"""FFmpeg 合成：帧序列 + SRT 字幕 + 背景音乐 → MP4。"""
from __future__ import annotations

import random
import subprocess
import sys
from pathlib import Path


def compose_video(
    frames_dir: str,
    fps: int,
    subtitle_path: str,
    music_dir: str,
    output_path: str,
    music_volume: float = 0.15,
) -> str:
    frames_dir = Path(frames_dir)
    subtitle_path = Path(subtitle_path)
    output_path = Path(output_path)
    output_path.parent.mkdir(parents=True, exist_ok=True)

    # 随机选一首背景音乐
    music_files = list(Path(music_dir).rglob("*.mp3"))
    if not music_files:
        raise FileNotFoundError(f"music_dir 下没有找到 MP3 文件: {music_dir}")
    music_file = random.choice(music_files)
    print(f"[screencast] 背景音乐: {music_file.name}")

    # FFmpeg 字幕路径在 Windows 下需要转义冒号（subtitles 滤镜内部语法）
    srt_path_str = str(subtitle_path).replace("\\", "/").replace(":", "\\:")

    subtitle_filter = (
        f"subtitles='{srt_path_str}'"
        ":force_style='FontName=Arial,FontSize=28,"
        "PrimaryColour=&H00FFFFFF,OutlineColour=&H00000000,"
        "Outline=2,Shadow=1,Alignment=2'"
    )

    # 视频和音频都放进 filter_complex，避免 -vf 与 -filter_complex 混用报错
    filter_complex = (
        f"[0:v]{subtitle_filter}[vout];"
        f"[1:a]volume={music_volume},apad[aout]"
    )

    cmd = [
        "ffmpeg", "-y",
        "-framerate", str(fps),
        "-i", str(frames_dir / "frame_%06d.png"),
        "-i", str(music_file),
        "-filter_complex", filter_complex,
        "-map", "[vout]",
        "-map", "[aout]",
        "-c:v", "libx264",
        "-preset", "fast",
        "-crf", "23",
        "-c:a", "aac",
        "-b:a", "128k",
        "-shortest",
        "-pix_fmt", "yuv420p",
        str(output_path),
    ]

    print(f"[screencast] 运行 FFmpeg 合成...")
    result = subprocess.run(cmd, capture_output=True, text=True, encoding="utf-8", errors="replace")
    if result.returncode != 0:
        print(result.stderr, file=sys.stderr)
        raise RuntimeError(f"FFmpeg 合成失败，退出码 {result.returncode}")

    print(f"[screencast] 输出: {output_path}")
    return str(output_path)