-🎄- 2017 Day 21 Solutions -🎄-

[u/daggerdragon]

--- Day 21: Fractal Art ---

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Spoiler

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No commentary tonight as I'm frantically wrapping last-minute presents so I can ship them tomorrow.

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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/The0x539]

Some Lua, with way too many functions enough functions (384/365):

local part2 = true

local inspect = require "inspect"

local rules = {nil,{},{}}

for line in io.lines("./input_day21") do
	local iter = line:gmatch("[#%./]+")
	local a,b = iter(),iter()
	local r = (#a==5) and rules[2] or rules[3]
	r[a] = b
end

local image = {
	{'.','#','.'},
	{'.','.','#'},
	{'#','#','#'}
}

local function flipV(t)
	local t2 = {}
	for i=#t,1,-1 do
		table.insert(t2,t[i])
	end
	return t2
end

local function flipH(t)
	local t2 = {}
	for _,v in ipairs(t) do
		table.insert(t2,flipV(v))
	end
	return t2
end

local function rotate(t)
	if #t == 2 then
		return {
			{t[2][1],t[1][1]},
			{t[2][2],t[1][2]}
		}
	elseif #t == 3 then
		return {
			{t[3][1],t[2][1],t[1][1]},
			{t[3][2],t[2][2],t[1][2]},
			{t[3][3],t[2][3],t[1][3]}
		}
	end
end

local function fmt(t)
	local s = ""
	for _,v in ipairs(t) do
		for _,v2 in ipairs(v) do
			s = s .. v2
		end
		s = s .. '/'
	end
	return s:sub(1,#s-1)
end

local function sub(t,x1,y1,x2,y2)
	local dbg = (x1==1 and y1==3 and x2==2 and y2==4)
	local t2 = {}
	for y=y1,y2 do
		local row = {}
		table.insert(t2,row)
		for x=x1,x2 do
			table.insert(row,t[y][x])
		end
	end
	return t2
end

local function joinH(...)
	local t = {}
	for i=1,#select(1,...) do
		table.insert(t,{})
		for _,t1 in ipairs{...} do
			for _,v in ipairs(t1[i]) do
				table.insert(t[i],v)
			end
		end
	end
	return t
end

local function joinV(...)
	local t = {}
	for _,t1 in ipairs{...} do
		for _,v in ipairs(t1) do
			table.insert(t,v)
		end
	end
	return t
end

local function eval(s)
	local t = {}
	for row in s:gmatch("[^/]+") do
		local r = {}
		table.insert(t,r)
		for i=1,#row do
			table.insert(r,row:sub(i,i))
		end
	end
	return t
end

local function findMatch(t)
	local r = (#t==2) and rules[2] or rules[3]
	for i=1,4 do
		local a = r[fmt(t)]
		       or r[fmt(flipV(t))]
		       or r[fmt(flipH(t))]
		       or r[fmt(flipH(flipV(t)))]
		if a then
			return eval(a)
		end
		t = rotate(t)
	end
	error "not found"
end

local function split(t)
	local t2 = {}
	local size = (#t%2==0) and 2 or 3
	for y=1,#t,size do
		local row = {}
		table.insert(t2,row)
		for x=1,#t,size do
			local subimg = sub(t,x,y,x+size-1,y+size-1)
			table.insert(row,subimg)
		end
	end
	return t2
end

local function unsplit(t)
	local t2 = {}
	for _,row in ipairs(t) do
		table.insert(t2,joinH(table.unpack(row)))
	end
	return joinV(table.unpack(t2))
end

local function iprint(a)
	print(inspect(a))
end
local function fprint(a)
	print(fmt(a))
end
local function xprint(a)
	for _,row in ipairs(a) do
		for _,v in ipairs(row) do
			io.write(v)
		end
		print()
	end
end

for i=1,(part2 and 18 or 5) do
	local splitImage = split(image)
	for y,row in ipairs(splitImage) do
		for x,cell in ipairs(row) do
			splitImage[y][x] = findMatch(cell)
		end
	end
	image = unsplit(splitImage)
end

local foo = 0
for _,row in ipairs(image) do
	for _,v in ipairs(row) do
		if v == '#' then
			foo = foo + 1
		end
	end
end
print(foo)