# Read-only history queries. All identifiers and SQL expressions are server-owned.
class CountsQuery
  EVENTS = ['create', 'update', 'discard', 'restore'].freeze
  AXES = ['event_type', 'month', 'user'].freeze
  COMMON_COLUMNS = ['created_at', 'resource_id', 'version_no', 'event_type', 'user'].freeze
  COLUMNS = {
      'posts' => COMMON_COLUMNS + ['title', 'url', 'tags', 'added_tags', 'removed_tags'],
      'tags' => COMMON_COLUMNS + ['name', 'category', 'aliases', 'parents', 'deprecated'] }.freeze
  KEYS = [
      'locale', 'mode', 'axis', 'page', 'limit', 'from', 'before',
      'user_id', 'event_type', 'resource_id', 'tag', 'columns'].freeze

  attr_reader :page, :limit, :mode, :axis, :columns

  def initialize(scope, resource, params)
    @scope = scope.except(:select, :order)
    @resource = resource
    @table = scope.klass.table_name
    @params = params
    raise ArgumentError unless (params.keys - KEYS).empty?
    raise ArgumentError unless params.values.all? { _1.is_a?(String) }

    @mode = params.fetch('mode', 'summary')
    @axis = params.fetch('axis', 'event_type')
    raise ArgumentError unless ['summary', 'details'].include?(mode) && AXES.include?(axis)

    @page = integer('page', 1)
    @limit = integer('limit', 20)
    raise ArgumentError if limit > 100 || page > 1_000_000

    @columns = params.fetch('columns', COLUMNS.fetch(resource).join(',')).split(',')
    if columns.empty? || columns.uniq != columns || (columns - COLUMNS.fetch(resource)).any?
      raise ArgumentError
    end
    apply_filters
  end

  def count = @scope.count

  def details(scope)
    scope.where(id: @scope.select("#{ @table }.id"))
        .order(Arel.sql("#{ @table }.created_at DESC, #{ @table }.id DESC"))
        .limit(limit).offset((page - 1) * limit)
  end

  def summary
    expression = case axis
                 when 'event_type' then "#{ @table }.event_type"
                 when 'user' then "#{ @table }.created_by_user_id"
                 when 'month' then "DATE_FORMAT(#{ @table }.created_at, '%Y-%m')"
                 end
    grouped = @scope.group(Arel.sql(expression))
    # A derived table counts groups without materialising every group in Ruby.
    sql = grouped.select(Arel.sql("#{ expression } AS group_key")).to_sql
    groups = @scope.klass.connection.select_value("SELECT COUNT(*) FROM (#{ sql }) counts_groups")
    rows = grouped.order(Arel.sql("#{ expression } ASC"))
        .limit(limit).offset((page - 1) * limit).count
    names = axis == 'user' ? User.where(id: rows.keys.compact).pluck(:id, :name).to_h : {}
    { group_count: groups.to_i,
      groups: rows.map { |key, value| { key:, name: names[key], count: value } } }
  end

  private

  def integer(key, default = nil)
    value = @params[key].presence
    return default if value.nil? && !default.nil?
    raise ArgumentError unless value&.match?(/\A[1-9][0-9]{0,9}\z/)

    value.to_i
  end

  def timestamp(key)
    value = @params[key].presence
    return nil unless value
    raise ArgumentError unless value.match?(/\A\d{4}-\d{2}-\d{2}T\d{2}:\d{2}(?::\d{2})?Z\z/)

    Date.iso8601(value.first(10))
    Time.iso8601(value.length == 17 ? value.sub(/Z\z/, ':00Z') : value)
  end

  def apply_filters
    first = timestamp('from')
    last = timestamp('before')
    raise ArgumentError if first && last && first >= last

    @scope = @scope.where("#{ @table }.created_at >= ?", first) if first
    @scope = @scope.where("#{ @table }.created_at < ?", last) if last
    if @params['resource_id'].present?
      id_column = @resource == 'posts' ? 'post_id' : 'tag_id'
      @scope = @scope.where(@table => { id_column => integer('resource_id') })
    end
    if @params['user_id'].present?
      user = @params['user_id'] == 'none' ? nil : integer('user_id')
      @scope = @scope.where(@table => { created_by_user_id: user })
    end
    if @params['event_type'].present?
      raise ArgumentError unless EVENTS.include?(@params['event_type'])

      @scope = @scope.where(@table => { event_type: @params['event_type'] })
    end
    return unless @params['tag'].present?
    raise ArgumentError unless @resource == 'posts'

    id = integer('tag')
    @scope = @scope.where(
        "JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('tag_id', CAST(? AS UNSIGNED))) OR " +
        "JSON_CONTAINS(prev.tags_json, JSON_OBJECT('tag_id', CAST(? AS UNSIGNED)))", id, id)
  end
end