Reverse bits of an integer

Algorithm goal

Explanation

(this is © from www.scala-algorithms.com)

Scala Concepts & Hints

Def Inside Def

A great aspect of Scala is being able to declare functions inside functions, making it possible to reduce repetition.

def exampleDef(input: String): String = {
  def surroundInputWith(char: Char): String = s"$char$input$char"
  surroundInputWith('-')
}

It is also frequently used in combination with Tail Recursion.

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)

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Tail Recursion

In Scala, tail recursion enables you to rewrite a mutable structure such as a while-loop, into an immutable algorithm.

def fibonacci(n: Int): Int = {
  @scala.annotation.tailrec
  def go(i: Int, previous: Int, beforePrevious: Int): Int =
    if (i >= n) previous else go(i + 1, previous + beforePrevious, previous)

  go(i = 1, previous = 1, beforePrevious = 0)
}

assert(fibonacci(8) == 21)

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Algorithm in Scala

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

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Test cases in Scala

assert(bitsOf(50) == "00000000000000000000000000110010")
assert(bitsOf(-50) == "11111111111111111111111111001110")
assert(bitsOf(reverse(-50)) == "01110011111111111111111111111111")
assert(bitsOf(reverse(50)) == "01001100000000000000000000000000")
assert(
  {
    val num = scala.util.Random.nextInt()
    reverse(num) == java.lang.Integer.reverse(num)
  },
  "Our reverse acts the same as Java's reverse for any int"
)
assert(
  {
    val num = scala.util.Random.nextInt()
    reverseO1(num) == java.lang.Integer.reverse(num)
  },
  "Reverse O(1) acts the same as Java's reverse for any int"
)
def bitsOf(int: Int): String = ???

def reverse(int: Int): Int = ???

def reverseO1(a: Int): Int = ???