Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15443#discussion_r83367702
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/StringHistogram.scala
 ---
    @@ -0,0 +1,209 @@
    +/*
    + * 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 java.nio.ByteBuffer
    +
    +import scala.collection.immutable.TreeMap
    +import scala.collection.mutable
    +
    +import com.google.common.primitives.{Ints, Longs}
    +
    +import org.apache.spark.sql.catalyst.InternalRow
    +import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
    +import 
org.apache.spark.sql.catalyst.analysis.TypeCheckResult.{TypeCheckFailure, 
TypeCheckSuccess}
    +import org.apache.spark.sql.catalyst.expressions.{Expression, 
ExpressionDescription}
    +import 
org.apache.spark.sql.catalyst.expressions.aggregate.StringHistogram.StringHistogramInfo
    +import org.apache.spark.sql.catalyst.util.ArrayBasedMapData
    +import org.apache.spark.sql.types._
    +import org.apache.spark.unsafe.types.UTF8String
    +
    +/**
    + * The StringHistogram function returns the histogram bins - (distinct 
value, frequency) pairs
    + * for a string column.
    + * @param child child expression that can produce column value with 
`child.eval(inputRow)`
    + * @param numBinsExpression The maximum number of bins. When the number of 
distinct values is
    + *                          larger than this, the function will return an 
empty result.
    + */
    +@ExpressionDescription(
    +  usage = "_FUNC_(col, numBins) - Returns histogram bins with the maximum 
number of bins allowed.")
    +case class StringHistogram(
    +    child: Expression,
    +    numBinsExpression: Expression,
    +    override val mutableAggBufferOffset: Int,
    +    override val inputAggBufferOffset: Int) extends 
TypedImperativeAggregate[StringHistogramInfo] {
    +
    +  def this(child: Expression, numBinsExpression: Expression) = {
    +    this(child, numBinsExpression, 0, 0)
    +  }
    +
    +  // Mark as lazy so that numBinsExpression is not evaluated during tree 
transformation.
    +  private lazy val numBins: Int = 
numBinsExpression.eval().asInstanceOf[Int]
    +
    +  override def checkInputDataTypes(): TypeCheckResult = {
    +    val defaultCheck = super.checkInputDataTypes()
    +    if (defaultCheck.isFailure) {
    +      defaultCheck
    +    } else if (!numBinsExpression.foldable) {
    +      TypeCheckFailure(s"The maximum number of bins provided must be a 
constant literal")
    +    } else if (numBins <= 0) {
    +      TypeCheckFailure(
    +        s"The maximum number of bins provided must be a positive integer 
literal (current value" +
    +          s" = $numBins)")
    +    } else {
    +      TypeCheckSuccess
    +    }
    +  }
    +
    +  override def update(buffer: StringHistogramInfo, input: InternalRow): 
Unit = {
    +    if (buffer.invalid) {
    +      return
    +    }
    +    val evaluated = child.eval(input)
    +    if (evaluated != null) {
    +      val str = evaluated.asInstanceOf[UTF8String]
    +      if (buffer.bins.contains(str)) {
    +        buffer.bins.update(str, buffer.bins(str) + 1)
    +      } else {
    +        if (buffer.bins.size >= numBins) {
    +          // clear the buffer and mark it as invalid
    +          buffer.bins.clear()
    +          buffer.invalid = true
    +        } else {
    +          buffer.bins.put(str, 1)
    +        }
    +      }
    +    }
    +  }
    +
    +  override def merge(buffer: StringHistogramInfo, other: 
StringHistogramInfo): Unit = {
    +    if (buffer.invalid || other.invalid) {
    +      buffer.bins.clear()
    +      return
    +    }
    +    other.bins.foreach { case (key, value) =>
    +      if (buffer.bins.contains(key)) {
    +        buffer.bins.update(key, buffer.bins(key) + value)
    +      } else {
    +        if (buffer.bins.size >= numBins) {
    +          // clear the buffer and mark it as invalid
    +          buffer.bins.clear()
    +          buffer.invalid = true
    +          return
    +        } else {
    +          buffer.bins.put(key, value)
    +        }
    +      }
    +    }
    +  }
    +
    +  override def eval(buffer: StringHistogramInfo): Any = {
    +    val sorted = TreeMap[UTF8String, Long](buffer.bins.toSeq: _*)
    +    ArrayBasedMapData(sorted.keys.toArray, sorted.values.toArray)
    +  }
    +
    +  override def serialize(buffer: StringHistogramInfo): Array[Byte] = {
    +    StringHistogram.serializer.serialize(buffer)
    +  }
    +
    +  override def deserialize(storageFormat: Array[Byte]): 
StringHistogramInfo = {
    +    StringHistogram.serializer.deserialize(storageFormat)
    +  }
    +
    +  override def createAggregationBuffer(): StringHistogramInfo = new 
StringHistogramInfo()
    +
    +  override def withNewMutableAggBufferOffset(newMutableAggBufferOffset: 
Int): StringHistogram = {
    +    copy(mutableAggBufferOffset = newMutableAggBufferOffset)
    +  }
    +
    +  override def withNewInputAggBufferOffset(newInputAggBufferOffset: Int): 
StringHistogram = {
    +    copy(inputAggBufferOffset = newInputAggBufferOffset)
    +  }
    +
    +  override def inputTypes: Seq[AbstractDataType] = Seq(StringType, 
IntegerType)
    +
    +  override def nullable: Boolean = false
    +
    +  override def dataType: DataType = MapType(StringType, LongType)
    +
    +  override def children: Seq[Expression] = Seq(child, numBinsExpression)
    +
    +  override def prettyName: String = "string_histogram"
    +}
    +
    +object StringHistogram {
    +
    +  /**
    +   * StringHistogramInfo maintains frequency of each distinct value.
    +   * @param invalid This is to mark the HashMap as invalid when its size 
(ndv of the column)
    +   *                exceeds numBins, because we won't generate string 
histograms in that case.
    +   * @param bins A HashMap to maintain frequency of each distinct value.
    +   */
    +  case class StringHistogramInfo(
    +      var invalid: Boolean = false,
    --- End diff --
    
    it's really weird to see a `var` in the constructor of case class, how about
    ```
    case class xxx {
      var invalide = false
    }
    ```


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