size < and at implemented, iterator still scaring me

Map.hpp

@@ -139,9 +139,14 @@ public:
   // who cares we're using vector
   ~Map() {}
 
-  size_t size() const { return this->store.size(); }
+  size_t size() const {
+    std::size_t count = 0;
+    for (auto &m_pair : this->store) {
+      count += m_pair.has_value() ? 1 : 0;
+    }
+    return count;
+  }
   bool empty() const { return this->store.empty(); }
-  // TODO: iterator creation
   Iterator begin() {
     std::size_t start = 0;
     while (2 * (start - store.begin()) + 1 < store.size()) {
@@ -194,9 +199,27 @@ public:
       }
     }
   }
-  Mapped_T &at(const Key_T &key) {}
+
-  const Mapped_T &at(const Key_T &key) const {}
+  Mapped_T &at(const Key_T &key) {
-  Mapped_T &operator[](const Key_T &) {}
+    std::size_t i = 0;
+    while (this->store.at(i).has_value()) {
+      switch (true) {
+      case this->store.at(i).first == key:
+        return this->store.at(i).second;
+      case this->store.at(i).first < key:
+        i = 2 * i + 1;
+        break;
+      case this->store.at(i).first > key:
+        i = 2 * i + 2;
+        break;
+      }
+    }
+    throw std::out_of_range{""};
+  }
+  const Mapped_T &at(const Key_T &key) const { return this->at(key); }
+  Mapped_T &operator[](const Key_T &key) { return this->at(key); }
+
+  // TODO: single insert
   std ::pair<Iterator, bool> insert(const ValueType &) {}
   template <typename IT_T> void insert(IT_T range_beg, IT_T range_end) {
     std::for_each(range_beg, range_end,
@@ -225,7 +248,40 @@ public:
   friend bool operator!=(const Map &lhs, const Map &rhs) {
     return !(lhs == rhs);
   }
-  // TODO < operator
+  friend bool operator<(const Map &lhs, const Map &rhs) {
-  friend bool operator<(const Map &lhs, const Map &rhs) { return false; }
+    std::size_t lhs_i = 0;
+    std::size_t rhs_i = 0;
+    for (; lhs_i < lhs.store.size() && rhs_i < rhs.store.size();
+         lhs_i++, rhs_i++) {
+      bool lhs_exhaust = false;
+      while (!lhs.store[lhs_i].has_value()) {
+        lhs_i++;
+        if (lhs.store.size() >= lhs_i) {
+          lhs_exhaust = true;
+          break;
+        }
+      }
+
+      bool rhs_exhaust = false;
+      while (!rhs.store[rhs_i].has_value()) {
+        rhs_i++;
+        if (rhs.store.size() >= rhs_i) {
+          rhs_exhaust = true;
+          break;
+        }
+      }
+
+      if (lhs_exhaust && !rhs_exhaust) {
+        return true;
+      }
+      if (lhs_exhaust || rhs_exhaust) {
+        break;
+      }
+      if (lhs.store[lhs_i] != rhs.store[rhs_i]) {
+        return lhs.store[lhs_i] < rhs.store[rhs_i];
+      }
+    }
+    return false;
+  }
 };
 } // namespace cs440