[#851] util/rand: use single random source

It is much more convenient to skip source creation.
Also fix some bugs:
1. `cryptoSource.Int63()` now returns number in [0, 1<<63) as required
   by `rand.Source` interface.
2. Replace `cryptoSource.Uint63()` with `cryptoSource.Uint64` to allow
   generate uint64 numbers directly (see rand.Source64 docs).

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/util/rand/rand.go

@@ -6,41 +6,39 @@ import (
 	mrand "math/rand"
 )
 
-type cryptoSource struct{}
-
-// Read is alias for crypto/rand.Read.
-var Read = crand.Read
-
-// New constructs the source of random numbers.
-func New() *mrand.Rand {
-	return mrand.New(&cryptoSource{})
-}
-
-func (s *cryptoSource) Seed(int64) {}
-
-func (s *cryptoSource) Int63() int64 {
-	return int64(s.Uint63())
-}
-
-func (s *cryptoSource) Uint63() uint64 {
-	buf := make([]byte, 8)
-	if _, err := crand.Read(buf); err != nil {
-		return 0
-	}
-
-	return binary.BigEndian.Uint64(buf)
-}
+var source = mrand.New(&cryptoSource{})
 
 // Uint64 returns a random uint64 value.
-func Uint64(r *mrand.Rand, max int64) uint64 {
-	if max <= 0 {
-		return 0
-	}
-
-	var i int64 = -1
-	for i < 0 {
-		i = r.Int63n(max)
-	}
-
-	return uint64(i)
+func Uint64() uint64 {
+	return source.Uint64()
+}
+
+// Uint64 returns a random uint32 value.
+func Uint32() uint32 {
+	return source.Uint32()
+}
+
+// Shuffle randomizes the order of elements.
+// n is the number of elements. Shuffle panics if n < 0.
+// swap swaps the elements with indexes i and j.
+func Shuffle(n int, swap func(i, j int)) {
+	source.Shuffle(n, swap)
+}
+
+// cryptoSource is math/rand.Source which takes entropy via crypto/rand.
+type cryptoSource struct{}
+
+// Seed implements math/rand.Source.
+func (s *cryptoSource) Seed(int64) {}
+
+// Int63 implements math/rand.Source.
+func (s *cryptoSource) Int63() int64 {
+	return int64(s.Uint64() >> 1)
+}
+
+// Uint64 implements math/rand.Source64.
+func (s *cryptoSource) Uint64() uint64 {
+	var buf [8]byte
+	_, _ = crand.Read(buf[:]) // always returns nil
+	return binary.BigEndian.Uint64(buf[:])
 }