# frozen_string_literal: true
require "graphql/pagination/connection"

module GraphQL
  module Pagination
    # A generic class for working with database query objects.
    class RelationConnection < Pagination::Connection
      def nodes
        load_nodes
        @nodes
      end

      def has_previous_page
        load_nodes
        @has_previous_page
      end

      def has_next_page
        load_nodes
        @has_next_page
      end

      def cursor_for(item)
        load_nodes
        # index in nodes + existing offset + 1 (because it's offset, not index)
        offset = nodes.index(item) + 1 + (@paged_nodes_offset || 0) + (relation_offset(items) || 0)
        context.schema.cursor_encoder.encode(offset.to_s)
      end

      private

      # @param relation [Object] A database query object
      # @return [Integer, nil] The offset value, or nil if there isn't one
      def relation_offset(relation)
        raise "#{self.class}#relation_offset(relation) must return the offset value for a #{relation.class} (#{relation.inspect})"
      end

      # @param relation [Object] A database query object
      # @return [Integer, nil] The limit value, or nil if there isn't one
      def relation_limit(relation)
        raise "#{self.class}#relation_limit(relation) must return the limit value for a #{relation.class} (#{relation.inspect})"
      end

      # @param relation [Object] A database query object
      # @return [Integer, nil] The number of items in this relation (hopefully determined without loading all records into memory!)
      def relation_count(relation)
        raise "#{self.class}#relation_count(relation) must return the count of records for a #{relation.class} (#{relation.inspect})"
      end

      # @param relation [Object] A database query object
      # @return [Object] A modified query object which will return no records
      def null_relation(relation)
        raise "#{self.class}#null_relation(relation) must return an empty relation for a #{relation.class} (#{relation.inspect})"
      end

      def offset_from_cursor(cursor)
        decode(cursor).to_i
      end

      # Abstract this operation so we can always ignore inputs less than zero.
      # (Sequel doesn't like it, understandably.)
      def set_offset(relation, offset_value)
        if offset_value >= 0
          relation.offset(offset_value)
        else
          relation.offset(0)
        end
      end

      # Abstract this operation so we can always ignore inputs less than zero.
      # (Sequel doesn't like it, understandably.)
      def set_limit(relation, limit_value)
        if limit_value > 0
          relation.limit(limit_value)
        elsif limit_value == 0
          null_relation(relation)
        else
          relation
        end
      end

      # Populate all the pagination info _once_,
      # It doesn't do anything on subsequent calls.
      def load_nodes
        @nodes ||= begin
          paginated_nodes = items
          after_offset = after && offset_from_cursor(after)
          before_offset = before && offset_from_cursor(before)

          if after_offset
            previous_offset = relation_offset(items) || 0
            paginated_nodes = set_offset(paginated_nodes, previous_offset + after_offset)
          end

          if before_offset && after_offset
            if after_offset < before_offset
              # Get the number of items between the two cursors
              space_between = before_offset - after_offset - 1
              paginated_nodes = set_limit(paginated_nodes, space_between)
            else
              # TODO I think this is untested
              # The cursors overextend one another to an empty set
              paginated_nodes = null_relation(paginated_nodes)
            end
          elsif before_offset
            # Use limit to cut off the tail of the relation
            paginated_nodes = set_limit(paginated_nodes, before_offset - 1)
          end

          sliced_nodes_count = relation_count(paginated_nodes)

          if first && (relation_limit(paginated_nodes).nil? || relation_limit(paginated_nodes) > first)
            # `first` would create a stricter limit that the one already applied, so add it
            paginated_nodes = set_limit(paginated_nodes, first)
          end

          if last
            if (lv = relation_limit(paginated_nodes))
              if last <= lv
                # `last` is a smaller slice than the current limit, so apply it
                offset = (relation_offset(paginated_nodes) || 0) + (lv - last)
                paginated_nodes = set_offset(paginated_nodes, offset)
                paginated_nodes = set_limit(paginated_nodes, last)
              end
            else
              # No limit, so get the last items
              offset = (relation_offset(paginated_nodes) || 0) + sliced_nodes_count - [last, sliced_nodes_count].min
              paginated_nodes = set_offset(paginated_nodes, offset)
              paginated_nodes = set_limit(paginated_nodes, last)
            end
          end

          # Apply max page size if nothing else was applied
          if max_page_size && !first && !last
            if relation_limit(paginated_nodes).nil? || relation_limit(paginated_nodes) > max_page_size
              paginated_nodes = set_limit(paginated_nodes, max_page_size)
            end
          end

          @has_next_page = !!(
            (before_offset && before_offset > 0) ||
            (first && sliced_nodes_count > first)
          )

          @has_previous_page = !!(
            (after_offset && after_offset > 0) ||
            (last && sliced_nodes_count > last)
          )

          @paged_nodes_offset = relation_offset(paginated_nodes)
          # Return an array so we can consistently use `.index(node)` on it
          paginated_nodes.to_a
        end
      end
    end
  end
end