Quick exit for non-zero slice buffers (#12812)

lucene/CHANGES.txt

@@ -178,6 +178,8 @@ Improvements
 
 * GITHUB#12870: Tighten synchronized loop in DirectoryTaxonomyReader#getOrdinal. (Stefan Vodita)
 
+* GITHUB#12812: Avoid overflows and false negatives in int slice buffer filled-with-zeros assertion. (Stefan Vodita)
+
 Optimizations
 ---------------------
 (No changes)

lucene/memory/src/java/org/apache/lucene/index/memory/MemoryIndex.java

@@ -179,6 +179,19 @@ public class MemoryIndex {
       super(allocator);
     }
 
+    /**
+     * For slices, buffers must be filled with zeros, so that we can find a slice's end based on a
+     * non-zero final value.
+     */
+    private static boolean assertSliceBuffer(int[] buffer) {
+      for (int value : buffer) {
+        if (value != 0) {
+          return false;
+        }
+      }
+      return true;
+    }
+
     /**
      * Creates a new int slice with the given starting size and returns the slices offset in the
      * pool.
@@ -197,14 +210,6 @@ public class MemoryIndex {
       return upto;
     }
 
-    private static boolean assertSliceBuffer(int[] buffer) {
-      int count = 0;
-      for (int i = 0; i < buffer.length; i++) {
-        count += buffer[i]; // for slices the buffer must only have 0 values
-      }
-      return count == 0;
-    }
-
     /** Allocates a new slice from the given offset */
     private int allocSlice(final int[] slice, final int sliceOffset) {
       final int level = slice[sliceOffset] & 15;