Add Java-side cache with large-request bypass for native RNG

src/main/java/com/ibm/crypto/plus/provider/base/ExtendedRandom.java

@@ -16,6 +16,16 @@ public final class ExtendedRandom {
     OCKContext ockContext;
     final long ockPRNGContextId;
 
+    // 128KB cache ?
+    private static final int MEGABYTE_CACHE_SIZE = 128 * 1024;
+
+    // 16KB threshold ?
+    private static final int BYPASS_THRESHOLD = 16 * 1024;
+
+    private byte[] megaByteCache;
+    private int cachePos; // Next unread index in cache
+    private int megaByteCacheLength;
+
     public static ExtendedRandom getInstance(OCKContext ockContext, String algName, OpenJCEPlusProvider provider)
             throws OCKException {
         if (ockContext == null) {
@@ -45,9 +55,42 @@ public synchronized void nextBytes(byte[] bytes) throws OCKException {
         if (bytes == null) {
             throw new IllegalArgumentException("bytes is null");
         }
+        int len = bytes.length;
+        if (len == 0) {
+            return;
+        }
 
-        if (bytes.length > 0) {
+        // 1) LARGE REQUEST BYPASS:
+        // Fill destination directly to avoid cache->dest copy cost.
+        if (len >= BYPASS_THRESHOLD) {
             NativeInterface.EXTRAND_nextBytes(ockContext.getId(), ockPRNGContextId, bytes);
+
+            // Invalidate cache so next small request refills fresh.
+            cachePos = megaByteCacheLength = 0;
+            return;
+        }
+
+        // 2) SMALL/MEDIUM REQUEST:
+        // Serve from cache, refilling as needed.
+        int outPos = 0;
+        int remaining = len;
+
+        while (remaining > 0) {
+            int available = megaByteCacheLength - cachePos;
+
+            // If cache is empty (or not initialized), refill it.
+            if (available <= 0) {
+                refillMegaByteCache();
+                available = megaByteCacheLength - cachePos;
+            }
+
+            // Copy as much as we can from cache into output.
+            int toCopy = Math.min(available, remaining);
+            System.arraycopy(megaByteCache, cachePos, bytes, outPos, toCopy);
+
+            cachePos += toCopy;
+            outPos += toCopy;
+            remaining -= toCopy;
         }
     }
 
@@ -75,4 +118,16 @@ private Runnable cleanOCKResources(long ockPRNGContextId, OCKContext ockContext)
             }
         };
     }
+
+    private void refillMegaByteCache() throws OCKException {
+        if (megaByteCache == null) {
+            megaByteCache = new byte[MEGABYTE_CACHE_SIZE];
+        }
+
+        // Fill the entire cache from native.
+        NativeInterface.EXTRAND_nextBytes(ockContext.getId(), ockPRNGContextId, megaByteCache);
+
+        cachePos = 0;
+        megaByteCacheLength = megaByteCache.length;
+    }
 }