# frozen_string_literal: true
module GraphQL
  module Execution
    class Lazy
      # Helpers for dealing with data structures containing {Lazy} instances
      # @api private
      module Resolve
        # Mutate `value`, replacing {Lazy} instances in place with their resolved values
        # @return [void]

        # This object can be passed like an array, but it doesn't allocate an
        # array until it's used.
        #
        # There's one crucial difference: you have to _capture_ the result
        # of `#<<`. (This _works_ with arrays but isn't required, since it has a side-effect.)
        # @api private
        module NullAccumulator
          def self.<<(item)
            [item]
          end

          def self.empty?
            true
          end
        end

        def self.resolve(value)
          lazies = resolve_in_place(value)
          deep_sync(lazies)
        end

        def self.resolve_in_place(value)
          acc = each_lazy(NullAccumulator, value)

          if acc.empty?
            Lazy::NullResult
          else
            Lazy.new {
              acc.each_with_index { |ctx, idx|
                acc[idx] = ctx.value.value
              }
              resolve_in_place(acc)
            }
          end
        end

        # If `value` is a collection,
        # add any {Lazy} instances in the collection
        # to `acc`
        # @return [void]
        def self.each_lazy(acc, value)
          case value
          when Hash
            value.each do |key, field_result|
              acc = each_lazy(acc, field_result)
            end
          when Array
            value.each do |field_result|
              acc = each_lazy(acc, field_result)
            end
          when Query::Context::SharedMethods
            field_value = value.value
            case field_value
            when Lazy
              acc = acc << value
            when Enumerable # shortcut for Hash & Array
              acc = each_lazy(acc, field_value)
            end
          end

          acc
        end

        # Traverse `val`, triggering resolution for each {Lazy}.
        # These {Lazy}s are expected to mutate their owner data structures
        # during resolution! (They're created with the `.then` calls in `resolve_in_place`).
        # @return [void]
        def self.deep_sync(val)
          case val
          when Lazy
            deep_sync(val.value)
          when Array
            val.each { |v| deep_sync(v.value) }
          when Hash
            val.each { |k, v| deep_sync(v.value) }
          end
        end
      end
    end
  end
end