Scala algorithm: Compute the steps to transform an anagram only using swaps

Published

Algorithm goal

An anagram is a rearrangement of letters in a word to produce another word, eg. spar becomes rasp. Find the steps to transform this anagram; eg spar -> spra -> srpa -> rspa -> rsap -> rasp.

There may be multiple solutions to each rearrangement (depending on where you start, for example).

Test cases in Scala

assert(alignPosition("THE EYES", "THEY SEE", 0) == Nil)
assert(
  alignPosition("THE EYES", "THEY SEE", 3) ==
    List("THE EYES", "THE YEES", "THEY EES")
)
assert(
  computeSwaps("spar", "rasp").toList.flatten ==
    List("spar", "spra", "srpa", "rspa", "rsap", "rasp")
)
assert(
  computeSwaps("THE EYES", "THEY SEE").toList.flatten ==
    List("THE EYES", "THE YEES", "THEY EES", "THEY ESE", "THEY SEE")
)

Algorithm in Scala

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

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Explanation

One approach to this is to split this into two functionalities: to align individual positions (eg find the steps to move the r in spar to front), and then to iterate through with this function over the whole string to the end.

Computing the swaps is done using scanLeft, which is more or less Scala's way of doing a while-loop. (this is © from www.scala-algorithms.com)

We collect all these alignments, and this gives us our final result.

Scala concepts & Hints

  1. 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)

  2. 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")

  3. Range

    The (1 to n) syntax produces a "Range" which is a representation of a sequence of numbers.

    assert((1 to 5).toString == "Range 1 to 5")

assert((1 to 5).reverse.toString() == "Range 5 to 1 by -1")

assert((1 to 5).toList == List(1, 2, 3, 4, 5))

  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. 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)

  6. View

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

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