Prevent unlikely race condition

src/main/java/net/minestom/server/utils/collection/IndexMap.java

@@ -1,30 +1,109 @@
 package net.minestom.server.utils.collection;
 
-import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
+import it.unimi.dsi.fastutil.HashCommon;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.NotNull;
 
+import static it.unimi.dsi.fastutil.HashCommon.arraySize;
+import static it.unimi.dsi.fastutil.HashCommon.maxFill;
+
+// Most of the code comes from fastutil's Object2IntOpenHashMap
 @ApiStatus.Internal
-public final class IndexMap<T> {
-    private final Object2IntOpenHashMap<T> indexMap;
-    private int lastIndex = 0; // Synchronized on INDEX_MAP
+public final class IndexMap<K> {
+    private K[] key;
+    private int[] value;
+    private int lastIndex;
+
+    private int maxFill, size;
+    private final float f = 0.75f;
 
     public IndexMap() {
-        this.indexMap = new Object2IntOpenHashMap<>();
-        indexMap.defaultReturnValue(-1);
+        final int n = arraySize(16, f);
+        maxFill = maxFill(n, f);
+        key = (K[]) new Object[n + 1];
+        value = new int[n + 1];
     }
 
-    public int get(@NotNull T key) {
-        int index = indexMap.getInt(key);
+    public int get(@NotNull K key) {
+        final int hash = HashCommon.mix(key.hashCode());
+        int index = getInt(key, hash);
         if (index == -1) {
-            synchronized (indexMap) {
-                index = indexMap.getInt(key);
-                if (index == -1) {
-                    index = lastIndex++;
-                    indexMap.put(key, index);
-                }
+            synchronized (this) {
+                index = getInt(key, hash);
+                if (index == -1) put(key, (index = lastIndex++), hash);
             }
         }
         return index;
     }
+
+    private int getInt(final K k, int hash) {
+        final K[] key = this.key;
+        final int[] value = this.value;
+        if (key.length != value.length)
+            return -1; // Race condition, continue to slow path
+
+        final int mask = key.length - 2;
+        K curr;
+        int pos;
+        // The starting point.
+        if ((curr = key[pos = hash & mask]) == null) return -1;
+        if (k.equals(curr)) return value[pos];
+        // There's always an unused entry.
+        while (true) {
+            if ((curr = key[pos = (pos + 1) & mask]) == null) return -1;
+            if (k.equals(curr)) return value[pos];
+        }
+    }
+
+    private void put(final K k, final int v, int hash) {
+        int pos = find(k, hash);
+        if (pos >= 0) {
+            value[pos] = v;
+        } else {
+            pos = -(pos + 1);
+            key[pos] = k;
+            value[pos] = v;
+            if (size++ >= maxFill) rehash(arraySize(size + 1, f));
+        }
+    }
+
+    private int find(final K k, int hash) {
+        final K[] key = this.key;
+        final int mask = key.length - 2;
+
+        K curr;
+        int pos;
+        // The starting point.
+        if ((curr = key[pos = hash & mask]) == null)
+            return -(pos + 1);
+        if (k.equals(curr)) return pos;
+        // There's always an unused entry.
+        while (true) {
+            if ((curr = key[pos = (pos + 1) & mask]) == null) return -(pos + 1);
+            if (k.equals(curr)) return pos;
+        }
+    }
+
+    private void rehash(final int newN) {
+        final K[] key = this.key;
+        final int[] value = this.value;
+        final int n = key.length - 1;
+
+        final int mask = newN - 1; // Note that this is used by the hashing macro
+        final K[] newKey = (K[]) new Object[newN + 1];
+        final int[] newValue = new int[newN + 1];
+
+        int i = n, pos;
+        for (int j = size; j-- != 0; ) {
+            while (((key[--i]) == null)) ;
+            if (!((newKey[pos = HashCommon.mix(key[i].hashCode()) & mask]) == null))
+                while (!((newKey[pos = (pos + 1) & mask]) == null)) ;
+            newKey[pos] = key[i];
+            newValue[pos] = value[i];
+        }
+        newValue[newN] = value[n];
+        this.maxFill = maxFill(n, f);
+        this.key = newKey;
+        this.value = newValue;
+    }
 }

src/test/java/net/minestom/server/utils/collection/IndexMapTest.java

@@ -0,0 +1,18 @@
+package net.minestom.server.utils.collection;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class IndexMapTest {
+    @Test
+    public void test() {
+        IndexMap<String> map = new IndexMap<>();
+        for (int i = 0; i < 1000; i++) {
+            assertEquals(i, map.get("test" + i));
+            for (int j = 0; j < i; j++) {
+                assertEquals(j, map.get("test" + j));
+            }
+        }
+    }
+}