diff --git a/include/mcl/container/hmap.hpp b/include/mcl/container/hmap.hpp
index 1212f02..17e880f 100644
--- a/include/mcl/container/hmap.hpp
+++ b/include/mcl/container/hmap.hpp
@@ -5,6 +5,8 @@
 #pragma once
 
 #include <bit>
+#include <cstddef>
+#include <functional>
 #include <utility>
 
 #include "mcl/assert.hpp"
@@ -14,12 +16,12 @@
 #include "mcl/stdint.hpp"
 
 #if defined(MCL_ARCHITECTURE_ARM64)
-#    include "arm_neon.h"
+#    include <arm_neon.h>
 #endif
 
 namespace mcl {
 
-template<typename K, typename T>
+template<typename KeyType, typename MappedType, typename Hash, typename Pred>
 class hmap;
 
 namespace detail {
@@ -73,20 +75,25 @@ struct meta_byte_group {
         return vmaxvq_u8(data) == 0xff;
     }
 
+    bool is_all_empty_or_tombstone()
+    {
+        return vminvq_u8(vandq_u8(data, vdupq_n_u8(0x80))) == 0x80;
+    }
+
     uint8x16_t data;
 };
 
 #    define MCL_HMAP_MATCH_META_BYTE_GROUP(MATCH, ...)                                                             \
         {                                                                                                          \
-            const uint64x2_t match_result = MATCH;                                                                 \
+            const uint64x2_t match_result{MATCH};                                                                  \
                                                                                                                    \
             for (u64 match_result_v{match_result[0]}; match_result_v != 0; match_result_v &= match_result_v - 1) { \
-                const size_t match_index = std::countr_zero(match_result_v) / 8;                                   \
+                const size_t match_index{static_cast<size_t>(std::countr_zero(match_result_v) / 8)};               \
                 __VA_ARGS__                                                                                        \
             }                                                                                                      \
                                                                                                                    \
             for (u64 match_result_v{match_result[1]}; match_result_v != 0; match_result_v &= match_result_v - 1) { \
-                const size_t match_index = 8 + std::countr_zero(match_result_v) / 8;                               \
+                const size_t match_index{static_cast<size_t>(8 + std::countr_zero(match_result_v) / 8)};           \
                 __VA_ARGS__                                                                                        \
             }                                                                                                      \
         }
@@ -103,12 +110,17 @@ union slot_union {
 
 }  // namespace detail
 
-template<typename K, typename T>
+template<bool IsConst, typename KeyType, typename MappedType, typename Hash, typename Pred>
 class hmap_iterator {
+    using base_value_type = std::pair<const KeyType, MappedType>;
+    using slot_type = detail::slot_union<base_value_type>;
+
 public:
+    using key_type = KeyType;
+    using mapped_type = MappedType;
     using iterator_category = std::forward_iterator_tag;
     using difference_type = std::ptrdiff_t;
-    using value_type = std::pair<K, T>;
+    using value_type = std::conditional_t<IsConst, std::add_const_t<base_value_type>, base_value_type>;
     using pointer = value_type*;
     using const_pointer = const value_type*;
     using reference = value_type&;
@@ -156,9 +168,7 @@ public:
     }
 
 private:
-    friend class hmap<K, T>;
-
-    using slot_type = detail::slot_union<value_type>;
+    friend class hmap<KeyType, MappedType, Hash, Pred>;
 
     hmap_iterator(detail::meta_byte* mb_ptr, slot_type* slot_ptr)
         : mb_ptr{mb_ptr}, slot_ptr{slot_ptr}
@@ -184,41 +194,57 @@ private:
         }
     }
 
-    detail::meta_byte* mb_ptr = nullptr;
-    slot_type* slot_ptr = nullptr;
+    detail::meta_byte* mb_ptr{nullptr};
+    slot_type* slot_ptr{nullptr};
 };
 
-template<typename KeyType, typename MappedType>
+template<typename KeyType, typename MappedType, typename Hash = std::hash<KeyType>, typename Pred = std::equal_to<KeyType>>
 class hmap {
 public:
     using key_type = KeyType;
     using mapped_type = MappedType;
-    using value_type = std::pair<key_type, mapped_type>;
-    using iterator = hmap_iterator<key_type, mapped_type>;
+    using hasher = Hash;
+    using key_equal = Pred;
+    using value_type = std::pair<const key_type, mapped_type>;
+    using reference = value_type&;
+    using const_reference = const value_type&;
+    using pointer = value_type*;
+    using const_pointer = const value_type*;
+    using size_type = std::size_t;
+    using difference_type = std::ptrdiff_t;
+
+    using iterator = hmap_iterator<false, key_type, mapped_type, hasher, key_equal>;
+    using const_iterator = hmap_iterator<true, key_type, mapped_type, hasher, key_equal>;
 
 private:
     using slot_type = detail::slot_union<value_type>;
     static_assert(!std::is_reference_v<key_type>);
     static_assert(!std::is_reference_v<mapped_type>);
 
-    static constexpr size_t group_size = 16;
+    static constexpr size_t group_size{16};
 
 public:
     hmap()
     {
         initialize_members(1);
     }
+    hmap(const hmap&);
+    hmap(hmap&&);
+    hmap& operator=(const hmap&);
+    hmap& operator=(hmap&&);
 
     ~hmap()
     {
-        for (auto iter = begin(); iter != end(); ++iter) {
-            iter->~value_type();
-        }
+        clear();
     }
 
+    [[nodiscard]] bool empty() const noexcept { return full_slots == 0; }
+    size_type size() const noexcept { return full_slots; }
+    size_type max_size() const noexcept { return static_cast<size_type>(std::numeric_limits<difference_type>::max()); }
+
     iterator begin()
     {
-        iterator result{get_iterator_at(0)};
+        iterator result{iterator_at(0)};
         result.skip_empty_or_tombstone();
         return result;
     }
@@ -226,34 +252,26 @@ public:
     {
         return {};
     }
-
-    iterator find(const key_type& key)
+    const_iterator cbegin() const
     {
-        const size_t hash = std::hash<key_type>{}(key);
-        const detail::meta_byte mb = detail::meta_byte_from_hash(hash);
-
-        size_t group_index = detail::group_index_from_hash(hash, group_index_mask);
-
-        while (true) {
-            detail::meta_byte_group g{mbs.get() + group_index * group_size};
-
-            MCL_HMAP_MATCH_META_BYTE_GROUP(g.match(mb), {
-                const size_t item_index{group_index * group_size + match_index};
-
-                if (slots[item_index].value.first == key) [[likely]] {
-                    return get_iterator_at(item_index);
-                }
-            });
-
-            if (g.is_any_empty()) [[likely]] {
-                return {};
-            }
-
-            group_index = (group_index + 1) & group_index_mask;
-        }
+        const_iterator result{const_iterator_at(0)};
+        result.skip_empty_or_tombstone();
+        return result;
+    }
+    const_iterator cend() const
+    {
+        return {};
+    }
+    const_iterator begin() const
+    {
+        return cbegin();
+    }
+    const_iterator end() const
+    {
+        return cend();
     }
 
-    template<typename K, typename... Args>
+    template<typename K = key_type, typename... Args>
     std::pair<iterator, bool> try_emplace(K&& k, Args&&... args)
     {
         auto [item_index, item_found] = find_key_or_empty_slot(k);
@@ -263,10 +281,10 @@ public:
                 std::forward_as_tuple(std::forward<K>(k)),
                 std::forward_as_tuple(std::forward<Args>(args)...));
         }
-        return {get_iterator_at(item_index), !item_found};
+        return {iterator_at(item_index), !item_found};
     }
 
-    template<typename K, typename V>
+    template<typename K = key_type, typename V = mapped_type>
     std::pair<iterator, bool> insert_or_assign(K&& k, V&& v)
     {
         auto [item_index, item_found] = find_key_or_empty_slot(k);
@@ -277,27 +295,40 @@ public:
                 std::forward<K>(k),
                 std::forward<V>(v));
         }
-        return {get_iterator_at(item_index), !item_found};
+        return {iterator_at(item_index), !item_found};
     }
 
+    void erase(const_iterator position)
+    {
+        if (position == cend()) {
+            return;
+        }
+
+        const size_t item_index{static_cast<size_t>(std::distance(mbs.get(), position.mb_ptr))};
+        const size_t group_index{item_index / group_size};
+        const detail::meta_byte_group g{mbs.get() + group_index * group_size};
+
+        erase_impl(item_index, std::move(g));
+    }
+    void erase(iterator position)
+    {
+        if (position == end()) {
+            return;
+        }
+
+        const size_t item_index{static_cast<size_t>(std::distance(mbs.get(), position.mb_ptr))};
+        const size_t group_index{item_index / group_size};
+        const detail::meta_byte_group g{mbs.get() + group_index * group_size};
+
+        erase_impl(item_index, std::move(g));
+    }
     template<typename K = key_type>
-    mapped_type& operator[](K&& k)
+    size_t erase(const K& key)
     {
-        return try_emplace(std::forward<K>(k)).first->second;
-    }
+        const size_t hash{hasher{}(key)};
+        const detail::meta_byte mb{detail::meta_byte_from_hash(hash)};
 
-private:
-    iterator get_iterator_at(size_t item_index)
-    {
-        return {mbs.get() + item_index, slots.get() + item_index};
-    }
-
-    std::pair<size_t, bool> find_key_or_empty_slot(const key_type& key)
-    {
-        const size_t hash = std::hash<key_type>{}(key);
-        const detail::meta_byte mb = detail::meta_byte_from_hash(hash);
-
-        size_t group_index = detail::group_index_from_hash(hash, group_index_mask);
+        size_t group_index{detail::group_index_from_hash(hash, group_index_mask)};
 
         while (true) {
             detail::meta_byte_group g{mbs.get() + group_index * group_size};
@@ -305,7 +336,140 @@ private:
             MCL_HMAP_MATCH_META_BYTE_GROUP(g.match(mb), {
                 const size_t item_index{group_index * group_size + match_index};
 
-                if (slots[item_index].value.first == key) [[likely]] {
+                if (key_equal{}(slots[item_index].value.first, key)) [[likely]] {
+                    erase_impl(item_index, std::move(g));
+                    return 1;
+                }
+            });
+
+            if (g.is_any_empty()) [[likely]] {
+                return 0;
+            }
+
+            group_index = (group_index + 1) & group_index_mask;
+        }
+    }
+
+    template<typename K = key_type>
+    iterator find(const K& key)
+    {
+        const size_t hash{hasher{}(key)};
+        const detail::meta_byte mb{detail::meta_byte_from_hash(hash)};
+
+        size_t group_index{detail::group_index_from_hash(hash, group_index_mask)};
+
+        while (true) {
+            detail::meta_byte_group g{mbs.get() + group_index * group_size};
+
+            MCL_HMAP_MATCH_META_BYTE_GROUP(g.match(mb), {
+                const size_t item_index{group_index * group_size + match_index};
+
+                if (key_equal{}(slots[item_index].value.first, key)) [[likely]] {
+                    return iterator_at(item_index);
+                }
+            });
+
+            if (g.is_any_empty()) [[likely]] {
+                return {};
+            }
+
+            group_index = (group_index + 1) & group_index_mask;
+        }
+    }
+    template<typename K = key_type>
+    const_iterator find(const K& key) const
+    {
+        const size_t hash{hasher{}(key)};
+        const detail::meta_byte mb{detail::meta_byte_from_hash(hash)};
+
+        size_t group_index{detail::group_index_from_hash(hash, group_index_mask)};
+
+        while (true) {
+            detail::meta_byte_group g{mbs.get() + group_index * group_size};
+
+            MCL_HMAP_MATCH_META_BYTE_GROUP(g.match(mb), {
+                const size_t item_index{group_index * group_size + match_index};
+
+                if (key_equal{}(slots[item_index].value.first, key)) [[likely]] {
+                    return const_iterator_at(item_index);
+                }
+            });
+
+            if (g.is_any_empty()) [[likely]] {
+                return {};
+            }
+
+            group_index = (group_index + 1) & group_index_mask;
+        }
+    }
+    template<typename K = key_type>
+    bool contains(const K& key) const
+    {
+        return find(key) != end();
+    }
+    template<typename K = key_type>
+    size_t count(const K& key) const
+    {
+        return contains(key) ? 1 : 0;
+    }
+
+    template<typename K = key_type>
+    mapped_type& operator[](K&& k)
+    {
+        return try_emplace(std::forward<K>(k)).first->second;
+    }
+    template<typename K = key_type>
+    mapped_type& at(K&& k)
+    {
+        const auto iter{find(k)};
+        if (iter == end()) {
+            throw std::out_of_range("hmap::at: key not found");
+        }
+        return iter->second;
+    }
+    template<typename K = key_type>
+    const mapped_type& at(K&& k) const
+    {
+        const auto iter{find(k)};
+        if (iter == end()) {
+            throw std::out_of_range("hmap::at: key not found");
+        }
+        return iter->second;
+    }
+
+    void clear()
+    {
+        for (auto iter{begin()}; iter != end(); ++iter) {
+            iter->~value_type();
+        }
+
+        clear_metadata();
+    }
+
+private:
+    iterator iterator_at(size_t item_index)
+    {
+        return {mbs.get() + item_index, slots.get() + item_index};
+    }
+    const_iterator const_iterator_at(size_t item_index)
+    {
+        return {mbs.get() + item_index, slots.get() + item_index};
+    }
+
+    std::pair<size_t, bool> find_key_or_empty_slot(const key_type& key)
+    {
+        const size_t hash{hasher{}(key)};
+        const detail::meta_byte mb{detail::meta_byte_from_hash(hash)};
+
+        size_t group_index{detail::group_index_from_hash(hash, group_index_mask)};
+
+        while (true) {
+            detail::meta_byte_group g{mbs.get() + group_index * group_size};
+
+            MCL_HMAP_MATCH_META_BYTE_GROUP(g.match(mb), {
+                const size_t item_index{group_index * group_size + match_index};
+
+                if (key_equal{}(slots[item_index].value.first, key)) [[likely]] {
                     return {item_index, true};
                 }
             });
@@ -324,7 +488,7 @@ private:
             grow_and_rehash();
         }
 
-        size_t group_index = detail::group_index_from_hash(hash, group_index_mask);
+        size_t group_index{detail::group_index_from_hash(hash, group_index_mask)};
 
         while (true) {
             detail::meta_byte_group g{mbs.get() + group_index * group_size};
@@ -335,6 +499,7 @@ private:
                 if (mbs[item_index] == detail::meta_byte::empty) [[likely]] {
                     --empty_slots;
                 }
+                ++full_slots;
 
                 mbs[item_index] = detail::meta_byte_from_hash(hash);
 
@@ -345,25 +510,38 @@ private:
         }
     }
 
+    void erase_impl(size_t item_index, detail::meta_byte_group&& g)
+    {
+        slots[item_index].value->~value_type();
+
+        --full_slots;
+        if (g.is_any_empty()) {
+            mbs[item_index] = detail::meta_byte::empty;
+            ++empty_slots;
+        } else {
+            mbs[item_index] = detail::meta_byte::tombstone;
+        }
+    }
+
     void grow_and_rehash()
     {
-        const size_t new_group_count = 2 * (group_index_mask + 1);
+        const size_t new_group_count{2 * (group_index_mask + 1)};
 
         pow2_resize(new_group_count);
     }
 
     void pow2_resize(size_t new_group_count)
     {
-        auto iter = begin();
+        auto iter{begin()};
 
-        const auto old_mbs = std::move(mbs);
-        const auto old_slots = std::move(slots);
+        const auto old_mbs{std::move(mbs)};
+        const auto old_slots{std::move(slots)};
 
         initialize_members(new_group_count);
 
         for (; iter != iterator{}; ++iter) {
-            const size_t hash = std::hash<key_type>{}(iter->first);
-            const size_t item_index = find_empty_slot_to_insert(hash);
+            const size_t hash{hasher{}(iter->first)};
+            const size_t item_index{find_empty_slot_to_insert(hash)};
 
             new (&slots[item_index].value) value_type(std::move(iter.slot_ptr->value));
             iter.slot_ptr->value.~value_type();
@@ -375,16 +553,26 @@ private:
         // DEBUG_ASSERT(group_count != 0 && std::ispow2(group_count));
 
         group_index_mask = group_count - 1;
-        empty_slots = group_count * group_size * 7 / 8;
         mbs = std::unique_ptr<detail::meta_byte[]>{new (std::align_val_t(group_size)) detail::meta_byte[group_count * group_size + 1]};
         slots = std::unique_ptr<slot_type[]>{new slot_type[group_count * group_size]};
 
+        clear_metadata();
+    }
+
+    void clear_metadata()
+    {
+        const size_t group_count{group_index_mask + 1};
+
+        empty_slots = group_count * group_size * 7 / 8;
+        full_slots = 0;
+
         std::memset(mbs.get(), static_cast<int>(detail::meta_byte::empty), group_count * group_size);
         mbs[group_count * group_size] = detail::meta_byte::end_sentinel;
     }
 
     size_t group_index_mask;
     size_t empty_slots;
+    size_t full_slots;
     std::unique_ptr<detail::meta_byte[]> mbs;
     std::unique_ptr<slot_type[]> slots;
 };
diff --git a/tests/hmap.cpp b/tests/hmap.cpp
index 62989a1..5518574 100644
--- a/tests/hmap.cpp
+++ b/tests/hmap.cpp
@@ -15,8 +15,11 @@ TEST_CASE("mcl::hmap", "[hmap]")
 
     constexpr int count = 100000;
 
+    REQUIRE(double_map.empty());
+
     for (int i = 0; i < count; ++i) {
         double_map[i] = i * 2;
+        REQUIRE(double_map.size() == i + 1);
     }
 
     for (int i = 0; i < count; ++i) {
@@ -36,4 +39,12 @@ TEST_CASE("mcl::hmap", "[hmap]")
         (void)k;
         REQUIRE(v == 1);
     }
+
+    REQUIRE(!double_map.empty());
+    double_map.clear();
+    REQUIRE(double_map.empty());
+
+    for (auto [k, v] : double_map) {
+        REQUIRE(false);
+    }
 }