Sonar Sweep - Advent of Code 2021 Day 1

 

Record Bar 33 1/3 was a very interesting record listening bar that I visited. 

I was the only patron that early evening and owner/bar-keep/DJ took requests

Record Bar, 33 1/3 rpm

I am finally catching up to Day 1 of this year's Advent of Code. In this problem, we are asked to determine how many times the depth of the ocean floor increases. Part B of the problem asks us modify the solution to consider a sliding window of depths.

If we were to plot the example given in the problem we can see the instances in which the depth increases. There are 7 such increases.

To solve part a, we need to compare the previous depth to the current one and increase a counter, as shown in the following function -

Integer prev = null;
Integer counter = 0; 

File myObj = new File(inputFile);
Scanner reader = new Scanner(myObj);
while (reader.hasNextLine()) {
    Integer depth = Integer.parseInt(reader.nextLine());
    if (prev != null && depth > prev)
        counter++;
    prev = depth;
}
reader.close();

The problem gets interesting when we need to solve for increases in a 3 element sliding window. We now need to keep track of the sum of depths in the previous sliding window, and compare it to the current one.

In order to solve part b, we maintain a queue of elements representing the sliding window. Depths can be added and removed as they come or go out of window, and this action is used to adjust the sum of depths in the sliding window

In the following diagram, as the sliding window moves, the depth of 4 is removed from the queue and subtracted from the sum. Also, the depth of 1 is pushed into the queue and added to the sum

The following function uses an ArrayQueue to keep track of the depths in a sliding window. Once the sliding window is full, we start to compare the previous sums (prev). The current sliding window sum is updated by subtracting the depth that has gone out of window

Integer prev = null;
Integer counter = 0;

Integer slidingWindowSum = 0;
ArrayDeque<Integer> slidingWindow = new ArrayDeque<>();

File myObj = new File(inputFile);
Scanner reader = new Scanner(myObj);
while (reader.hasNextLine()) {
    Integer depth = Integer.parseInt(reader.nextLine());

    // track the depth in the sliding window queue and also track sliding window sum
    slidingWindow.add(depth);
    slidingWindowSum += depth;

    // if we are tracking enough depths we can start comparing the prev sliding sum
    if (slidingWindow.size() > slidingWindowSize) {
        slidingWindowSum -= slidingWindow.pop(); // subtract depth out of window
        if (prev < slidingWindowSum)
            counter++;
    }
    prev = slidingWindowSum;
}

Full source code can be found here - 

https://github.com/jasoncoelho/adventOfCode2021/blob/main/1/run.java