# Stack Safety, a Scala language concept

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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.

### A fix using Tail Recursion

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

So, so long as you can rewrite a recursive function into an iterative function, you're good to go!

However, do not fret, there is a way to rewrite even recursive functions into stack-safe ones, called 'Defunctionalisation', which we must cover in future.

### A fix using foldLeft and foldRight

Tail recursion can be expressed in FoldLeft-FoldRight, and vice-versa; which is better depends on how it looks in the end. It's also related to State machine

### A fix using Range

Here, we benefit from Scala's utilities that allow specialisation

### A fix using a direct formula

Ideal if you can avoid an iteration and come up with a formula that runs in O(1)