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Day 3: Diagnostic Report

https://adventofcode.com/2021/day/3
Challenge
The submarine has been making some odd creaking noises, so you ask it to produce a diagnostic report just in case.
The diagnostic report (your puzzle input) consists of a list of binary numbers which, when decoded properly, can tell you many useful things about the conditions of the submarine. The first parameter to check is the power consumption.
You need to use the binary numbers in the diagnostic report to generate two new binary numbers (called the gamma rate and the epsilon rate). The power consumption can then be found by multiplying the gamma rate by the epsilon rate.
Each bit in the gamma rate can be determined by finding the most common bit in the corresponding position of all numbers in the diagnostic report. For example, given the following diagnostic report:
00100
11110
10110
10111
10101
01111
00111
11100
10000
11001
00010
01010
Considering only the first bit of each number, there are five 0 bits and seven 1 bits. Since the most common bit is 1, the first bit of the gamma rate is 1.
The most common second bit of the numbers in the diagnostic report is 0, so the second bit of the gamma rate is 0.
The most common value of the third, fourth, and fifth bits are 1, 1, and 0, respectively, and so the final three bits of the gamma rate are 110.
So, the gamma rate is the binary number 10110, or 22 in decimal.
The epsilon rate is calculated in a similar way; rather than use the most common bit, the least common bit from each position is used. So, the epsilon rate is 01001, or 9 in decimal. Multiplying the gamma rate (22) by the epsilon rate (9) produces the power consumption, 198.
Use the binary numbers in your diagnostic report to calculate the gamma rate and epsilon rate, then multiply them together. What is the power consumption of the submarine? (Be sure to represent your answer in decimal, not binary.)

Part Two

Next, you should verify the life support rating, which can be determined by multiplying the oxygen generator rating by the CO2 scrubber rating.
Both the oxygen generator rating and the CO2 scrubber rating are values that can be found in your diagnostic report - finding them is the tricky part. Both values are located using a similar process that involves filtering out values until only one remains. Before searching for either rating value, start with the full list of binary numbers from your diagnostic report and consider just the first bit of those numbers. Then:
The bit criteria depends on which type of rating value you want to find:
For example, to determine the oxygen generator rating value using the same example diagnostic report from above:
Then, to determine the CO2 scrubber rating value from the same example above:
Finally, to find the life support rating, multiply the oxygen generator rating (23) by the CO2 scrubber rating (10) to get 230.
Use the binary numbers in your diagnostic report to calculate the oxygen generator rating and CO2 scrubber rating, then multiply them together. What is the life support rating of the submarine? (Be sure to represent your answer in decimal, not binary.)

Part A

First we'll count the number of 0s and 1s in each slot and store the total as slots.
import { strings } from "./util";

const slots: number[][] = [];
strings().forEach((bs) => {
  bs.split("").forEach((bit, idx) => {
    slots[idx] ||= [0, 0];
    slots[idx][parseInt(bit)]++;
  });
});
For our sample input, slots will correspond to the following.
[
  [5, 7],
  [7, 5],
  [4, 8],
  [5, 7],
  [7, 5],
]
To get the gamma of this two-dimensional array, we will:
const gamma = parseInt(
  slots
    .map(([zeroes, ones]) =>
      zeroes > ones ? "0" : "1"
    )
    .join(""),
  2
);
Computing epsilon is the same (except we flip 0 and 1)
const epsilon = parseInt(
  slots
    .map(([zeroes, ones]) =>
      zeroes > ones ? "1" : "0"
    )
    .join(""),
  2
);
Then we multiply 'em to get the answer
console.log(gamma * epsilon);

Part B

The instructions are a little convoluted, but are as follows:

Before searching for either rating value, start with the full list of binary numbers from your diagnostic report and consider just the first bit of those numbers. Then:

The bit criteria depends on which type of rating value you want to find:
Let's write a function that given a list of numbers and a bit index, returns the oxygen generator rating.
Start with a base case:
import { strings } from "./util";

const input = strings();

function oxygenGeneratorRating(
  nums: string[],
  idx: number
): number {
  if (nums.length === 1) {
    return parseInt(nums[0], 2);
  }

  // ...
}
Then we want to count the zeroes and ones in the given position idx. We'll be clever and use a reduce.
const [zeroes, ones] = nums.reduce(
  ([zeroes, ones], num) =>
    num[idx] === "0"
      ? [zeroes + 1, ones]
      : [zeroes, ones + 1],
  [0, 0]
);
Once we've counted the zeroes and ones, we want to trim down the list of numbers and call the rating function again.
if (zeroes > ones) {
  return oxygenGeneratorRating(
    nums.filter((num) => num[idx] === "0"),
    idx + 1
  );
} else {
  return oxygenGeneratorRating(
    nums.filter((num) => num[idx] === "1"),
    idx + 1
  );
}
The code for co2ScrubberRating is pretty much exactly the same, except when there are more zeroes than ones we want to filter for numbers with a 1 in the current position, and 0 otherwise. So the if-statement at the end gets flipped.
We could probably abstract oxygenGeneratorRating into getRating(nums, idx, ????), but we'll just copy this code and never look back ūüėé
function co2ScrubberRating(
  nums: string[],
  idx: number
): number {
  if (nums.length === 1) {
    return parseInt(nums[0], 2);
  }

  const [zeroes, ones] = nums.reduce(
    ([zeroes, ones], num) =>
      num[idx] === "0"
        ? [zeroes + 1, ones]
        : [zeroes, ones + 1],
    [0, 0]
  );

  if (zeroes > ones) {
    return co2ScrubberRating(
      nums.filter((num) => num[idx] === "1"),
      idx + 1
    );
  } else {
    return co2ScrubberRating(
      nums.filter((num) => num[idx] === "0"),
      idx + 1
    );
  }
}
Then we multiply the two readings together to get our result.
console.log(
  oxygenGeneratorRating(input, 0) *
    co2ScrubberRating(input, 0)
)
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