package updates

import "sync"

// outputTailMax bounds the package-manager output carried back in a
// PatchResult. The end of the output is where apt and dnf say what failed, so
// the newest bytes are the ones kept.
const outputTailMax = 64 << 10

// tailBuffer is an io.Writer that retains only the last max bytes written.
// Stdout and stderr are both pointed at one, so it is safe for concurrent use.
type tailBuffer struct {
	mu  sync.Mutex
	max int
	buf []byte
}

func newTailBuffer(max int) *tailBuffer { return &tailBuffer{max: max} }

func (t *tailBuffer) Write(p []byte) (int, error) {
	t.mu.Lock()
	defer t.mu.Unlock()
	t.buf = append(t.buf, p...)
	if over := len(t.buf) - t.max; over > 0 {
		t.buf = append([]byte(nil), t.buf[over:]...)
	}
	return len(p), nil
}

func (t *tailBuffer) String() string {
	t.mu.Lock()
	defer t.mu.Unlock()
	return string(t.buf)
}