def _pos_expr(pos: str):
    pos = (pos or "top-right").lower()
    if pos == "top-left":
        return "10:10"
    if pos == "top-right":
        return "W-w-10:10"
    if pos == "bottom-left":
        return "10:H-h-10"
    if pos == "bottom-right":
        return "W-w-10:H-h-10"
    if pos == "bottom-center":
        return "(W-w)/2:H-h-20"
    return "(W-w)/2:(H-h)/2"

def _drawtext(text: str, pos: str):
    # keep escaping minimal (CPU-friendly); avoid fancy fonts
    t = (text or "").replace(":", r"\:").replace("'", r"\'")
    p = (pos or "bottom-center").lower()
    if p == "top-left":
        x, y = "10", "10"
    elif p == "top-right":
        x, y = "w-tw-10", "10"
    elif p == "bottom-left":
        x, y = "10", "h-th-10"
    elif p == "bottom-right":
        x, y = "w-tw-10", "h-th-10"
    elif p == "center":
        x, y = "(w-tw)/2", "(h-th)/2"
    else:  # bottom-center
        x, y = "(w-tw)/2", "h-th-20"

    return f"drawtext=text='{t}':x={x}:y={y}:fontsize=36:fontcolor=white:box=1:boxcolor=black@0.5"

def build_ffmpeg_cmd(local_files: list[str], plan: dict) -> str:
    """
    Returns a one-line ffmpeg command WITHOUT output path.
    Output path is forced by the server.
    Convention: local_files[0] is the main input -> input/<downloaded name>
    """

    main = local_files[0]  # input/<name>
    cmd = ["ffmpeg", "-i", main]

    # Extra inputs: logo + audio mix tracks
    logo = plan.get("logo_overlay")
    audio_mix = plan.get("audio_mix") or []

    extra_inputs = []
    if logo and logo.get("file"):
        extra_inputs.append(logo["file"])

    for t in audio_mix:
        f = t.get("file")
        if f:
            extra_inputs.append(f)

    # unique but keep order
    seen = set()
    uniq_extra = []
    for f in extra_inputs:
        if f != main and f not in seen:
            seen.add(f)
            uniq_extra.append(f)

    for f in uniq_extra:
        cmd += ["-i", f]

    # Simple trim controls (input-level)
    trim = plan.get("trim")
    if trim and trim.get("start") is not None:
        cmd += ["-ss", str(trim["start"])]
    if trim and trim.get("end") is not None:
        cmd += ["-to", str(trim["end"])]

    # Build filters
    vf = []
    af = []
    effects = plan.get("effects") or {}
    vf += effects.get("video_filters") or []
    af += effects.get("audio_filters") or []

    if plan.get("resize"):
        vf.append(f"scale={plan['resize']}")
    if plan.get("crop"):
        vf.append(f"crop={plan['crop']}")
    if plan.get("fps"):
        vf.append(f"fps={int(plan['fps'])}")

    subs = plan.get("subtitles")
    if subs and subs.get("file"):
        vf.append(f"subtitles={subs['file']}")

    # Text overlays
    for o in (plan.get("text_overlays") or []):
        vf.append(_drawtext(o.get("text", ""), o.get("pos", "bottom-center")))

    # Now decide if we need filter_complex:
    # - if logo overlay exists
    # - if audio mix exists
    # - if we have any vf/af but need mapping explicitly
    needs_complex = bool(logo and logo.get("file")) or bool(audio_mix)

    if not needs_complex:
        # Simple case: use -vf/-af
        if vf:
            cmd += ["-vf", ",".join(vf)]
        if af:
            cmd += ["-af", ",".join(af)]

        # Output type decision
        out_ext = (plan.get("output_ext") or "mp4").lower()
        if out_ext in ("mp3", "wav"):
            if out_ext == "mp3":
                cmd += ["-vn", "-c:a", "libmp3lame", "-b:a", "192k"]
            else:
                cmd += ["-vn", "-c:a", "pcm_s16le"]
        elif out_ext == "gif":
            cmd += ["-vf", "fps=12,scale=640:-1:flags=lanczos"]
        else:
            cmd += ["-c:v", "libx264", "-preset", "veryfast", "-crf", "23", "-c:a", "aac", "-b:a", "192k"]

        return " ".join(cmd)

    # filter_complex path
    fc = []
    v_in = "[0:v]"
    a_in = "[0:a]"

    # apply base vf
    if vf:
        fc.append(f"{v_in}{','.join(vf)}[v0]")
        v_in = "[v0]"

    # logo overlay: logo is input index 1 if present (but audio tracks may also be added)
    # we added logo first in uniq_extra if present, so it should be input 1 when exists
    current_v = v_in
    next_label = 1

    if logo and logo.get("file"):
        logo_idx = 1  # by construction
        ov = f"[{logo_idx}:v]"
        scale = (logo.get("scale") or "").strip()
        if scale:
            fc.append(f"{ov}scale={scale}[lg]")
            ov = "[lg]"

        opacity = logo.get("opacity")
        if opacity is not None:
            # apply alpha via colorchannelmixer if image supports alpha; still okay for most PNGs
            fc.append(f"{ov}colorchannelmixer=aa={float(opacity)}[lga]")
            ov = "[lga]"

        xy = _pos_expr(logo.get("pos", "top-right"))
        fc.append(f"{current_v}{ov}overlay={xy}[v1]")
        current_v = "[v1]"

    # audio mix: base + N tracks
    # audio tracks start after logo if logo exists; otherwise start at input 1
    audio_start_idx = 2 if (logo and logo.get("file")) else 1

    mix_inputs = []
    base_audio_label = a_in

    # apply base af
    if af:
        fc.append(f"{base_audio_label}{','.join(af)}[a0]")
        base_audio_label = "[a0]"

    mix_inputs.append(base_audio_label)

    for i, tr in enumerate(audio_mix):
        idx = audio_start_idx + i
        vol = tr.get("volume", 0.2)
        loop = tr.get("loop", False)

        ain = f"[{idx}:a]"
        if loop:
            # loop audio by enabling -stream_loop at input-level is better,
            # but we keep it simple and let short loops still mix; user can provide longer music.
            pass
        fc.append(f"{ain}volume={float(vol)}[am{i}]")
        mix_inputs.append(f"[am{i}]")

    if len(mix_inputs) == 1:
        final_a = mix_inputs[0]
    else:
        # amix
        fc.append(f"{''.join(mix_inputs)}amix=inputs={len(mix_inputs)}:duration=longest:dropout_transition=2[aout]")
        final_a = "[aout]"

    cmd += ["-filter_complex", ";".join(fc)]
    cmd += ["-map", current_v, "-map", final_a]

    out_ext = (plan.get("output_ext") or "mp4").lower()
    if out_ext in ("mp3", "wav"):
        if out_ext == "mp3":
            cmd += ["-vn", "-c:a", "libmp3lame", "-b:a", "192k"]
        else:
            cmd += ["-vn", "-c:a", "pcm_s16le"]
    elif out_ext == "gif":
        cmd += ["-map", current_v, "-vf", "fps=12,scale=640:-1:flags=lanczos"]
    else:
        cmd += ["-c:v", "libx264", "-preset", "veryfast", "-crf", "23", "-c:a", "aac", "-b:a", "192k"]

    return " ".join(cmd)