require "open3"

module Streams
  # Converte cover_image in MP4 slate (H.264 720p + AAC mono), allineato a infra/scripts/generate_slate.sh.
  class GenerateCoverSlate
    class Error < StandardError; end

    SLATE_DURATION_SECS = 30
    VIDEO_FILTER = "scale=1280:720:force_original_aspect_ratio=decrease,pad=1280:720:(ow-iw)/2:(oh-ih)/2:color=0x0a0a0e".freeze

    def self.call(record)
      new(record).call
    end

    def initialize(record)
      @record = record
    end

    def call
      return @record unless @record.cover_image.attached?

      @image_temp = download_cover_image
      @mp4_temp = encode_slate(@image_temp)
      attach_slate(@mp4_temp)
      write_to_slates_dir(@mp4_temp)
      @record
    rescue Error
      raise
    rescue StandardError => e
      raise Error, e.message
    ensure
      cleanup_tempfiles
    end

    private

    def download_cover_image
      ext = extension_for(@record.cover_image.content_type)
      path = temp_path("cover-in", ext)
      @record.cover_image.blob.open(tmpdir: Dir.tmpdir) do |file|
        FileUtils.cp(file.path, path)
      end
      path
    end

    def encode_slate(image_path)
      out = temp_path("cover-slate", ".mp4")
      cmd = ffmpeg_args(image_path, out)
      stdout, stderr, status = Open3.capture3(*cmd)
      unless status.success? && File.exist?(out) && File.size(out).positive?
        detail = stderr.to_s.strip.presence || stdout.to_s.strip
        raise Error, "ffmpeg slate failed: #{detail}"
      end

      out
    end

    def ffmpeg_args(input, output)
      [
        "ffmpeg", "-nostdin", "-hide_banner", "-loglevel", "error",
        "-loop", "1", "-framerate", "30", "-i", input,
        "-f", "lavfi", "-i", "anullsrc=r=48000:cl=mono",
        "-filter:v", VIDEO_FILTER,
        "-map", "0:v", "-map", "1:a",
        "-t", SLATE_DURATION_SECS.to_s,
        "-c:v", "libx264", "-pix_fmt", "yuv420p", "-profile:v", "baseline", "-level", "3.1",
        "-x264-params", "keyint=30:min-keyint=30:scenecut=0:bframes=0",
        "-g", "30", "-keyint_min", "30", "-force_key_frames", "expr:gte(t,n_forced*1)",
        "-preset", "fast",
        "-c:a", "aac", "-b:a", "128k", "-ac", "1", "-shortest", "-y", output
      ]
    end

    def attach_slate(mp4_path)
      @record.cover_slate.purge if @record.cover_slate.attached?
      File.open(mp4_path, "rb") do |io|
        @record.cover_slate.attach(
          io: io,
          filename: slate_filename,
          content_type: "video/mp4"
        )
      end
      @record.reload
    end

    def write_to_slates_dir(mp4_path)
      dest = CoverSlatePaths.expected_path(@record)
      return unless dest

      FileUtils.mkdir_p(File.dirname(dest))
      FileUtils.cp(mp4_path, dest)
    rescue Errno::EACCES, Errno::EROFS => e
      Rails.logger.info("[GenerateCoverSlate] skip disk copy #{dest}: #{e.class}")
    end

    def slate_filename
      "#{@record.class.name.underscore}-#{@record.id}.mp4"
    end

    def extension_for(content_type)
      case content_type
      when "image/png" then ".png"
      when "image/webp" then ".webp"
      else ".jpg"
      end
    end

    def temp_path(prefix, ext)
      path = File.join(Dir.tmpdir, "#{prefix}-#{@record.class.name}-#{@record.id}-#{SecureRandom.hex(4)}#{ext}")
      @tempfiles ||= []
      @tempfiles << path
      path
    end

    def cleanup_tempfiles
      Array(@tempfiles).each do |path|
        FileUtils.rm_f(path)
      end
    end
  end
end