package supervisor

import "sync"

// ringBuffer keeps the last n lines of output.
//
// Engine logs are the only diagnosis available when a shard fails to load, but
// they are also unbounded — a model loading for ten minutes can produce
// megabytes. Keeping a bounded tail makes `ctl logs` useful without letting a
// chatty engine consume the agent's memory.
type ringBuffer struct {
	mu    sync.Mutex
	lines []string
	next  int
	full  bool
}

func newRingBuffer(n int) *ringBuffer {
	if n <= 0 {
		n = 256
	}
	return &ringBuffer{lines: make([]string, n)}
}

func (r *ringBuffer) add(line string) {
	r.mu.Lock()
	defer r.mu.Unlock()
	r.lines[r.next] = line
	r.next = (r.next + 1) % len(r.lines)
	if r.next == 0 {
		r.full = true
	}
}

// tail returns up to n most recent lines, oldest first.
func (r *ringBuffer) tail(n int) []string {
	r.mu.Lock()
	defer r.mu.Unlock()

	size := r.next
	if r.full {
		size = len(r.lines)
	}
	if n <= 0 || n > size {
		n = size
	}

	out := make([]string, 0, n)
	for i := size - n; i < size; i++ {
		idx := i
		if r.full {
			idx = (r.next + i) % len(r.lines)
		}
		out = append(out, r.lines[idx])
	}
	return out
}