# Scala algorithm: Reverse bits of an integer

Published

## Algorithm goal

Reverse the bits of an integer, eg 2 becomes 1073741824 as 2 is 0b[30 times of 0-bit]10, so 2 reversed is 0b01[30 times of 0-bit].

## Test cases in Scala

``````assert(bitsOf(2) == "00000000000000000000000000000010")
assert(bitsOf(reverse(2)) == "01000000000000000000000000000000")
assert(reverse(2) == 1073741824)
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"
)
``````

## Algorithm in Scala

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

## Explanation

We take the approach of taking bit-by-bit from the end and using a left binary shift to move that bit to the left side at every iteration.

Note that there is a more efficient implementation that is O(1), but we would be copy-pasting a standard Java algorithm at this point; see `java.lang.Integer.reverse`. (this is Â© from www.scala-algorithms.com)

## Scala concepts & Hints

1. ### 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('-')
}

assert(exampleDef("test") == "-test-")
``````

It is also frequently used in combination with Tail Recursion.

2. ### 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)
``````
3. ### 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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