Step-by-step guide

Step 1 - Algorithm

readLines

readLines() : open the file and return each line as String
for each value (one value by line), find another value with a result of 2020 for the addition
display the result of their multiplication

Step 2 - What do we need ?

Function to run our code
Variables to store the lines
For loop to iterate over the lines
If statement to test if addition equals to 2020
String template to print the final result

Let’s learn this quickly

Then go back to main course

Step 3 - simple/naive implementation

Read lines from the input file to a variable

val lines = File("src/main/kotlin/day1/input.txt")
  .readLines()

Convert to Int

val lines = File("src/main/kotlin/day1/input.txt")
  .readLines().map { it.toInt() }

Iterate over the lines

for (line1 in lines) {
  for(line2 in lines) {
      println(line1 + " " + line2)
  }
}

Test if the addition of two numbers are equal

for (line1 in lines) {
  for(line2 in lines) {
      if(line1 + line2 == 2020) {
          println("OK")
          return
      }
  }
}

Display the multiplication of the two previously found numbers

for (line1 in lines) {
  for(line2 in lines) {
      if(line1 + line2 == 2020) {
          println(line1 * line2)
          return
      }
  }
}

Congratulations we have solved the challenge !!!

Step 4 - Refactor / improve our code

Store the result of the current number minus 2020 associated to the current number
```
val complements = lines.associateWith { 2020 - it }
```
Find one resulting numbers in the lines

val pair = lines.firstNotNullOf { number ->
    val complement = complements[number]
    if(complement !=  null)
        Pair(number, complement)
    else null
}

Print the first value
```
println(pair.first * pair.second)
```

Based on JetBrains work available here