package video

import (
	"bytes"
	"context"
	"fmt"
	"os/exec"
	"strconv"
	"strings"
)

// RemuxArgs builds the ffmpeg arguments for a lossless copy into a faststart
// mp4. Streams are limited to the first video and optional first audio track;
// subtitles and extra tracks are dropped so players do not stumble on them.
func RemuxArgs(in, out string) []string {
	return []string{
		"-hide_banner", "-nostdin", "-y",
		"-i", in,
		"-map", "0:v:0", "-map", "0:a:0?",
		"-c", "copy",
		"-movflags", "+faststart",
		out,
	}
}

// TranscodeArgs builds the ffmpeg arguments for a browser-safe h264/aac mp4.
// The video is scaled to fit maxLongSide without upscaling and forced to
// even dimensions; ffmpeg applies display rotation during re-encoding.
func TranscodeArgs(in, out string, maxLongSide int) []string {
	scale := fmt.Sprintf(
		"scale=w='min(%d,iw)':h='min(%d,ih)':force_original_aspect_ratio=decrease:force_divisible_by=2",
		maxLongSide, maxLongSide)
	return []string{
		"-hide_banner", "-nostdin", "-y",
		"-i", in,
		"-map", "0:v:0", "-map", "0:a:0?",
		"-c:v", "libx264", "-preset", "veryfast", "-crf", "23",
		"-pix_fmt", "yuv420p", "-vf", scale,
		"-c:a", "aac", "-b:a", "128k",
		"-movflags", "+faststart",
		out,
	}
}

// PosterArgs builds the ffmpeg arguments to extract a single poster frame at
// atSecond, already scaled so downstream image pipelines decode a bounded frame.
func PosterArgs(in, out string, atSecond int) []string {
	scale := fmt.Sprintf("scale=w='min(%d,iw)':h=-2", MaxLongSide)
	return []string{
		"-hide_banner", "-nostdin", "-y",
		"-ss", strconv.Itoa(atSecond),
		"-i", in,
		"-frames:v", "1",
		"-vf", scale,
		"-f", "image2",
		out,
	}
}

// Run executes ffmpeg with the given arguments and returns an error carrying
// the captured stderr on failure.
func Run(ctx context.Context, ffmpegPath string, args ...string) error {
	cmd := exec.CommandContext(ctx, ffmpegPath, args...)
	var errb bytes.Buffer
	cmd.Stderr = &errb
	if err := cmd.Run(); err != nil {
		return fmt.Errorf("ffmpeg %s: %w: %s", strings.Join(args, " "), err, strings.TrimSpace(errb.String()))
	}
	return nil
}