Scala algorithm: Run-length encoding (RLE) Encoder

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Algorithm goal

Run-length encoding is one of the most basic compression methods, which is especially useful where there long runs of a particular character or a group of characters. It could be particularly useful for example in column-based databases which have potentially many repeating values.

Run-length turns a sequence of characters into effectively a sequence of count-character pairs. For example, WWWAAW becomes 3W2A1W.

This algorithm performs the Encoding part; see RunLengthEncodingDecoder for the Decoding part.

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Algorithm in Scala

29 lines of Scala (version 2.13), showing how concise Scala can be!

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There is a Tail Recursion approach that can be taken; while more obvious to implement, it has two disadvantages:

  • It is more complex to read and reason about
  • It it is not possible to make it work in a streaming fashion (because who wants to run out of memory?).

We present an immutable collection-based approach that only relies on plain and lazy transformations without worrying about index accessesand is also very efficient through the use of View, and because of that, is very easy to adapt to Streamed approaches. (this is © from

There are 2 stages to consider: computing the cumulative counts of each character (WWWAAW -> 1 -> W, 2 -> W, 3 -> W, 1 -> A, 2 -> A, 1 -> W), and then detecting transitions between the characters (we only emit a record once we have reached the maximum count, and we reach that when a different character is reached). Typically when dealing with transitions, consider the Sliding / Sliding Window concept.

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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. scanLeft and scanRight

    Scala's `scan` functions enable you to do folds like foldLeft and foldRight, while collecting the intermediate results

    assert(List(1, 2, 3, 4, 5).scanLeft(0)(_ + _) == List(0, 1, 3, 6, 10, 15))
  5. Sliding / Sliding Window

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

  6. Stack Safety

    Stack safety is present where a function cannot crash due to overflowing the limit of number of recursive calls.

    This function will work for n = 5, but will not work for n = 2000 (crash with java.lang.StackOverflowError) - however there is a way to fix it :-)

    In Scala Algorithms, we try to write the algorithms in a stack-safe way, where possible, so that when you use the algorithms, they will not crash on large inputs. However, stack-safe implementations are often more complex, and in some cases, overly complex, for the task at hand.

    def sum(from: Int, until: Int): Int =
      if (from == until) until else from + sum(from + 1, until)
    def thisWillSucceed: Int = sum(1, 5)
    def thisWillFail: Int = sum(1, 300)
  7. View

    The .view syntax creates a structure that mirrors another structure, until "forced" by an eager operation like .toList, .foreach, .forall, .count.

Scala Algorithms: The most comprehensive library of algorithms in standard pure-functional Scala

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Fully unit-tested, with explanations and relevant concepts; new algorithms published about once a week.

  1. Find minimum missing positive number in a sequence
  2. Longest increasing sub-sequence length
  3. Compute the length of longest valid parentheses
  4. Check if an array is a palindrome
  5. Monitor success rate of a process that may fail
  6. Find combinations adding up to N (non-unique)
  7. Remove duplicates from an unsorted List
  8. Find combinations adding up to N (unique)
  9. Find k closest elements to a value in a sorted Array
  10. Make a queue using stacks (Lists in Scala)
  11. Single-elimination tournament tree
  12. Quick Sort sorting algorithm in pure immutable Scala
  13. Compute a Roman numeral for an Integer, and vice-versa
  14. Matching parentheses algorithm with foldLeft and a state machine
  15. Traverse a tree Breadth-First, immutably
  16. Read a matrix as a spiral
  17. Remove duplicates from a sorted list (state machine)
  18. Merge Sort: stack-safe, tail-recursive, in pure immutable Scala, N-way
  19. Binary search a generic Array
  20. Merge Sort: in pure immutable Scala
  21. Make a queue using Maps
  22. Is an Array a permutation?
  23. Count number of contiguous countries by colors
  24. Add numbers without using addition (plus sign)
  25. Tic Tac Toe MinMax solve
  26. Run-length encoding (RLE) Encoder
  27. Print Alphabet Diamond
  28. Balanced parentheses algorithm with tail-call recursion optimisation
  29. Reverse a String's words efficiently
  30. Count number of changes (manipulations) needed to make an anagram with foldLeft and a MultiSet
  31. Count passing cars
  32. Counting inversions of a sequence (array) using a Merge Sort
  33. Compute nth row of Pascal's triangle
  34. Run-length encoding (RLE) Decoder
  35. Check if a number is a palindrome
  36. In a range of numbers, count the numbers divisible by a specific integer
  37. Find the index of a substring ('indexOf')
  38. Reshape a matrix
  39. Closest pair of coordinates in a 2D plane
  40. Find the contiguous slice with the minimum average
  41. Compute maximum sum of subarray (Kadane's algorithm)
  42. Binary search in a rotated sorted array
  43. Rotate Array right in pure-functional Scala - using an unusual immutable efficient approach
  44. Length of the longest common substring
  45. Tic Tac Toe board check
  46. Find an unpaired number in an array
  47. Check if a String is a palindrome
  48. Count binary gap size of a number using tail recursion
  49. Remove duplicates from a sorted list (Sliding)
  50. Find sub-array with the maximum sum
  51. Find the minimum absolute difference of two partitions
  52. Find maximum potential profit from an array of stock price
  53. Fibonacci in purely functional immutable Scala
  54. Fizz Buzz in purely functional immutable Scala
  55. Count factors/divisors of an integer
  56. Compute single-digit sum of digits
  57. Traverse a tree Depth-First
  58. Reverse bits of an integer
  59. QuickSelect Selection Algorithm (kth smallest item/order statistic)
  60. Rotate a matrix by 90 degrees clockwise

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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