y9san9 / aoc23

Solving Advent of Code in pure Kotlin without use of mutability

Advent Of Code 2023

Restrictions

• 

• No mutable state at all (var variables, mutable lists, etc.)

• To be done...

Navigation

• Day 1

  • Part 1
  • Part 2

• Day 2

  • Part 1
  • Part 2

• Day 3

  • Part 1
  • Part 2

• Day 4

  • Part 1
  • Part 2

• Day 5

  • Part 1
  • Part 2

• Day 6

  • Part 1
  • Part 2

• Day 7

  • Part 1
  • Part 2

• Day 8

  • Part 1
  • Part 2

• Day 9

  • Part 1
  • Part 2

• Day 10

  • Part 1
  • Part 2

Immutability

I know that lots of stdlib functions (like map, sumOf, etc.) use mutable state under the hood. But that mutable stays in place meaning that I can implement such functions without any mutability, so I still allowed to use them.

Loops

You may ask how one will create loops which obviously use a concept of a mutable variable and are obviously required to solve almost any task?

They will be emulated using tail-recursive functions like this:

fun loop(index: Int) {
    println(index)
    loop(index = index + 1)
}

This creates infinite loop without any mutability.

• But it will throw stackoverflow exception!!! – One may say

And they will be absolutely correct. It will throw exception unless we add a special modifier called tailrec which optimizes tail recursion and unwraps it back to loops after compilation. So:

tailrec fun loop(index: Int) {
    println(index)
    loop(index = index + 1)
}

This will not throw any exception and will infinitely print natural numbers.