module ActiveRecord
  module Calculations
    # Count the records.
    #
    #   Person.count
    #   # => the total count of all people
    #
    #   Person.count(:age)
    #   # => returns the total count of all people whose age is present in database
    #
    #   Person.count(:all)
    #   # => performs a COUNT(*) (:all is an alias for '*')
    #
    #   Person.distinct.count(:age)
    #   # => counts the number of different age values
    #
    # If #count is used with {Relation#group}[rdoc-ref:QueryMethods#group],
    # it returns a Hash whose keys represent the aggregated column,
    # and the values are the respective amounts:
    #
    #   Person.group(:city).count
    #   # => { 'Rome' => 5, 'Paris' => 3 }
    #
    # If #count is used with {Relation#group}[rdoc-ref:QueryMethods#group] for multiple columns, it returns a Hash whose
    # keys are an array containing the individual values of each column and the value
    # of each key would be the #count.
    #
    #   Article.group(:status, :category).count
    #   # =>  {["draft", "business"]=>10, ["draft", "technology"]=>4,
    #          ["published", "business"]=>0, ["published", "technology"]=>2}
    #
    # If #count is used with {Relation#select}[rdoc-ref:QueryMethods#select], it will count the selected columns:
    #
    #   Person.select(:age).count
    #   # => counts the number of different age values
    #
    # Note: not all valid {Relation#select}[rdoc-ref:QueryMethods#select] expressions are valid #count expressions. The specifics differ
    # between databases. In invalid cases, an error from the database is thrown.
    def count(column_name = nil)
      return super() if block_given?
      calculate(:count, column_name)
    end

    # Calculates the average value on a given column. Returns +nil+ if there's
    # no row. See #calculate for examples with options.
    #
    #   Person.average(:age) # => 35.8
    def average(column_name)
      calculate(:average, column_name)
    end

    # Calculates the minimum value on a given column. The value is returned
    # with the same data type of the column, or +nil+ if there's no row. See
    # #calculate for examples with options.
    #
    #   Person.minimum(:age) # => 7
    def minimum(column_name)
      calculate(:minimum, column_name)
    end

    # Calculates the maximum value on a given column. The value is returned
    # with the same data type of the column, or +nil+ if there's no row. See
    # #calculate for examples with options.
    #
    #   Person.maximum(:age) # => 93
    def maximum(column_name)
      calculate(:maximum, column_name)
    end

    # Calculates the sum of values on a given column. The value is returned
    # with the same data type of the column, +0+ if there's no row. See
    # #calculate for examples with options.
    #
    #   Person.sum(:age) # => 4562
    def sum(column_name = nil)
      return super() if block_given?
      calculate(:sum, column_name)
    end

    # This calculates aggregate values in the given column. Methods for #count, #sum, #average,
    # #minimum, and #maximum have been added as shortcuts.
    #
    #   Person.calculate(:count, :all) # The same as Person.count
    #   Person.average(:age) # SELECT AVG(age) FROM people...
    #
    #   # Selects the minimum age for any family without any minors
    #   Person.group(:last_name).having("min(age) > 17").minimum(:age)
    #
    #   Person.sum("2 * age")
    #
    # There are two basic forms of output:
    #
    # * Single aggregate value: The single value is type cast to Integer for COUNT, Float
    #   for AVG, and the given column's type for everything else.
    #
    # * Grouped values: This returns an ordered hash of the values and groups them. It
    #   takes either a column name, or the name of a belongs_to association.
    #
    #      values = Person.group('last_name').maximum(:age)
    #      puts values["Drake"]
    #      # => 43
    #
    #      drake  = Family.find_by(last_name: 'Drake')
    #      values = Person.group(:family).maximum(:age) # Person belongs_to :family
    #      puts values[drake]
    #      # => 43
    #
    #      values.each do |family, max_age|
    #        ...
    #      end
    def calculate(operation, column_name)
      if has_include?(column_name)
        relation = construct_relation_for_association_calculations
        relation = relation.distinct if operation.to_s.downcase == "count"

        relation.calculate(operation, column_name)
      else
        perform_calculation(operation, column_name)
      end
    end

    # Use #pluck as a shortcut to select one or more attributes without
    # loading a bunch of records just to grab the attributes you want.
    #
    #   Person.pluck(:name)
    #
    # instead of
    #
    #   Person.all.map(&:name)
    #
    # Pluck returns an Array of attribute values type-casted to match
    # the plucked column names, if they can be deduced. Plucking an SQL fragment
    # returns String values by default.
    #
    #   Person.pluck(:name)
    #   # SELECT people.name FROM people
    #   # => ['David', 'Jeremy', 'Jose']
    #
    #   Person.pluck(:id, :name)
    #   # SELECT people.id, people.name FROM people
    #   # => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]
    #
    #   Person.distinct.pluck(:role)
    #   # SELECT DISTINCT role FROM people
    #   # => ['admin', 'member', 'guest']
    #
    #   Person.where(age: 21).limit(5).pluck(:id)
    #   # SELECT people.id FROM people WHERE people.age = 21 LIMIT 5
    #   # => [2, 3]
    #
    #   Person.pluck('DATEDIFF(updated_at, created_at)')
    #   # SELECT DATEDIFF(updated_at, created_at) FROM people
    #   # => ['0', '27761', '173']
    #
    # See also #ids.
    #
    def pluck(*column_names)
      if loaded? && (column_names.map(&:to_s) - @klass.attribute_names - @klass.attribute_aliases.keys).empty?
        return records.pluck(*column_names)
      end

      if has_include?(column_names.first)
        construct_relation_for_association_calculations.pluck(*column_names)
      else
        relation = spawn
        relation.select_values = column_names.map { |cn|
          @klass.has_attribute?(cn) || @klass.attribute_alias?(cn) ? arel_attribute(cn) : cn
        }
        result = klass.connection.select_all(relation.arel, nil, bound_attributes)
        result.cast_values(klass.attribute_types)
      end
    end

    # Pluck all the ID's for the relation using the table's primary key
    #
    #   Person.ids # SELECT people.id FROM people
    #   Person.joins(:companies).ids # SELECT people.id FROM people INNER JOIN companies ON companies.person_id = people.id
    def ids
      pluck primary_key
    end

    private

      def has_include?(column_name)
        eager_loading? || (includes_values.present? && column_name && column_name != :all)
      end

      def perform_calculation(operation, column_name)
        operation = operation.to_s.downcase

        # If #count is used with #distinct (i.e. `relation.distinct.count`) it is
        # considered distinct.
        distinct = distinct_value

        if operation == "count"
          column_name ||= select_for_count
          column_name = primary_key if column_name == :all && distinct
          distinct = nil if column_name =~ /\s*DISTINCT[\s(]+/i
        end

        if group_values.any?
          execute_grouped_calculation(operation, column_name, distinct)
        else
          execute_simple_calculation(operation, column_name, distinct)
        end
      end

      def aggregate_column(column_name)
        return column_name if Arel::Expressions === column_name

        if @klass.has_attribute?(column_name.to_s) || @klass.attribute_alias?(column_name.to_s)
          @klass.arel_attribute(column_name)
        else
          Arel.sql(column_name == :all ? "*" : column_name.to_s)
        end
      end

      def operation_over_aggregate_column(column, operation, distinct)
        operation == "count" ? column.count(distinct) : column.send(operation)
      end

      def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
        column_alias = column_name

        if operation == "count" && (limit_value || offset_value)
          # Shortcut when limit is zero.
          return 0 if limit_value == 0

          query_builder = build_count_subquery(spawn, column_name, distinct)
        else
          # PostgreSQL doesn't like ORDER BY when there are no GROUP BY
          relation = unscope(:order).distinct!(false)

          column = aggregate_column(column_name)

          select_value = operation_over_aggregate_column(column, operation, distinct)

          column_alias = select_value.alias
          column_alias ||= @klass.connection.column_name_for_operation(operation, select_value)
          relation.select_values = [select_value]

          query_builder = relation.arel
        end

        result = @klass.connection.select_all(query_builder, nil, bound_attributes)
        row    = result.first
        value  = row && row.values.first
        type   = result.column_types.fetch(column_alias) do
          type_for(column_name)
        end

        type_cast_calculated_value(value, type, operation)
      end

      def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
        group_attrs = group_values

        if group_attrs.first.respond_to?(:to_sym)
          association  = @klass._reflect_on_association(group_attrs.first)
          associated   = group_attrs.size == 1 && association && association.belongs_to? # only count belongs_to associations
          group_fields = Array(associated ? association.foreign_key : group_attrs)
        else
          group_fields = group_attrs
        end
        group_fields = arel_columns(group_fields)

        group_aliases = group_fields.map { |field| column_alias_for(field) }
        group_columns = group_aliases.zip(group_fields)

        if operation == "count" && column_name == :all
          aggregate_alias = "count_all"
        else
          aggregate_alias = column_alias_for([operation, column_name].join(" "))
        end

        select_values = [
          operation_over_aggregate_column(
            aggregate_column(column_name),
            operation,
            distinct).as(aggregate_alias)
        ]
        select_values += self.select_values unless having_clause.empty?

        select_values.concat group_columns.map { |aliaz, field|
          if field.respond_to?(:as)
            field.as(aliaz)
          else
            "#{field} AS #{aliaz}"
          end
        }

        relation = except(:group).distinct!(false)
        relation.group_values  = group_fields
        relation.select_values = select_values

        calculated_data = @klass.connection.select_all(relation, nil, relation.bound_attributes)

        if association
          key_ids     = calculated_data.collect { |row| row[group_aliases.first] }
          key_records = association.klass.base_class.where(association.klass.base_class.primary_key => key_ids)
          key_records = Hash[key_records.map { |r| [r.id, r] }]
        end

        Hash[calculated_data.map do |row|
          key = group_columns.map { |aliaz, col_name|
            type = type_for(col_name) do
              calculated_data.column_types.fetch(aliaz, Type.default_value)
            end
            type_cast_calculated_value(row[aliaz], type)
          }
          key = key.first if key.size == 1
          key = key_records[key] if associated

          type = calculated_data.column_types.fetch(aggregate_alias) { type_for(column_name) }
          [key, type_cast_calculated_value(row[aggregate_alias], type, operation)]
        end]
      end

      # Converts the given keys to the value that the database adapter returns as
      # a usable column name:
      #
      #   column_alias_for("users.id")                 # => "users_id"
      #   column_alias_for("sum(id)")                  # => "sum_id"
      #   column_alias_for("count(distinct users.id)") # => "count_distinct_users_id"
      #   column_alias_for("count(*)")                 # => "count_all"
      def column_alias_for(keys)
        if keys.respond_to? :name
          keys = "#{keys.relation.name}.#{keys.name}"
        end

        table_name = keys.to_s.downcase
        table_name.gsub!(/\*/, "all")
        table_name.gsub!(/\W+/, " ")
        table_name.strip!
        table_name.gsub!(/ +/, "_")

        @klass.connection.table_alias_for(table_name)
      end

      def type_for(field, &block)
        field_name = field.respond_to?(:name) ? field.name.to_s : field.to_s.split(".").last
        @klass.type_for_attribute(field_name, &block)
      end

      def type_cast_calculated_value(value, type, operation = nil)
        case operation
        when "count"   then value.to_i
        when "sum"     then type.deserialize(value || 0)
        when "average" then value.respond_to?(:to_d) ? value.to_d : value
        else type.deserialize(value)
        end
      end

      def select_for_count
        if select_values.present?
          return select_values.first if select_values.one?
          select_values.join(", ")
        else
          :all
        end
      end

      def build_count_subquery(relation, column_name, distinct)
        column_alias = Arel.sql("count_column")
        subquery_alias = Arel.sql("subquery_for_count")

        aliased_column = aggregate_column(column_name == :all ? 1 : column_name).as(column_alias)
        relation.select_values = [aliased_column]
        subquery = relation.arel.as(subquery_alias)

        sm = Arel::SelectManager.new relation.engine
        select_value = operation_over_aggregate_column(column_alias, "count", distinct)
        sm.project(select_value).from(subquery)
      end
  end
end