohmann::json{
-            {"max_problem_id", max_problem_id}
-        };
-        batch.Put("@", json.dump());
-        json = nlohmann::json{
             {"content", problem    },
             {"answer",  answer     },
             {"error",   check_error}
@@ -178,7 +200,6 @@ extern "C"
 
         auto it = problems_db->NewIterator(leveldb::ReadOptions());
         for (it->SeekToFirst(); it->Valid(); it->Next()) {
-            if (!it->key().compare("@")) continue;
             problems.emplace_back(std::stoi(it->key().ToString()));
         }