Use ChaCha20 caching in FastRandomContext

Author: sipa

src/random.cpp

@@ -605,12 +605,9 @@ void FastRandomContext::RandomSeed()
 
 uint256 FastRandomContext::rand256() noexcept
 {
-    if (bytebuf_size < 32) {
-        FillByteBuffer();
-    }
+    if (requires_seed) RandomSeed();
     uint256 ret;
-    memcpy(ret.begin(), bytebuf + 64 - bytebuf_size, 32);
-    bytebuf_size -= 32;
+    rng.Keystream(ret.data(), ret.size());
     return ret;
 }
 
@@ -624,7 +621,7 @@ std::vector<unsigned char> FastRandomContext::randbytes(size_t len)
     return ret;
 }
 
-FastRandomContext::FastRandomContext(const uint256& seed) noexcept : requires_seed(false), bytebuf_size(0), bitbuf_size(0)
+FastRandomContext::FastRandomContext(const uint256& seed) noexcept : requires_seed(false), bitbuf_size(0)
 {
     rng.SetKey(seed.begin(), 32);
 }
@@ -675,7 +672,7 @@ bool Random_SanityCheck()
     return true;
 }
 
-FastRandomContext::FastRandomContext(bool fDeterministic) noexcept : requires_seed(!fDeterministic), bytebuf_size(0), bitbuf_size(0)
+FastRandomContext::FastRandomContext(bool fDeterministic) noexcept : requires_seed(!fDeterministic), bitbuf_size(0)
 {
     if (!fDeterministic) {
         return;
@@ -688,12 +685,9 @@ FastRandomContext& FastRandomContext::operator=(FastRandomContext&& from) noexce
 {
     requires_seed = from.requires_seed;
     rng = from.rng;
-    std::copy(std::begin(from.bytebuf), std::end(from.bytebuf), std::begin(bytebuf));
-    bytebuf_size = from.bytebuf_size;
     bitbuf = from.bitbuf;
     bitbuf_size = from.bitbuf_size;
     from.requires_seed = true;
-    from.bytebuf_size = 0;
     from.bitbuf_size = 0;
     return *this;
 }

src/random.h

@@ -145,23 +145,11 @@ class FastRandomContext
     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 @@ class FastRandomContext
     /** 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. */