Advent of Code 2022 - Day 1 - Kotlin Edition

Day 1 - Calorie Counting

Part 1

Task

Santa's reindeer typically eat regular reindeer food, but they need a lot of magical energy to deliver presents on Christmas. For that, their favorite snack is a special type of star fruit that only grows deep in the jungle. The Elves have brought you on their annual expedition to the grove where the fruit grows.

To supply enough magical energy, the expedition needs to retrieve a minimum of fifty stars by December 25th. Although the Elves assure you that the grove has plenty of fruit, you decide to grab any fruit you see along the way, just in case.

Collect stars by solving puzzles. Two puzzles will be made available on each day in the Advent calendar; the second puzzle is unlocked when you complete the first. Each puzzle grants one star. Good luck!

The jungle must be too overgrown and difficult to navigate in vehicles or access from the air; the Elves' expedition traditionally goes on foot. As your boats approach land, the Elves begin taking inventory of their supplies. One important consideration is food - in particular, the number of Calories each Elf is carrying (your puzzle input).

The Elves take turns writing down the number of Calories contained by the various meals, snacks, rations, etc. that they've brought with them, one item per line. Each Elf separates their own inventory from the previous Elf's inventory (if any) by a blank line.

For example, suppose the Elves finish writing their items' Calories and end up with the following list:

1000
2000
3000

4000

5000
6000

7000
8000
9000

10000

This list represents the Calories of the food carried by five Elves:

• The first Elf is carrying food with 1000, 2000, and 3000 Calories, a total of 6000 Calories.

• The second Elf is carrying one food item with 4000 Calories.

• The third Elf is carrying food with 5000 and 6000 Calories, a total of 11000 Calories.

• The fourth Elf is carrying food with 7000, 8000, and 9000 Calories, a total of 24000 Calories.

• The fifth Elf is carrying one food item with 10000 Calories.

In case the Elves get hungry and need extra snacks, they need to know which Elf to ask: they'd like to know how many Calories are being carried by the Elf carrying the most Calories. In the example above, this is 24000 (carried by the fourth Elf).

Find the Elf carrying the most Calories. How many total Calories is that Elf carrying?

Solution

We will start by implementing the function that takes a List (our input file) and aggregate it into a List that represents the Calories each Elf is carrying.

The solution will be as follow:

• use the fold function over the list, and start with a list with 0 Calories.

• For each step, if the current line isn't empty we will update the Calories stored in the latest entry in the list.

• If the is empty, we will create a new entry in the list, and set it to 0 - this will represent the next elf in the list.

• Find the maximum value in the list of Calories and return it.
  

private fun caloriesCount(input: List<String>) =
    input.fold(mutableListOf(0)) { elves, snack ->
        if (snack.isEmpty()) elves.add(0)
        else elves[elves.lastIndex] = elves.last() + snack.toInt()
        elves
    }

fun solvePart1(): Int =
    caloriesCount(input).maxOf { it }

We can now run our test cases:

@Test
fun `Part 1 - Example`() {
    val day01 = Day01(exampleInput)
    assertEquals(24000, day01.solvePart1())
}

@Test
fun `Part 1 - Real Input`() {
    val day01 = Day01(resourceAsList("2022/day01.txt"))
    assertEquals(69177, day01.solvePart1())
}

Part 2

Task

By the time you calculate the answer to the Elves' question, they've already realized that the Elf carrying the most Calories of food might eventually run out of snacks.

To avoid this unacceptable situation, the Elves would instead like to know the total Calories carried by the top three Elves carrying the most Calories. That way, even if one of those Elves runs out of snacks, they still have two backups.

In the example above, the top three Elves are the fourth Elf (with 24000 Calories), then the third Elf (with 11000 Calories), then the fifth Elf (with 10000 Calories). The sum of the Calories carried by these three elves is 45000.

Find the top three Elves carrying the most Calories. How many Calories are those Elves carrying in total?

Solution

• We will re-use the caloriesCount() function from the previous step

• Sort the results

• Take the top 3 calories in the list and sum them together
  

fun solvePart2(): Int =
    caloriesCount(input)
        .sorted()
        .takeLast(3)
        .sum()

We can now combine both solutions as follow:

 @Test
fun `Part 2 - Example`() {
    val day01 = Day01(exampleInput)
    assertEquals(45000, day01.solvePart2())
}

@Test
fun `Part 2 - Real Input`() {
    val day01 = Day01(resourceAsList("2022/day01.txt"))
    assertEquals(207456, day01.solvePart2())
}

All of the code that was presented in the article, along with the utility classes that are used are available in my GitHub account.

See you on the next challenge!

Comments

[Hwajun Lee]

Hi, it was nice post! This post helps me alot to study Kotlin.
I want to ask about caloriesCount function in part 1.
When you use fold function for it, in the end there is elves.
I do wonder what is this doing?
If I understand correclty, fold function need initialize value and operation.

[Yonatan Karp-Rudin]

Hi @dasogom !
I'm really happy to hear that, as the main reason I'm writing about Kotlin is to make it more usable for others (especially in the backend).

As for your question, in Kotlin, as with many other languages that support functional programming when you run a lambda function (an anonymous function) the return value of the function is the last line of code within it.

Since what fold is doing is to take the iterate over the list, and aggregate the information by passing the results of the previous iteration, the important thing for us was to pass the list of elves from one iteration to another.

So to summarize, the lambda function would either update the calories of a specific elf in the list, or create a new entry for the next elf, and return the new list as the input of the next iteration. as you can see, our call starts with the initialized value of mutableListOf(0) which means a list that can be mutated and initialized with 1 element of the value 0

I hope that answers your question, please let me know if you have more of them!

Also feel free to follow my blog directly as usually I'm posting there earlier :)