package transcript

import (
	"fmt"
	"io"
	"os"
	"sync"
)

// FileWriter appends each Message to a text file as a human-readable line,
// flushed to disk immediately. Unlike Broadcaster.Publish (which drops
// messages for a slow/stalled SSE client on purpose), FileWriter never
// drops — this is the durable record a crash shouldn't lose data from, so
// every Write is synchronous and fsync'd before returning.
type FileWriter struct {
	mu sync.Mutex
	f  *os.File
}

// NewFileWriter opens path for appending, creating it if needed. Appending
// (not truncating) means restarting transcriptor-ai against the same path
// continues the same transcript file rather than discarding it.
func NewFileWriter(path string) (*FileWriter, error) {
	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
	if err != nil {
		return nil, fmt.Errorf("open transcript file: %w", err)
	}
	return &FileWriter{f: f}, nil
}

// Write appends one line for msg and fsyncs before returning.
func (fw *FileWriter) Write(msg Message) error {
	fw.mu.Lock()
	defer fw.mu.Unlock()
	line := fmt.Sprintf("[%s] [%s] %s\n", msg.Timestamp.Format("15:04:05"), msg.Track, msg.Text)
	if _, err := io.WriteString(fw.f, line); err != nil {
		return fmt.Errorf("write transcript line: %w", err)
	}
	if err := fw.f.Sync(); err != nil {
		return fmt.Errorf("sync transcript file: %w", err)
	}
	return nil
}

func (fw *FileWriter) Close() error {
	fw.mu.Lock()
	defer fw.mu.Unlock()
	return fw.f.Close()
}