[GitHub] spark pull request #17359: [SPARK-20028][SQL] Add aggreagate expression nGra...

[viirya <gi...@git.apache.org>]

Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/17359#discussion_r107343519
  
    --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/NGrams.scala ---
    @@ -0,0 +1,258 @@
    +/*
    + * 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 java.util.HashMap
    +
    +import org.apache.spark.serializer.KryoSerializer
    +import org.apache.spark.SparkConf
    +import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
    +import org.apache.spark.sql.catalyst.analysis.TypeCheckResult._
    +import org.apache.spark.sql.catalyst.expressions.{Expression, ExpressionDescription, ImplicitCastInputTypes, Literal}
    +import org.apache.spark.sql.catalyst.expressions.aggregate.NGrams.NGramBuffer
    +import org.apache.spark.sql.catalyst.InternalRow
    +import org.apache.spark.sql.catalyst.util.{ArrayBasedMapData, GenericArrayData}
    +import org.apache.spark.sql.types._
    +import org.apache.spark.unsafe.types.UTF8String
    +
    +/**
    + * Return the top-k n-grams in rows that consist of sequences of strings.
    + */
    +@ExpressionDescription(
    +  usage = """
    +    _FUNC_(expr, n, k, accuracy) - Estimates the top-k n-grams in rows that consist of sequences
    +      of strings, represented as arrays of strings, or arrays of arrays of strings. 'accuracy' is an
    +      optional precision factor that controls memory usage.
    +      The parameter 'n' specifies what type of n-grams are being estimated. Unigrams are n = 1, and
    +      bigrams are n = 2. Generally, n will not be greater than about 5. The 'k' parameter specifies
    +      how many of the highest-frequency n-grams will be returned by the UDAF. The optional precision
    +      factor 'accuracy' specifies how much memory to use for estimation; more memory will give
    +      more accurate frequency counts, but could crash the JVM. The value will be the max between
    +      'accuracy'(0 if it's not specified) and 1000/k, which indicates the max number of n-grams
    +      which are kept in the internal HashMap.
    +      The output is an array of maps with the top-k n-grams and corresponding frequency.
    +  """,
    +  extended = """
    +    Examples:
    +      > SELECT ngrams(array('abc', 'abc', 'bcd', 'abc', 'bcd'), 2, 4);
    +       [["abc","bcd"]:2.0},
    --- End diff --
    
    hmm, doesn't "abc" show 3 times?


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