BLI: new C++ hash table data structures

This commit adds some new hashing based data structures to blenlib.
All of them use open addressing with probing currently.
Furthermore, they support small object optimization, but it is not
customizable yet. I'll add support for this when necessary.
The following main data structures are included:

**Set**
A collection of values, where every value must exist at most once.
This is similar to a Python `set`.

**SetVector**
A combination of a Set and a Vector. It supports fast search for
elements and maintains insertion order when there are no deletes.
All elements are stored in a continuous array. So they can be
iterated over using a normal `ArrayRef`.

**Map**
A set of key-value-pairs, where every key must exist at most once.
This is similar to a Python `dict`.

**StringMap**
A special map for the case when the keys are strings. This case is
fairly common and allows for some optimizations. Most importantly,
many unnecessary allocations can be avoided by storing strings in
a single buffer. Furthermore, the interface of this class uses
`StringRef` to avoid unnecessary conversions.

This commit is a continuation of rB369d5e8ad2bb7.

1 files changed, 302 insertions, 0 deletions

diff --git a/source/blender/blenlib/BLI_open_addressing.h b/source/blender/blenlib/BLI_open_addressing.h
new file mode 100644
index 00000000000..3bdacae18d1
--- /dev/null
+++ b/source/blender/blenlib/BLI_open_addressing.h
@@ -0,0 +1,302 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/** \file
+ * \ingroup bli
+ *
+ * This class offers a useful abstraction for other containers that implement hash tables using
+ * open addressing. It handles the following aspects:
+ *   - Allocation and deallocation of the open addressing array.
+ *   - Optional small object optimization.
+ *   - Keeps track of how many elements and dummies are in the table.
+ *
+ * The nice thing about this abstraction is that it does not get in the way of any performance
+ * optimizations. The data that is actually stored in the table is still fully defined by the
+ * actual hash table implementation.
+ */
+
+#pragma once
+
+#include <cmath>
+
+#include "BLI_utildefines.h"
+#include "BLI_memory_utils_cxx.h"
+#include "BLI_math_base.h"
+#include "BLI_allocator.h"
+
+namespace BLI {
+
+template<typename Item, uint32_t ItemsInSmallStorage = 1, typename Allocator = GuardedAllocator>
+class OpenAddressingArray {
+ private:
+  static constexpr auto slots_per_item = Item::slots_per_item;
+  static constexpr float max_load_factor = 0.5f;
+
+  /* Invariants:
+   *   2^m_item_exponent = m_item_amount
+   *   m_item_amount * slots_per_item = m_slots_total
+   *   m_slot_mask = m_slots_total - 1
+   *   m_slots_set_or_dummy < m_slots_total
+   */
+
+  /* Array containing the actual hash table. Might be a pointer to the inlined storage. */
+  Item *m_items;
+  /* Number of items in the hash table. Must be a power of two. */
+  uint32_t m_item_amount;
+  /* Exponent of the current item amount. */
+  uint8_t m_item_exponent;
+  /* Number of elements that could be stored in the table when the load factor is 1. */
+  uint32_t m_slots_total;
+  /* Number of elements that are not empty. */
+  uint32_t m_slots_set_or_dummy;
+  /* Number of dummy entries. */
+  uint32_t m_slots_dummy;
+  /* Max number of slots that can be non-empty according to the load factor. */
+  uint32_t m_slots_usable;
+  /* Can be used to map a hash value into the range of valid slot indices. */
+  uint32_t m_slot_mask;
+  Allocator m_allocator;
+  char m_local_storage[sizeof(Item) * ItemsInSmallStorage];
+
+ public:
+  explicit OpenAddressingArray(uint8_t item_exponent = 0)
+  {
+    m_slots_total = (1 << item_exponent) * slots_per_item;
+    m_slots_set_or_dummy = 0;
+    m_slots_dummy = 0;
+    m_slots_usable = m_slots_total * max_load_factor;
+    m_slot_mask = m_slots_total - 1;
+    m_item_amount = m_slots_total / slots_per_item;
+    m_item_exponent = item_exponent;
+
+    if (m_item_amount <= ItemsInSmallStorage) {
+      m_items = this->small_storage();
+    }
+    else {
+      m_items = (Item *)m_allocator.allocate_aligned(
+          sizeof(Item) * m_item_amount, std::alignment_of<Item>::value, __func__);
+    }
+
+    for (uint32_t i = 0; i < m_item_amount; i++) {
+      new (m_items + i) Item();
+    }
+  }
+
+  ~OpenAddressingArray()
+  {
+    if (m_items != nullptr) {
+      for (uint32_t i = 0; i < m_item_amount; i++) {
+        m_items[i].~Item();
+      }
+      if (!this->is_in_small_storage()) {
+        m_allocator.deallocate((void *)m_items);
+      }
+    }
+  }
+
+  OpenAddressingArray(const OpenAddressingArray &other)
+  {
+    m_slots_total = other.m_slots_total;
+    m_slots_set_or_dummy = other.m_slots_set_or_dummy;
+    m_slots_dummy = other.m_slots_dummy;
+    m_slots_usable = other.m_slots_usable;
+    m_slot_mask = other.m_slot_mask;
+    m_item_amount = other.m_item_amount;
+    m_item_exponent = other.m_item_exponent;
+
+    if (m_item_amount <= ItemsInSmallStorage) {
+      m_items = this->small_storage();
+    }
+    else {
+      m_items = (Item *)m_allocator.allocate_aligned(
+          sizeof(Item) * m_item_amount, std::alignment_of<Item>::value, __func__);
+    }
+
+    uninitialized_copy_n(other.m_items, m_item_amount, m_items);
+  }
+
+  OpenAddressingArray(OpenAddressingArray &&other) noexcept
+  {
+    m_slots_total = other.m_slots_total;
+    m_slots_set_or_dummy = other.m_slots_set_or_dummy;
+    m_slots_dummy = other.m_slots_dummy;
+    m_slots_usable = other.m_slots_usable;
+    m_slot_mask = other.m_slot_mask;
+    m_item_amount = other.m_item_amount;
+    m_item_exponent = other.m_item_exponent;
+    if (other.is_in_small_storage()) {
+      m_items = this->small_storage();
+      uninitialized_relocate_n(other.m_items, m_item_amount, m_items);
+    }
+    else {
+      m_items = other.m_items;
+    }
+
+    other.m_items = nullptr;
+    other.~OpenAddressingArray();
+    new (&other) OpenAddressingArray();
+  }
+
+  OpenAddressingArray &operator=(const OpenAddressingArray &other)
+  {
+    if (this == &other) {
+      return *this;
+    }
+    this->~OpenAddressingArray();
+    new (this) OpenAddressingArray(other);
+    return *this;
+  }
+
+  OpenAddressingArray &operator=(OpenAddressingArray &&other)
+  {
+    if (this == &other) {
+      return *this;
+    }
+    this->~OpenAddressingArray();
+    new (this) OpenAddressingArray(std::move(other));
+    return *this;
+  }
+
+  /* Prepare a new array that can hold a minimum of min_usable_slots elements. All entries are
+   * empty. */
+  OpenAddressingArray init_reserved(uint32_t min_usable_slots) const
+  {
+    float min_total_slots = (float)min_usable_slots / max_load_factor;
+    uint32_t min_total_items = (uint32_t)std::ceil(min_total_slots / (float)slots_per_item);
+    uint8_t item_exponent = log2_ceil_u(min_total_items);
+    OpenAddressingArray grown(item_exponent);
+    grown.m_slots_set_or_dummy = this->slots_set();
+    return grown;
+  }
+
+  /**
+   * Amount of items in the array times the number of slots per item.
+   */
+  uint32_t slots_total() const
+  {
+    return m_slots_total;
+  }
+
+  /**
+   * Amount of slots that are initialized with some value that is not empty or dummy.
+   */
+  uint32_t slots_set() const
+  {
+    return m_slots_set_or_dummy - m_slots_dummy;
+  }
+
+  /**
+   * Amount of slots that can be used before the array should grow.
+   */
+  uint32_t slots_usable() const
+  {
+    return m_slots_usable;
+  }
+
+  /**
+   * Update the counters after one empty element is used for a newly added element.
+   */
+  void update__empty_to_set()
+  {
+    m_slots_set_or_dummy++;
+  }
+
+  /**
+   * Update the counters after one previously dummy element becomes set.
+   */
+  void update__dummy_to_set()
+  {
+    m_slots_dummy--;
+  }
+
+  /**
+   * Update the counters after one previously set element becomes a dummy.
+   */
+  void update__set_to_dummy()
+  {
+    m_slots_dummy++;
+  }
+
+  /**
+   * Access the current slot mask for this array.
+   */
+  uint32_t slot_mask() const
+  {
+    return m_slot_mask;
+  }
+
+  /**
+   * Access the item for a specific item index.
+   * Note: The item index is not necessarily the slot index.
+   */
+  const Item &item(uint32_t item_index) const
+  {
+    return m_items[item_index];
+  }
+
+  Item &item(uint32_t item_index)
+  {
+    return m_items[item_index];
+  }
+
+  uint8_t item_exponent() const
+  {
+    return m_item_exponent;
+  }
+
+  uint32_t item_amount() const
+  {
+    return m_item_amount;
+  }
+
+  bool should_grow() const
+  {
+    return m_slots_set_or_dummy >= m_slots_usable;
+  }
+
+  Item *begin()
+  {
+    return m_items;
+  }
+
+  Item *end()
+  {
+    return m_items + m_item_amount;
+  }
+
+  const Item *begin() const
+  {
+    return m_items;
+  }
+
+  const Item *end() const
+  {
+    return m_items + m_item_amount;
+  }
+
+ private:
+  Item *small_storage() const
+  {
+    return reinterpret_cast<Item *>((char *)m_local_storage);
+  }
+
+  bool is_in_small_storage() const
+  {
+    return m_items == this->small_storage();
+  }
+};
+
+}  // namespace BLI