Use ChaCha20 caching in FastRandomContext

src/random.h

@@ -145,23 +145,11 @@ private:
     bool requires_seed;
     ChaCha20 rng;
 
-    unsigned char bytebuf[64];
-    int bytebuf_size;
-
     uint64_t bitbuf;
     int bitbuf_size;
 
     void RandomSeed();
 
-    void FillByteBuffer()
-    {
-        if (requires_seed) {
-            RandomSeed();
-        }
-        rng.Keystream(bytebuf, sizeof(bytebuf));
-        bytebuf_size = sizeof(bytebuf);
-    }
-
     void FillBitBuffer()
     {
         bitbuf = rand64();
@@ -185,10 +173,10 @@ public:
     /** Generate a random 64-bit integer. */
     uint64_t rand64() noexcept
     {
-        if (bytebuf_size < 8) FillByteBuffer();
-        uint64_t ret = ReadLE64(bytebuf + 64 - bytebuf_size);
-        bytebuf_size -= 8;
-        return ret;
+        if (requires_seed) RandomSeed();
+        unsigned char buf[8];
+        rng.Keystream(buf, 8);
+        return ReadLE64(buf);
     }
 
     /** Generate a random (bits)-bit integer. */