package trace

import (
	"encoding/json"
	"fmt"
	"io"
	"sync"
)

// Writer persists a completed span. Implementations must be safe for concurrent
// use, as WriteSpan may be called from several goroutines. A Writer such as
// *JSONRecorder provides structured local logging for the M9 panel.
type Writer interface {
	// WriteSpan persists a single completed span and returns any encoding or
	// write error.
	WriteSpan(rec SpanRecord) error
}

// JSONRecorder writes each completed span as one line of JSON (JSONL) to an
// underlying io.Writer. It is safe for concurrent use.
type JSONRecorder struct {
	mu  sync.Mutex
	enc *json.Encoder
}

// NewJSONRecorder builds a JSONRecorder that appends one JSON object per span to
// out.
func NewJSONRecorder(out io.Writer) *JSONRecorder {
	return &JSONRecorder{enc: json.NewEncoder(out)}
}

// WriteSpan encodes rec as JSON followed by a newline. Concurrent calls are
// serialized so a single span object is never interleaved with another.
func (r *JSONRecorder) WriteSpan(rec SpanRecord) error {
	r.mu.Lock()
	defer r.mu.Unlock()
	if err := r.enc.Encode(rec); err != nil {
		return fmt.Errorf("trace: encode span %q: %w", rec.ID, err)
	}
	return nil
}