Scala algorithm: Remove duplicates from a sorted list (Sliding)

Published

Algorithm goal

In Streaming data, data may come in duplicated; it could be due to various factors such as duplicated data from sources and idempotency for redundancy; for consumption though we may need to deduplicate the data for at-most-once processing. Some deduplicators retain state of long-gone elements (in which case .distinct will suffice, but have a memory cost), but in this case we are looking at only consecutive duplicate elements.

This is an alternative approach to RemoveDuplicatesFromSortedListStateMachine

Test cases in Scala

assert(removeDuplicates(List.empty[Int]) == List.empty[Int])
assert(removeDuplicates(List(1)) == List(1))
assert(removeDuplicates(List(1, 1)) == List(1))
assert(removeDuplicates(List(1, 2)) == List(1, 2))
assert(removeDuplicates(List(1, 1, 2)) == List(1, 2))
assert(removeDuplicates(List(1, 1, 1, 2)) == List(1, 2))
assert(removeDuplicates(List(1, 1, 2, 3, 3)) == List(1, 2, 3))
assert(removeDuplicatesWithMarker(List(1, 1, 2, 3, 3)) == List(1, 2, 3))

Algorithm in Scala

23 lines of Scala (compatible versions 2.13 & 3.0), showing how concise Scala can be!

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Explanation

The approach here is a little different to the State Machine approach:

The idea is that by running a sliding window, we can check pairs of elements for their equality, and if they are not equal, emit the newly found one. (this is © from www.scala-algorithms.com)

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Scala concepts & Hints

  1. Collect

    'collect' allows you to use Pattern Matching, to filter and map items.

    assert("Hello World".collect {
  case character if Character.isUpperCase(character) => character.toLower
} == "hw")

  2. Option Type

    The 'Option' type is used to describe a computation that either has a result or does not. In Scala, you can 'chain' Option processing, combine with lists and other data structures. For example, you can also turn a pattern-match into a function that return an Option, and vice-versa!

    assert(Option(1).flatMap(x => Option(x + 2)) == Option(3))

assert(Option(1).flatMap(x => None) == None)

  3. Pattern Matching

    Pattern matching in Scala lets you quickly identify what you are looking for in a data, and also extract it.

    assert("Hello World".collect {
  case character if Character.isUpperCase(character) => character.toLower
} == "hw")

  4. Sliding / Sliding Window

    Get fixed-length sliding sub-sequences (sliding windows) from another sequence

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Explore the 21 most useful Scala concepts

To save you going through various tutorials, we cherry-picked the most useful Scala concepts in a consistent form.

  1. Class Inside Class
  2. Class Inside Def
  3. Collect
  4. Def Inside Def
  5. Drop, Take, dropRight, takeRight
  6. foldLeft and foldRight
  7. For-comprehension
  8. Lazy List
  9. Option Type
  10. Ordering
  11. Partial Function
  12. Pattern Matching
  13. Range
  14. scanLeft and scanRight
  15. Sliding / Sliding Window
  16. Stack Safety
  17. State machine
  18. Tail Recursion
  19. Type Class
  20. View
  21. Zip

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