From 82582497a74a821a4fabf1d114fee660aa3756d7 Mon Sep 17 00:00:00 2001
From: FICTURE7 <ficture7@gmail.com>
Date: Fri, 9 Apr 2021 19:47:09 +0400
Subject: [PATCH] Add EntryTable<TEntry>

---
 ARMeilleure/Common/EntryTable.cs | 100 +++++++++++++++++++++++++++++++
 1 file changed, 100 insertions(+)
 create mode 100644 ARMeilleure/Common/EntryTable.cs

diff --git a/ARMeilleure/Common/EntryTable.cs b/ARMeilleure/Common/EntryTable.cs
new file mode 100644
index 000000000..eef8445a5
--- /dev/null
+++ b/ARMeilleure/Common/EntryTable.cs
@@ -0,0 +1,100 @@
+using System;
+
+namespace ARMeilleure.Common
+{
+    /// <summary>
+    /// Represents a fixed size table of the type <typeparamref name="TEntry"/>, whose entries will remain at the same
+    /// address through out the table's lifetime.
+    /// </summary>
+    /// <typeparam name="TEntry">Type of the entry in the table</typeparam>
+    class EntryTable<TEntry> where TEntry : unmanaged
+    {
+        private int _freeHint;
+        private readonly TEntry[] _table;
+        private readonly BitMap _allocated;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="EntryTable{TValue}"/> class with the specified capacity.
+        /// </summary>
+        /// <param name="capacity">Capacity of the table</param>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="capacity"/> is less than 0</exception>
+        public EntryTable(int capacity)
+        {
+            if (capacity < 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(capacity));
+            }
+
+            _freeHint = 0;
+            _allocated = new BitMap();
+            _table = GC.AllocateArray<TEntry>(capacity, pinned: true);
+        }
+
+        /// <summary>
+        /// Tries to allocate an entry in the <see cref="EntryTable{TValue}"/>. Returns <see langword="true"/> if
+        /// success; otherwise returns <see langword="false"/>.
+        /// </summary>
+        /// <param name="index">Index of entry allocated in the table</param>
+        /// <returns><see langword="true"/> if success; otherwise <see langword="false"/></returns>
+        public bool TryAllocate(out int index)
+        {
+            lock (_allocated)
+            {
+                if (_allocated.IsSet(_freeHint))
+                {
+                    _freeHint = _allocated.FindFirstUnset();
+                }
+
+                if (_freeHint < _table.Length)
+                {
+                    index = checked(_freeHint++);
+
+                    _allocated.Set(index);
+
+                    return true;
+                }
+            }
+
+            index = 0;
+
+            return false;
+        }
+
+        /// <summary>
+        /// Frees the entry at the specified <paramref name="index"/>.
+        /// </summary>
+        /// <param name="index">Index of entry to free</param>
+        public void Free(int index)
+        {
+            lock (_allocated)
+            {
+                _allocated.Clear(index);
+            }
+        }
+
+        /// <summary>
+        /// Gets a reference to the entry at the specified allocated <paramref name="index"/>.
+        /// </summary>
+        /// <param name="index">Index of the entry</param>
+        /// <returns>Reference to the entry at the specified index</returns>
+        /// <exception cref="ArgumentException">Entry at <paramref name="index"/> is not allocated</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is outside of the table</exception>
+        public ref TEntry GetValue(int index)
+        {
+            if (index < 0 || index >= _table.Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index));
+            }
+
+            lock (_allocated)
+            {
+                if (!_allocated.IsSet(index))
+                {
+                    throw new ArgumentException("Entry at the specified index was not allocated", nameof(index));
+                }
+            }
+
+            return ref _table[index];
+        }
+    }
+}