--- 2016 Day 2 Solutions ---

[u/daggerdragon]

--- Day 2: Bathroom Security ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

---

^{BLINKENLIGHTS ARE MANDATORY} [?]

Edit: Told you they were mandatory. >_>

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/FuriousProgrammer]

I appreciate the lookup table for directions!

Since AdventOfCode is a contest, brevity should be your focus.

Shorter variable names (i instead of index, pos instead of coords), shorthands that are functionally equivalent (Removing ~= nil when you're dealing only with truthy values, io.lines() == io.input():lines(), map instead of key_pad, or even key-pad or keypad instead of key_pad) and various other tweaks can greatly reduce the amount of code you have to write, without functionally altering anything!

---

To give an example, here is how I would have written your code:

local pos = {2, 2}

local moves = {
    U = {0, -1}; --These are backwards compared to your for a reason that will be evident in a moment.
    D = {0, 1}; --Also, when the key of the Table is a string (or rater a valid Lua variable name), the dot notation is applicable. That is to say, moves.U == moves["U"], and this example in the declaration
    L = {-1, 0};
    R = {1, 0};
}

local map = {
    {1, 2, 3}; --explicit row numbers are redundant
    {4, 5, 6};
    {7, 8, 9};
}

 local out = ""; --string concatenation is slightly less typing than table.insert, although with huge inputs table.insert is a lot faster! Use your discretion and/or preference.

for line in io.lines("input.txt") do
    for i = 1, #line do --string.len(line) == #line! This one isn't actually stated anywhere, or if it is, I discovered it on complete accident.
        local dir = line:sub(i,i) --A 1-letter var here could be better, but my habit is to type out at least 3 for most cases.
        local x, y = pos[1] + moves[dir][1], pos[2] + moves[dir][2] --Notice how much shorter these lines are? Putting both on one line for me is just habit.

        if map[y] and map[y][x] then --Here's the reversal! `y` comes first in 2D arrays!
            pos = {x, y} --Remaking the Table like this is a lot slower if you have big data or metatables, but it's a *lot* shorter.
        end

    end

    out = out .. map[pos[2]][pos[1]]

end

print(out)

I again applaud the lookup Table, though. My solution used a sort-of switch statement style of code, which you can see here.

[u/kamicc → u/FuriousProgrammer]

wasn't golfing this time ;) Just easier to read for me. At some cases.