math/rand/v2: implement io.Reader for ChaCha8

Fixes [reserved]

api/next/67059.txt

@@ -0,0 +1 @@
+pkg math/rand/v2, method (*ChaCha8) Read(p []byte) (n int, err error) #67059

src/internal/chacha8rand/chacha8.go

@@ -158,3 +158,23 @@ func Unmarshal(s *State, data []byte) error {
 	}
 	return nil
 }
+
+// Read reads random bytes from the state into p.
+func Read(s *State, p []byte) (n int, err error) {
+	for n = 0; n < len(p); n++ {
+		if n == len(p) {
+			return n, nil
+		}
+
+		for {
+			x, ok := s.Next()
+			if ok {
+				p[n] = byte(x)
+				n++
+				break
+			}
+			s.Refill()
+		}
+	}
+	return n, nil
+}

src/math/rand/v2/chacha8.go

@@ -44,3 +44,8 @@ func (c *ChaCha8) UnmarshalBinary(data []byte) error {
 func (c *ChaCha8) MarshalBinary() ([]byte, error) {
 	return chacha8rand.Marshal(&c.state), nil
 }
+
+// Read generates len(p) random bytes and writes them into p.
+func (c *ChaCha8) Read(p []byte) (n int, err error) {
+	return chacha8rand.Read(&c.state, p)
+}

src/math/rand/v2/chacha8_test.go

@@ -5,8 +5,11 @@
 package rand_test
 
 import (
+	"bytes"
+	"io"
 	. "math/rand/v2"
 	"testing"
+	"testing/iotest"
 )
 
 func TestChaCha8(t *testing.T) {
@@ -529,3 +532,79 @@ var chacha8marshal = []string{
 	"chacha8:\x00\x00\x00\x00\x00\x00\x00zK3\x9bB!,\x94\x9d\x975\xce'O_t\xee|\xb21\x87\xbb\xbb\xfd)\x8f\xe52\x01\vP\fk",
 	"chacha8:\x00\x00\x00\x00\x00\x00\x00{K3\x9bB!,\x94\x9d\x975\xce'O_t\xee|\xb21\x87\xbb\xbb\xfd)\x8f\xe52\x01\vP\fk",
 }
+
+func TestReadEmpty(t *testing.T) {
+	r := NewChaCha8(chacha8seed)
+	buf := make([]byte, 0)
+	n, err := r.Read(buf)
+	if err != nil {
+		t.Errorf("Read err into empty buffer; %v", err)
+	}
+	if n != 0 {
+		t.Errorf("Read into empty buffer returned unexpected n of %d", n)
+	}
+}
+
+func TestReadSeedReset(t *testing.T) {
+	r := NewChaCha8(chacha8seed)
+	b1 := make([]byte, 128)
+	_, err := r.Read(b1)
+	if err != nil {
+		t.Errorf("read: %v", err)
+	}
+
+	r.Seed(chacha8seed)
+	b2 := make([]byte, 128)
+	_, err = r.Read(b2)
+	if err != nil {
+		t.Errorf("read: %v", err)
+	}
+	if !bytes.Equal(b1, b2) {
+		t.Errorf("mismatch after re-seed:\n%x\n%x", b1, b2)
+	}
+}
+
+func TestReadByOneByte(t *testing.T) {
+	r := NewChaCha8(chacha8seed)
+	b1 := make([]byte, 100)
+	_, err := io.ReadFull(iotest.OneByteReader(r), b1)
+	if err != nil {
+		t.Errorf("read by one byte: %v", err)
+	}
+	r = NewChaCha8(chacha8seed)
+	b2 := make([]byte, 100)
+	_, err = r.Read(b2)
+	if err != nil {
+		t.Errorf("read: %v", err)
+	}
+	if !bytes.Equal(b1, b2) {
+		t.Errorf("read by one byte vs single read:\n%x\n%x", b1, b2)
+	}
+}
+
+func BenchmarkRead3(b *testing.B) {
+	r := NewChaCha8(chacha8seed)
+	buf := make([]byte, 3)
+	b.ResetTimer()
+	for n := b.N; n > 0; n-- {
+		r.Read(buf)
+	}
+}
+
+func BenchmarkRead64(b *testing.B) {
+	r := NewChaCha8(chacha8seed)
+	buf := make([]byte, 64)
+	b.ResetTimer()
+	for n := b.N; n > 0; n-- {
+		r.Read(buf)
+	}
+}
+
+func BenchmarkRead1000(b *testing.B) {
+	r := NewChaCha8(chacha8seed)
+	buf := make([]byte, 1000)
+	b.ResetTimer()
+	for n := b.N; n > 0; n-- {
+		r.Read(buf)
+	}
+}