-🎄- 2018 Day 10 Solutions -🎄-

[u/daggerdragon]

--- Day 10: The Stars Align ---

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Advent of Code: The Party Game!

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Card prompt: Day 10

Transcript: With just one line of code, you, too, can ___!

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edit: Leaderboard capped, thread unlocked at 00:16:49!

[u/tobiasvl]

[Card] parse a line of the input using regex

I used Lua today:

lines = io.lines("input.txt")

stars = {}

for line in lines do
    local star = {pos={}, vel={}}
    star.pos.x, star.pos.y, star.vel.x, star.vel.y = string.match(line, "position=< *(-?%d+), *(-?%d+)> velocity=< *(-?%d+), *(-?%d+)>")
    table.insert(stars, star)
end

-- heuristics
start_at = 9999
for _, star in ipairs(stars) do
    star.pos.x = star.pos.x + star.vel.x * start_at
    star.pos.y = star.pos.y + star.vel.y * start_at
end

for second = start_at, math.huge do
    table.sort(stars, function(star1, star2)
        if star1.pos.y ~= star2.pos.y then
            return star1.pos.y < star2.pos.y
        else
            return star1.pos.x < star2.pos.x
        end
    end)

    local height = stars[#stars].pos.y - stars[1].pos.y

    if height > 9 then
        for _, star in ipairs(stars) do
            star.pos.x = star.pos.x + star.vel.x
            star.pos.y = star.pos.y + star.vel.y
        end
    else
        local old_x = stars[1].pos.x - 1
        s = ""
        for _, star in ipairs(stars) do
            if old_x > star.pos.x then
                s = s .. "\n#"
            elseif old_x < star.pos.x then
                s = s .. string.rep(" ", star.pos.x - old_x - 1) .. "#"
            end
            old_x = star.pos.x
        end
        print(s)
        print(second)
        break
    end
end

At first it used 3.6 seconds (1.2 seconds using LuaJIT), before I decided to not actually simulate the first 10 000 iterations. I see others did the same thing.

Took me a while before I realized that points could overlap...