Advent of Code (in MiniScript), Day 3

#miniscript #adventofcode #programming

Let's tackle the Day 3 Advent of Code challenge in MiniScript!

Again a long story problem hides a relatively simple task: split each string into two equal halves, and find the one letter that occurs in both halves. Assign a value to that (1 through 26 for lowercase letters, 27 through 52 for uppercase letters), and print out the total.

My MiniScript code for this is nothing fancy... it just loops through the file, doing the task described above.

f = file.open("day3-input.txt")
total = 0
while true
    line = f.readLine
    if line == null then break // end of file
    // split the line into our two compartments
    compA = line[:line.len/2]
    compB = line[line.len/2:]
    // find the item that appears in both
    for c in compA
        if compB.indexOf(c) != null then break
    end for
    // find the "priority" value of this item
    if c >= "a" then
        priority = c.code - "a".code + 1
    else
        priority = c.code - "A".code + 27
    end if
    total = total + priority
    print compA + "/" + compB + " --> " + c + " worth " + priority + ", total " + total
end while

Core MiniScript doesn't have a "contains" function; you check for a substring by using indexOf, which returns null if the substring is not found. So to find the common letter, we iterate over each character of compA, and (using indexOf) see if compB contains the same letter.

Finally, note the use of .code to get the Unicode code point of each letter. That lets us subtract the code point for the start of the alphabet, then add the proper offset for lowercase or uppercase letters, to get the priority value for any letter.

Running this produced a bunch of output like

WsQgstQmvQJnssWsW/PzhRzhBjZBSBRZSnj --> n worth 14, total 8260
qwCNqFwDrrlDr/FPvRhTSPPzLRz --> F worth 32, total 8292
bppqwppCddlvfbDN/VgmMmtMfVVmfmVWW --> f worth 6, total 8298

...and the final total, 8298 in my case, was the right answer.

Part B turns out to be basically the same task, except that instead of dividing each line into two halves, we read groups of three lines at a time, and need to find the letter that occurs in all three. So I just took out my line-splitting code, and instead read in three lines at a time. To find the common letter, we still just iterate over each character in the first line, but now we check that it's contained in both of the other two lines.

f = file.open("day3-input.txt")
total = 0
while true
    // read the next three lines (one "group of elves")
    lineA = f.readLine
    if lineA == null then break // end of file
    lineB = f.readLine
    lineC = f.readLine
    // find the item that appears in all three
    for c in lineA
        if lineB.indexOf(c) != null and lineC.indexOf(c) != null then break
    end for
    // find the "priority" value of this item
    if c >= "a" then
        priority = c.code - "a".code + 1
    else
        priority = c.code - "A".code + 27
    end if
    total = total + priority
    print lineA + "/" + lineB + "/" + lineC + " --> " + c + 
      " worth " + priority + ", total " + total
end while

Running this produced more verbose output (I had to stretch my terminal window out to keep these lines from wrapping!), but the final total, which for me was 2708, was again the right answer.

So! That's the end of the third day, and we're all caught up. The next challenge drops tonight, at midnight in New York. As you've seen, these challenges aren't terribly hard; they're just couched in often confusing story problem. The trick is to see through all the story, and spot the core task, which (so far, at least) is usually pretty straightforward.

So why not give the next one a try on your own? MiniScript is a great language to do it in. Build your skills, have some fun, and play along with literally tens of thousands of others!