class PostImportPreviewer
  FIELDS = %w[title thumbnail_base original_created_from original_created_before duration tags parent_post_ids].freeze

  def initialize parsed:, url_column:, mappings: { }, existing: 'skip'
    @parsed = parsed
    @url_column = url_column.to_i
    @mappings = mappings.to_h.stringify_keys
    @existing = existing == 'error' ? 'error' : 'skip'
  end

  def preview
    urls = { }
    @parsed[:rows].map do |row|
      values = row[:values]
      attributes = FIELDS.to_h { |field| [field, mapped_value(field, values)] }
      url = values[@url_column].to_s.strip
      metadata, warnings = fetch_metadata(url)
      attributes = metadata.merge(attributes) { |_key, automatic, input| input.presence || automatic }
      post = Post.new(url:)
      post.valid?
      normal_url = post.url
      errors = post.errors.to_hash.transform_values(&:dup)
      errors[:url] = ['URL が重複しています．'] if normal_url.present? && urls.key?(normal_url)
      urls[normal_url] = true if normal_url.present?
      if normal_url.present? && Post.exists?(url: normal_url)
        @existing == 'error' ? errors[:url] = ['既存投稿です．'] : warnings << '既存投稿のためスキップします．'
      end
      validate_preview_tags(attributes['tags'], errors)
      validate_parents(attributes['parent_post_ids'], errors)
      { source_row: row[:source_row], url: normal_url || url, attributes:, warnings:, errors:,
        status: errors.present? ? 'error' : (warnings.present? ? 'warning' : 'ready'),
        existing: normal_url.present? && Post.exists?(url: normal_url) }
    end
  end

  private

  def mapped_value field, values
    mapping = @mappings[field]
    return '' if mapping.blank?
    return mapping['value'].to_s if mapping['kind'] == 'fixed'

    Array(mapping['columns']).filter_map { |index| values[index.to_i].to_s.presence }.join(field == 'tags' ? ' ' : '')
  end

  def fetch_metadata url
    return [{ }, ['URL が空です．']] if url.blank?

    data = PostMetadataFetcher.fetch(url)
    warnings = []
    warnings << 'タイトルを取得できませんでした．' if data[:title].blank?
    warnings << 'サムネールを取得できませんでした．' if data[:thumbnail_base].blank?
    [data.compact, warnings]
  rescue Preview::UrlSafety::UnsafeUrl, Preview::HttpFetcher::FetchFailed, Preview::HttpFetcher::ResponseTooLarge => e
    [{ }, ["メタデータを取得できませんでした: #{ e.message }"]]
  end

  def validate_preview_tags raw, errors
    names = raw.to_s.split
    return if names.empty?
    if names.any? { _1.downcase.start_with?('nico:') }
      errors[:tags] = ['ニコニコ・タグは直接指定できません．']
      return
    end
    canonical = names.map { TagName.canonicalise(_1.sub(/\[.*\]\z/, '')).first }
    if Tag.joins(:tag_name).where(tag_names: { name: canonical }, deprecated_at: nil).count < canonical.uniq.length
      errors[:tags] = ['存在しないタグ又は廃止済みタグがあります．']
    end
  rescue Tag::SectionLiteralParseError
    errors[:tags] = ['タグ区間の記法が不正です．']
  end

  def validate_parents raw, errors
    ids = raw.to_s.split.map { Integer(_1, exception: false) }
    return if ids.compact.empty? && raw.to_s.blank?
    errors[:parent_post_ids] = ['親投稿 Id. が不正です．'] and return if ids.any? { _1.nil? || _1 <= 0 }
    errors[:parent_post_ids] = ['存在しない親投稿 Id. があります．'] if Post.where(id: ids).count != ids.uniq.length
  end
end