Day 3: Diagnostic Report
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:

Keep only numbers selected by the bit criteria for the type of rating value for which you are searching. Discard numbers which do not match the bit criteria.

If you only have one number left, stop; this is the rating value for which you are searching.

Otherwise, repeat the process, considering the next bit to the right.
The bit criteria depends on which type of rating value you want to find:

To find oxygen generator rating, determine the most common value (
0
or1
) in the current bit position, and keep only numbers with that bit in that position. If0
and1
are equally common, keep values with a1
in the position being considered. 
To find CO2 scrubber rating, determine the least common value (
0
or1
) in the current bit position, and keep only numbers with that bit in that position. If0
and1
are equally common, keep values with a0
in the position being considered.
For example, to determine the oxygen generator rating value using the same example diagnostic report from above:

Start with all 12 numbers and consider only the first bit of each number. There are more
1
bits (7) than0
bits (5), so keep only the 7 numbers with a1
in the first position:11110
,10110
,10111
,10101
,11100
,10000
, and11001
. 
Then, consider the second bit of the 7 remaining numbers: there are more
0
bits (4) than1
bits (3), so keep only the 4 numbers with a0
in the second position:10110
,10111
,10101
, and10000
. 
In the third position, three of the four numbers have a
1
, so keep those three:10110
,10111
, and10101
. 
In the fourth position, two of the three numbers have a
1
, so keep those two:10110
and10111
. 
In the fifth position, there are an equal number of
0
bits and1
bits (one each). So, to find the oxygen generator rating, keep the number with a1
in that position:10111
. 
As there is only one number left, stop; the oxygen generator ratingis
10111
, or23
in decimal.
Then, to determine the CO2 scrubber rating value from the same example above:

Start again with all 12 numbers and consider only the first bit of each number. There are fewer
0
bits (5) than1
bits (7), so keep only the 5 numbers with a0
in the first position:00100
,01111
,00111
,00010
, and01010
. 
Then, consider the second bit of the 5 remaining numbers: there are fewer
1
bits (2) than0
bits (3), so keep only the 2 numbers with a1
in the second position:01111
and01010
. 
In the third position, there are an equal number of
0
bits and1
bits (one each). So, to find the CO2 scrubber rating, keep the number with a0
in that position:01010
. 
As there is only one number left, stop; the CO2 scrubber rating is
01010
, or10
in decimal.
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 twodimensional array, we will:

Loop through each of the
slots

For the given pair of numbers in each slot, the "bit" is
0
if there are more zeroes, and1
if there are more ones 
Join the bits as a bitstring

Convert the value from binary to decimal (I originally built the number with
reduce
but meh, just useparseInt
which takes a "radix")
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:
Keep only numbers selected by the bit criteria for the type of rating value for which you are searching. Discard numbers which do not match the bit criteria. If you only have one number left, stop; this is the rating value for which you are searching. Otherwise, repeat the process, considering the next bit to the right.The bit criteria depends on which type of rating value you want to find:
To find oxygen generator rating, determine the most common value (0
or1
) in the current bit position, and keep only numbers with that bit in that position. If0
and1
are equally common, keep values with a1
in the position being considered. To find CO2 scrubber rating, determine the least common value (0
or1
) in the current bit position, and keep only numbers with that bit in that position. If0
and1
are equally common, keep values with a0
in the position being considered.
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 there are more zeroes, keep only the numbers with a
0
in positionidx

If there are more ones, keep only the numbers with a
1
in positionidx
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 ifstatement 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
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)
)