class Pagy::Keyset

def self.new(set, **vars)
  if self == Pagy::Keyset
    if defined?(::ActiveRecord) && set.is_a?(::ActiveRecord::Relation)
      ActiveRecord
    elsif defined?(::Sequel) && set.is_a?(::Sequel::Dataset)
      Sequel
    else
      raise TypeError, "expected set to be an instance of ActiveRecord::Relation or Sequel::Dataset; got #{set.class}"
    end.new(set, **vars)
  else
    allocate.tap { |instance| instance.send(:initialize, set, **vars) }
  end
end

def after_latest_query
  operator   = { asc: '>', desc: '<' }
  directions = @keyset.values
  if @vars[:row_comparison]
    # Row comparison: check if your DB supports it (especially if you have mixed order rows)
    columns      = @keyset.keys
    placeholders = columns.map { |column| ":#{column}" }.join(', ')
    "( #{columns.join(', ')} ) #{operator[directions.first]} ( #{placeholders} )"
  else
    # Generic comparison: works for keysets ordered in mixed or same directions
    keyset = @keyset.to_a
    where  = []
    until keyset.empty?
      last_column, last_direction = keyset.pop
      query = +'( '
      query << (keyset.map { |column, _d| "#{column} = :#{column}" } \
                << "#{last_column} #{operator[last_direction]} :#{last_column}").join(' AND ')
      query << ' )'
      where << query
    end
    where.join(' OR ')
  end
end

def initialize(set, **vars)
  default = DEFAULT.slice(:limit, :page_param,                    # from pagy
                          :headers,                               # from headers extra
                          :jsonapi,                               # from jsonapi extra
                          :limit_param, :limit_max, :limit_extra) # from limit_extra
  assign_vars({ **default, page: nil }, vars)
  assign_limit
  @set    = set
  @page   = @vars[:page]
  @keyset = extract_keyset
  raise InternalError, 'the set must be ordered' if @keyset.empty?
  return unless @page
  latest  = JSON.parse(B64.urlsafe_decode(@page)).transform_keys(&:to_sym)
  @latest = @vars[:typecast_latest]&.(latest) || typecast_latest(latest)
  raise InternalError, 'page and keyset are not consistent' \
        unless @latest.keys == @keyset.keys
end

def next
  records
  return unless @more
  @next ||= B64.urlsafe_encode(latest_from(@records.last).to_json)
end

def records
  @records ||= begin
    @set    = apply_select if select?
    @set    = @vars[:after_latest]&.(@set, @latest) || after_latest if @latest
    records = @set.limit(@limit + 1).to_a
    @more   = records.size > @limit && !records.pop.nil?
    records
  end
end