A Teacher Looks at Advent of Code 2020 - Day 4

One of the nice things about Advent of Code is that it gets me to explore language features I haven't used yet. Today's problem got me to explore Clojure Spec which is a very cool validation library. There's a complete run through of the solution in Clojure in the video but here I'll talk about the problem in Python (mostly).

Today's problem is about validating passports. You start with a text file consisting of passport information. Each passport is one or more lines with each line having a bunch of key value pairs. For example, these two lines represent a passport for someone who's eye color (ecl) is gray (gry) and who was born (byr) in 1937:

ecl:gry pid:860033327 eyr:2020 hcl:#fffffd
byr:1937 iyr:2017 cid:147 hgt:183cm

The catch is that one passport can span multiple lines and that passports are separated by two consecutive newlines in the file.

A passport has 8 field types with one, Country of Origin (cid) being optional.

For part 1, a valid passport is one that contains all 7 required fields.

The video goes over a Clojure solution which, I think is cleaner but the idea is the same as the Python I'll talk about here.

Splitting the data into a list of potential passports is easy because you can split the string on two newlines:

data = open("../data/sample04.dat").read()
data=data.split("\n\n")

Now we have a list of string.

Next, we can split each string on whitespace so that each string in each sublist is a string in the form k:v:

data_list = []
for d in data:
    data_list.append([item for item in d.split()])

So, for example, data_list[0] might look like this:

['ecl:gry', 'pid:860033327', 'eyr:2020', 'hcl:#fffffd', 'byr:1937', 'iyr:2017', 'cid:147', 'hgt:183cm']

Finally, we can convert each passport into a dictionary:

data_dicts=[]
for d in data_list:
    temp = { item.split(":")[0]:item.split(":")[1] for item in d} 
    temp.pop('cid',None)
    data_dicts.append(temp)

The easiest way I came up with to check if a passport was valid was to make a set out of a list of required field names, make a set out of each potential passports field names (they're dictionary keys) and see if they're equal:

fields = set(["byr","iyr","eyr","hgt","hcl","ecl","pid"])
valid_passports =  [set(x.keys()) == fields for x in data_dicts]

I think the Clojure code is cleaner but it's much the same.

Part two added a twist - you now have to not only see if the required fields are there but you had to make sure they had valid data. For example, height had to start with a positive integer followed by either cm or in. If it was cm, the number had to be in a certain range and if it was in it had to be within a different range.

This didn't sound hard but could get tricky. For each field type you could write a function that took in the value and returned true or false depending on its validity - lots of ad hoc code. You could then loop over all the passports and test to see if all the conditions were met.

It turns out that Clojure has a really cool library - Clojure Spec that does just that. You set up validators for each field type and then one for an entire passport. Here's the code:

(s/def ::byr (s/and string? #(>= (u/parse-int %) 1920) #(<= (u/parse-int %) 2002)))
(s/def ::iyr (s/and string? #(>= (u/parse-int %) 2010) #(<= (u/parse-int %) 2020)))
(s/def ::eyr (s/and string? #(>= (u/parse-int %) 2020) #(<= (u/parse-int %) 2030)))
(s/def ::hgt (s/and string? hgt-test))
(s/def ::hcl (s/and string? #(re-find #"#[0-9a-f]{6}" %)))
(s/def ::ecl (s/and string? #(re-find #"amb|blu|brn|gry|grn|hzl|oth" %)))
(s/def ::pid (s/and string? #(re-find #"^[0-9]{9}$" % )))
(s/def ::cid string?)
(s/def ::passport (s/keys
                   :req [::byr ::iyr ::eyr ::hgt ::hcl ::ecl ::pid]
                   :opt [::cid]))

(s/valid? ::passport test-passport)

The last line would test to see if test-passport was valid. It's all covered in detail in the video.

Clojure spec wasn't required for this problem but I've been meaning to play with it for a while and it led to a clean and elegant way of testing passports.

Not sure if I'll get to more posts or even solve more problems - I'm trying to limit my own screen time over the weekends but we'll see.

If you want to check out all the Clojure goodness here it is: Enjoy!