/*
 * 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.AnalysisException
import org.apache.spark.sql.catalyst.expressions._
import org.apache.spark.sql.types._

/**
 * Returns the last value of `child` for a group of rows. If the last value of `child`
 * is `null`, it returns `null` (respecting nulls). Even if [[Last]] is used on an already
 * sorted column, if we do partial aggregation and final aggregation (when mergeExpression
 * is used) its result will not be deterministic (unless the input table is sorted and has
 * a single partition, and we use a single reducer to do the aggregation.).
 */
@ExpressionDescription(
  usage = """
    _FUNC_(expr[, isIgnoreNull]) - Returns the last value of `expr` for a group of rows.
      If `isIgnoreNull` is true, returns only non-null values.
  """)
case class Last(child: Expression, ignoreNullsExpr: Expression) extends DeclarativeAggregate {

  def this(child: Expression) = this(child, Literal.create(false, BooleanType))

  private val ignoreNulls: Boolean = ignoreNullsExpr match {
    case Literal(b: Boolean, BooleanType) => b
    case _ =>
      throw new AnalysisException("The second argument of First should be a boolean literal.")
  }

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

  override def nullable: Boolean = true

  // Last is not a deterministic function.
  override def deterministic: Boolean = false

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

  // Expected input data type.
  override def inputTypes: Seq[AbstractDataType] = Seq(AnyDataType, BooleanType)

  private lazy val last = AttributeReference("last", child.dataType)()

  private lazy val valueSet = AttributeReference("valueSet", BooleanType)()

  override lazy val aggBufferAttributes: Seq[AttributeReference] = last :: valueSet :: Nil

  override lazy val initialValues: Seq[Literal] = Seq(
    /* last = */ Literal.create(null, child.dataType),
    /* valueSet = */ Literal.create(false, BooleanType)
  )

  override lazy val updateExpressions: Seq[Expression] = {
    if (ignoreNulls) {
      Seq(
        /* last = */ If(IsNull(child), last, child),
        /* valueSet = */ Or(valueSet, IsNotNull(child))
      )
    } else {
      Seq(
        /* last = */ child,
        /* valueSet = */ Literal.create(true, BooleanType)
      )
    }
  }

  override lazy val mergeExpressions: Seq[Expression] = {
    // Prefer the right hand expression if it has been set.
    Seq(
      /* last = */ If(valueSet.right, last.right, last.left),
      /* valueSet = */ Or(valueSet.right, valueSet.left)
    )
  }

  override lazy val evaluateExpression: AttributeReference = last

  override def toString: String = s"last($child)${if (ignoreNulls) " ignore nulls"}"
}
