[GitHub] [kafka] cmccabe commented on a change in pull request #10334: MINOR: Fix BaseHashTable sizing

[GitBox <gi...@apache.org>]

cmccabe commented on a change in pull request #10334:
URL: https://github.com/apache/kafka/pull/10334#discussion_r596269367



##########
File path: metadata/src/main/java/org/apache/kafka/timeline/BaseHashTable.java
##########
@@ -56,12 +58,30 @@
         this.elements = new Object[expectedSizeToCapacity(expectedSize)];
     }
 
+    /**
+     * Calculate the capacity we should provision, given the expected size.
+     *
+     * Our capacity must always be a power of 2, and never less than 2.
+     */
     static int expectedSizeToCapacity(int expectedSize) {
-        if (expectedSize <= 1) {
-            return 2;
+        if (expectedSize >= MAX_CAPACITY / 2) {
+            return MAX_CAPACITY;
+        }
+        return Math.max(MIN_CAPACITY, roundUpToPowerOfTwo(expectedSize * 2));
+    }
+
+    private static int roundUpToPowerOfTwo(int i) {
+        if (i < 0) {
+            return 0;
         }
-        double sizeToFit = expectedSize / MAX_LOAD_FACTOR;
-        return (int) Math.min(MAX_CAPACITY, Math.ceil(Math.log(sizeToFit) / LN_2));
+        i = i - 1;
+        i |= i >> 1;
+        i |= i >> 2;
+        i |= i >> 4;
+        i |= i >> 8;
+        i |= i >> 16;
+        i = i + 1;
+        return i < 0 ? MAX_CAPACITY : i;

Review comment:
       Fair enough, we can use an implementation based on `numberOfLeadingZeros` :)




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