Return correct next state from Read

gigaparsec.go

@@ -127,8 +127,8 @@ type State[In any] struct {
 
 func (s State[In]) Read(dst []In) (n uint64, next State[In], err error) {
 	nread, err := s.r.ReadAt(dst, int64(s.pos))
-	if n > 0 {
+	if nread > 0 {
-		s.pos += uint64(n)
+		s.pos += uint64(nread)
 	}
 	return uint64(nread), s, err
 }

parser_test.go

@@ -48,8 +48,8 @@ func TestSlice(t *testing.T) {
 	}))
 	t.Run("fails at end of input", rapid.MakeCheck(func(t *rapid.T) {
 		s := rapid.SliceOfN(rapid.Byte(), 1, -1).Draw(t, "s")
-		inputLen := rapid.IntRange(0, len(s)-1).Draw(t, "inputLen")
+		input := rapid.Map(rapid.IntRange(0, len(s)-1),
-		input := s[:inputLen]
+			func(n int) []byte { return s[:n] }).Draw(t, "inputLen")
 		assertParseFails(t, input, gigaparsec.MatchSlice(s))
 	}))
 	t.Run("fails when read fails", rapid.MakeCheck(func(t *rapid.T) {
@@ -63,8 +63,8 @@ func TestSlice(t *testing.T) {
 	}))
 	t.Run("succeeds when contents match", rapid.MakeCheck(func(t *rapid.T) {
 		input := rapid.SliceOfN(rapid.Byte(), 1, -1).Draw(t, "input")
-		sLen := rapid.IntRange(0, len(input)).Draw(t, "sLen")
+		s := rapid.Map(rapid.IntRange(0, len(input)),
-		s := input[:sLen]
+			func(n int) []byte { return input[:n] }).Draw(t, "s")
 		start := gigaparsec.MakeState(bytes.NewReader(input))
 
 		result, err := gigaparsec.MatchSlice(s)(start)