import subprocess
import os

def generate_background(images, output_path):

    if not images:
        raise Exception("No images uploaded")

    temp_videos = []

    slide_duration = 15
    fade_duration = 1

    total_slide_duration = slide_duration + fade_duration

    fps = 30
    total_frames = int(total_slide_duration * fps)

    # =====================================================
    # GENERATE EACH SLIDE
    # =====================================================

    for i, img in enumerate(images):

        temp_video = f"/tmp/slide_{i}.mp4"
        temp_videos.append(temp_video)

        vf = (
            "scale=3840:2160:force_original_aspect_ratio=increase,"
            "crop=3840:2160,"
            f"zoompan="
            f"z='1+0.30*on/{total_frames}':"
            f"x='(iw-iw/zoom)/2':"
            f"y='(ih-ih/zoom)/2':"
            f"d={total_frames}:"
            "s=3840x2160:"
            f"fps={fps},"
            "scale=1920:1080:flags=lanczos"
        )

        cmd = [
            "ffmpeg",
            "-y",
            "-loop", "1",
            "-i", img,

            "-vf", vf,

            "-t", str(total_slide_duration),

            "-c:v", "libx264",
            "-preset", "ultrafast",
            "-crf", "22",
            "-pix_fmt", "yuv420p",

            temp_video
        ]

        print(f"Generating slide {i+1}/{len(images)}")

        subprocess.run(cmd, check=True)

    # =====================================================
    # SINGLE IMAGE
    # =====================================================

    if len(temp_videos) == 1:
        os.replace(temp_videos[0], output_path)
        return

    # =====================================================
    # XFADE COMBINE
    # =====================================================

    print("Combining slides...")

    cmd_concat = ["ffmpeg", "-y"]

    for v in temp_videos:
        cmd_concat.extend(["-i", v])

    filter_complex = ""

    current = "[0:v]"
    offset = slide_duration

    for i in range(1, len(temp_videos)):

        next_input = f"[{i}:v]"

        if i == len(temp_videos) - 1:
            out = ""
        else:
            out = f"[v{i}]"

        filter_complex += (
            f"{current}{next_input}"
            f"xfade="
            f"transition=fade:"
            f"duration={fade_duration}:"
            f"offset={offset}"
            f"{out};"
        )

        current = f"[v{i}]"
        offset += slide_duration

    filter_complex = filter_complex.rstrip(";")

    cmd_concat.extend([
        "-filter_complex", filter_complex,

        "-c:v", "libx264",
        "-preset", "ultrafast",
        "-crf", "22",
        "-pix_fmt", "yuv420p",

        output_path
    ])

    subprocess.run(cmd_concat, check=True)

    # =====================================================
    # CHECK RESULT
    # =====================================================

    probe = subprocess.run(
        [
            "ffprobe",
            "-v", "error",
            "-show_entries",
            "format=duration",
            "-of",
            "default=noprint_wrappers=1:nokey=1",
            output_path
        ],
        capture_output=True,
        text=True
    )

    print("\nSUCCESS")
    print(f"Duration: {probe.stdout.strip()} sec")

    # =====================================================
    # CLEANUP
    # =====================================================

    for v in temp_videos:
        if os.path.exists(v):
            os.remove(v)