# Advent 2019 Day 4

Day 4. Most of the day was spent working on the NY State CS standards to I didn't figure to have much time to work on the problem. Fortunately, I was able to knock out part 1 before work started and part 2 was a quick adjustment when I got back to it at the start of lunch.

Once again, it was a problem with a few interesting teacher side aspects.

I only wrote a Clojure solution today so that's what I'll use for my code examples.

The gist was that you were going to use a 6 digit integer as a password but only subset of the numbers between a start and end point. Only numbers in that range for which these two properties held:

- There were at least one repeated digit, that is 123445 is o since we have two consecutive 4s but 124354 isn't
- The digits are increasing. That is, given any digit, the digit to its right has either the same or a greater value.

You had to find the number of "valid passwords."

At first read this sounds like a math problem but it really isn't. Looking at the first constraint, if you convert the number to a string, looking to see that at least two consecutive characters are the same is pretty easy - it's just a simple loop.

It turns out that it's even easier using Regular Expressions. Regular expressions (regex) aren't usually an explicit part of a CS sequence but man are they useful. Basically they allow you to set up a pattern that will match text. Some examples:

pattern | Description | Matches | doesn't match |
---|---|---|---|

aa[0-9]bb | Matches aa a digit than bb | aa3bb | ax3b, aacbb |

a+bb[a-z] | one or more of a then two b then an a-z | aabbc | aabcb |

You can use parentheses to form a "match group" and then use "\1" to match the group, so this regular expression:

`([0-9])\1`

will match two any substring with two of digits, adjacent. Many regex
engines allow you to use **\d** instead of **[0-9]** as a shorthand for
match a single digit.

Here's the line that will take a list of potential passwords in a
variable **passwords** and return a list that only containsn the
passwords that meet the two consecutive of the same digit rule.

`(filter #(re-seq #"(\d)\1" %) passwords)`

I knew that Python has a **partition** function that would take a string
with a potential password (I convert the number to a string before
doing the regex test above). It would take a string "1245" and convert
it into something like this **(1,2), (2,3), (3,4), (4,5)**. Here's the instruction:

`(map #(partition 2 1 %) passwords)`

Next we test each pair in the partition to see that the first value in the pair is less than or equal to the second one:

`(map #(every? (fn [ [a b]] (<= (int a) (int b))) %) passwrods)`

Pull out the ones where the above is true and count them:

`(count (filter true?))`

And that's part one.

The Python equivalent would be easier to read for the non-lisper but the idea is the same and pretty straightforward.

What I like is that we just solved a number problem without math, just text processing.

Part 2 is more of the same. This time we still need a pair of adjacent same digits but runs of 3 or more didn't count. Now, 122234 wouldn't be a valid password because the run is of 3 while 1222344 would because while the repeated 2s don't count the 4s would.

Fortunately, this is just more text processing:

- Check to make sure the sequence is increasing (as above)
- Remove all the sequences of 3 or more repeats. This can also be
xdone using a regular expression search and replace. In Clojure
it's:
The [0-9]\1\1+ says match any sequence of 3 or more of the same digit and we replace it with an empty string.
`cleaned (string/replace s #"([0-9])\1\1+" "")`

- If the remaining string has a pair of adjacent same digits (as above) its a valid password.

And that's it. Both parts solved as a text processing problem. What I love here is that it seems to be a numeric problem but it has a text based solution. Certainly something worth talking about.

Another thing I thought about as a teacher but didn't pay attention to in my solution was efficiency. My range consisted of about 500,000 values to check. Not huge enough to make a time difference but it's worth noting that there are only about 500 that fulfill the "increasing" requirement but almost 200,000 that have a double digit subsequence.

This means it could be more efficient to look for increasing first and throw away most of the data and then do the digit test. This type of ordering can be a big deal for other problems so could be worth discussing.

Another issue is pipelining. Are you looping over the entire (or even culled) data set over and over or can you take each number and put it through a sequence of tests and/or transformations - the pipeline could also lead to a more efficient solution.

Lots of good teacher fodder.

In closing I'll mention one last thought - not mine but rather a suggestion I found while perusing the Advent of Code subreddit after I had submitted my solutions

Someone noted that since a valid number has increasing digits, if
digits appear more than once they **have** to be adjacent to each
other. If they weren't they'd violate the increasing restriction. This
means that you could solve this problem more numerically by:

- do the "increasing" test
- Find the sum of the counts of each digit value (0 through 9)
- Test to see if at least one digit appears twice.

Based on tomorrows schedule I doubt I'll get to AoC day 5 but we'll see.