/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.spark.sql.catalyst.expressions.aggregate

import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
import org.apache.spark.sql.catalyst.dsl.expressions._
import org.apache.spark.sql.catalyst.expressions._
import org.apache.spark.sql.catalyst.util.TypeUtils
import org.apache.spark.sql.types._

@ExpressionDescription(
  usage = "_FUNC_(expr) - Returns the mean calculated from values of a group.")
case class Average(child: Expression) extends DeclarativeAggregate with ImplicitCastInputTypes {

  override def prettyName: String = "avg"

  override def children: Seq[Expression] = child :: Nil

  override def nullable: Boolean = true

  // Return data type.
  override def dataType: DataType = resultType

  override def inputTypes: Seq[AbstractDataType] = Seq(NumericType)

  override def checkInputDataTypes(): TypeCheckResult =
    TypeUtils.checkForNumericExpr(child.dataType, "function average")

  private lazy val resultType = child.dataType match {
    case DecimalType.Fixed(p, s) =>
      DecimalType.bounded(p + 4, s + 4)
    case _ => DoubleType
  }

  private lazy val sumDataType = child.dataType match {
    case _ @ DecimalType.Fixed(p, s) => DecimalType.bounded(p + 10, s)
    case _ => DoubleType
  }

  private lazy val sum = AttributeReference("sum", sumDataType)()
  private lazy val count = AttributeReference("count", LongType)()

  override lazy val aggBufferAttributes = sum :: count :: Nil

  override lazy val initialValues = Seq(
    /* sum = */ Cast(Literal(0), sumDataType),
    /* count = */ Literal(0L)
  )

  override lazy val updateExpressions = Seq(
    /* sum = */
    Add(
      sum,
      Coalesce(Cast(child, sumDataType) :: Cast(Literal(0), sumDataType) :: Nil)),
    /* count = */ If(IsNull(child), count, count + 1L)
  )

  override lazy val mergeExpressions = Seq(
    /* sum = */ sum.left + sum.right,
    /* count = */ count.left + count.right
  )

  // If all input are nulls, count will be 0 and we will get null after the division.
  override lazy val evaluateExpression = child.dataType match {
    case DecimalType.Fixed(p, s) =>
      // increase the precision and scale to prevent precision loss
      val dt = DecimalType.bounded(p + 14, s + 4)
      Cast(Cast(sum, dt) / Cast(count, DecimalType.bounded(DecimalType.MAX_PRECISION, 0)),
        resultType)
    case _ =>
      Cast(sum, resultType) / Cast(count, resultType)
  }
}
