Advent of Code 2023 - Day Two

Advent Calendar.


Round Two, Fight!

Day Two was a cakewalk for the most part (aside from a few minutes scratching my head as I got my minimum/maximum backward).

I always do that. I always mess up some mundane detail

Here's my solution - we'll break down parts one and two in this post. GitHub

Part 1 - A Little Data Cleanup and a Hashtable

So the basics of the problem were you played a game - the game involved a bag of colored cubes (red, green, blue) and an elf pulls out a handful of cubes three times during the game. The elf asks you "If there were only supposed to be a maximum of 12 red, 13 green, and 14 blue cubes in the bag, which of the games am I dirty rotten filthy cheater" (some embellishment on my part maybe).

You've got a list of the games played and the cubes drawn... some games maybe the elf drew 14 red in one draw which would mean he was a filthy cheater. We know how many cubes should be in the bag. The elf wants to know the games that they didn't cheat on, so add up the game number if the game was fair. This is fairly simple with a fun little PowerShell trick:

 1$maxValue = @{
 2    red = 12
 3    green = 13
 4    blue = 14
 7$sum = 0
 8:outer for($i = 1; $i -le $puzzleInput.Count; $i++)
10    $gameId = $i
11    $pulls = ($puzzleInput[($i-1)])[(7 + ([string]$i).Length)..($puzzleInput[($i-1)].Length)] -join '' -split '; '
12    foreach($pull in $pulls)
13    {
14        $colors = $pull -split ", "
15        foreach($color in $colors)
16        {
17            $number, $colorid = $color -split ' '
18            if($maxValue[$colorid] -lt [int]$number){continue outer}
19        }
20    }
21    $sum += $gameId

The basic premise is - we'll create a hashtable with the maximum values for each color, then loop through each game (and each draw within the game - there are three per game). If we ever encounter a condition where the drawn number is greater than the max value, we throw the cubes back at them and continue on to the next game without checking the remaining draws (if any).

Since we're nesting loops, and we're checking inside the nest, it's best to just continue the outer loop (skip the remainder of that loop iteration and start with the next time through the loop). In PowerShell we can accomplish this by labeling our loop - I conveniently called the outer/parent loop :outer. Then when we reach a condition we want to continue on, we just call continue outer. Because we're adding the game id to the $sum var at the end of the :outer loop, it conveniently skips that step unless we never reach the condition of the elf cheating.

The other little "trick" being used here, is instead of doing some sort of switch statement on the color, we're referencing the max value in the hashtable for that color: $maxValue[$colorid] to keep our code clean.

Easy peasy, even if the elf is a scoundrel.

Part 2 - Where We Assume The Elf Isn't Cheating

Okay, so maybe the elf was wrong about the number of cubes in the bag, but he forgot how many were in there. Now he wants to know for each game what is the minimum number of cubes that had to be in the bag for that game to be possible. I think he wants to know because he wants me to take the knife away from his throat after swindling me out of a thousand bucks.

We'll use a hashtable again, but this time set it up for each game. Since he wants us to do multiplication of the colors for some odd reason, we'll give each color a default minimum of 1 (so that if a one color is never pulled during the game it doesn't throw off our multiplication table - 1 times anything is still just the anything).

 1$sum = 0
 2for($i = 1; $i -le $puzzleInput.Count; $i++)
 4    $pulls = ($puzzleInput[($i-1)])[(7 + ([string]$i).Length)..($puzzleInput[($i-1)].Length)] -join '' -split '; '
 5    $lowestValue = @{
 6        red = 1
 7        green = 1
 8        blue = 1
 9    }
10    foreach($pull in $pulls)
11    {
12        $colors = $pull -split ", "
13        foreach($color in $colors)
14        {
15            $number, $colorid = $color -split ' '
16            if($lowestValue[$colorid] -lt [int]$number){$lowestValue[$colorid] = [int]$number}
17        }
18    }
19    $gamePower = $lowestValue['red']*$lowestValue['blue']*$lowestValue['green']
20    $sum += $gamePower

We do basically the same thing here except this time instead of referencing the value for lookup, we'll also update the value with a new minimum if he drew more than the current known minimum. For example - if he drew 4 red cubes, but so far I've only seen him draw 2 red cubes, the new minimum value for the red cubes this game is 4. We don't have to continue on the loop this time because we do want to evaluate every draw/pull. After we evaluate each draw for the game then we multiply the minimum number of cubes for each color together and add it to the sum. I'm told this number will help me get to some destination unknown, but I'm convinced he just is asking for gibberish now.

Closing Thoughts

Closing Thought 1: Why didn't I use Regex to manipulate the data?
Meh... I'm lazy and doing some good old fashioned string manipulation seemed easy to me for today. Regex probably would have been cleaner than the nightmare that this is: ($puzzleInput[($i-1)])[(7 + ([string]$i).Length)..($puzzleInput[($i-1)].Length)] -join '' -split '; '

Closing Thought 2: You mentioned making a mundane mistake... what was it?
When I was doing part two, I had originally set the hashtable to [int]::MaxValue and when I found a lower value I updated it. All this accomplished was telling me the lowest number pulled/grabbed for each color during a game. I needed the max value pulled during a game. Twas a silly mistake.

Happy Scripting!