aoc/2024/06/solution.nix

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{lib, ...}: input: rec {
content = lib.trim input;
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chart = toChart content;
toChart = s: s
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|> lib.strings.splitString "\n"
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|> map (lib.stringToCharacters)
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;
startingPos = let
xs = map (lib.lists.findFirstIndex (char: char == "^") null) chart;
y = lib.lists.findFirstIndex (x: !isNull x) null xs;
x = builtins.elemAt xs y;
in
{inherit x y;}
;
posToString = {x, y}: "${toString x},${toString y}";
addVec = a: b: {x = a.x + b.x; y = a.y + b.y;};
index = i: list: builtins.elemAt list i;
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getChar = {x, y}: chart: chart
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|> index y
|> index x
;
height = builtins.length chart;
width = chart |> index 0 |> builtins.length;
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replace2d = {x, y}: updated: chart: let
newRow = chart |> index y |> replace x updated;
in
replace y newRow chart;
replace = i: updated: list:
let before = lib.sublist 0 i list;
after = lib.sublist (i + 1) (builtins.length list - i) list;
in
if i < 0 || i > builtins.length list then list else
before ++ [updated] ++ after;
progress = {visited ? {}, pos, dir, obs ? [], charts ? chart, ...}: let
newDir = if colliding then rotate dir else dir;
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newPos = addVec pos newDir;
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colliding = nextChar == "#";
nextChar = getChar fwd charts;
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fwd = addVec pos dir;
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hash = posToString pos;
exited = fwd.y >= height || fwd.x >= width || fwd.y < 0 || fwd.x < 0;
canObstruct =
# unique
(!lib.elem (posToString fwd) obs)
# inside map
&& (!exited)
# can't put an obstacle when there already is one
&& (!colliding)
# if I already visited the place in front, putting an obstacle there won't work, as I can't get to where I am now!
&& (!lib.hasAttr (posToString fwd) visited)
# see if I will end up in a loop turning right here
&& getLoop {inherit pos visited dir; charts = replace2d fwd "#" charts;};
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in
{
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visited = visited // {${hash} = visited.${hash} or [] ++ [dir];};
pos = if colliding then pos else newPos;
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dir = newDir;
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inherit exited charts;
obs = obs ++ (if canObstruct then [(posToString fwd)] else []);
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};
rotate = {x, y}: {x = -y; y = x;};
startingDir = {x = 0; y = -1;};
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getVisits = {pos ? startingPos
, dir ? startingDir
, visited ? {}
, obs ? []
, charts ? chart
, ...}:
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let
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nextStep = progress {inherit pos dir visited obs charts;};
in if nextStep.exited then nextStep else getVisits nextStep;
path = getVisits {};
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part1result = path.visited
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|> builtins.attrNames
|> builtins.length
;
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getLoop = state:
let
nextStep = progress state;
hash = posToString state.pos;
enteredLoop = lib.elem state.dir (state.visited.${hash} or []);
in if enteredLoop then true
else if nextStep.exited then false
else getLoop nextStep;
part2result = builtins.trace "This is very slow!" (path.obs |> builtins.length)
;
# visualisation stuff
hashToVec = s: s |> lib.splitString "," |> map lib.strings.toInt |> (v: {x = index 0 v; y = index 1 v;});
annotateObs = chart:
builtins.foldl'
(c: obs: let pos = hashToVec obs; in replace2d pos "O" c) chart path.obs;
annotatePath = chart:
builtins.foldl'
(c: hash: let
pos = hashToVec hash;
dir = index 0 path.visited.${hash};
sym = if dir.x == 1 then ">" else if dir.x == -1 then "<" else if dir.y == 1 then "v" else "^";
in replace2d pos sym c) chart
(builtins.attrNames path.visited);
chartToStr = c: c
|> map (lib.concatStringsSep "")
|> lib.concatStringsSep "\n"
;
annotated = chart |> annotatePath |> annotateObs |> chartToStr;
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# FASTER!
startingObsMap = getObsMap chart;
getObsMap = let
findObs = lib.lists.findFirstIndex (b: b == "#") null;
in chart: chart
|> lib.imap0 (y: line: line
|> lib.imap0 (x: char: if char != "#" then null else {
right = if y + 1 >= height || x + 1 >= width then null else (chart
|> index (y + 1)
|> lib.sublist (x + 1) width
|> findObs
|> maybeAdd (x + 1)
);
left = if y < 1 || x <= 0 then null else (chart
|> index (y - 1)
|> lib.sublist 0 (x)
|> lib.reverseList
|> findObs
|> maybeSub (x - 1)
);
down = if x < 1 then null else (chart
|> lib.sublist (y + 1) (height)
|> map (index (x - 1))
|> findObs
|> maybeAdd (y + 1)
);
up = if x + 1 >= width then null else (searchUp (b: b == "#") {x = x + 1; y = y - 1;} chart);
})
)
;
maybe = f: b: if isNull b then null else f b;
maybeAdd = v: builtins.add v |> maybe;
maybeSub = v: builtins.sub v |> maybe;
searchUp = matcher: {x, y}: chart: chart
|> lib.sublist 0 (y + 1)
|> map (index (x))
|> lib.reverseList
|> lib.lists.findFirstIndex matcher null
|> maybeSub (y)
;
firstOb = {
y = searchUp (i: !isNull i) startingPos startingObsMap;
x = startingPos.x;
};
index2d = {x, y}: m: m |> index y |> index x;
lookRight = obsMap: prev: {
x = (index2d prev obsMap).right;
y = prev.y + 1;
};
lookDown = obsMap: prev: {
x = prev.x - 1;
y = (index2d prev obsMap).down;
};
lookLeft = obsMap: prev: {
x = (index2d prev obsMap).left;
y = prev.y - 1;
};
lookUp = obsMap: prev: {
x = prev.x + 1;
y = (index2d prev obsMap).up;
};
getNextDir = i: [lookRight lookDown lookLeft lookUp]
|> index (lib.mod i 4)
;
getFastpath = {i ? 0, pos ? firstOb, obsMap ? startingObsMap, hist ? []}: let
nextPos = (getNextDir i) obsMap pos;
newHist = hist ++ [point];
point = pos // {dir = lib.mod i 4;};
looped = lib.elem point hist;
in if isNull pos.x || isNull pos.y || looped
then {path = hist ++ [pos]; inherit looped;}
else getFastpath {i = i + 1; pos = nextPos; hist = newHist; inherit obsMap;}
;
fastpath = getFastpath {};
allObs = builtins.genList (i: {
x = lib.mod i width;
y = i / height;
}) (height * width)
|> builtins.filter (pos: index2d pos chart == ".")
|> map ({x, y}: replace2d {inherit x y;} "#" chart)
;
# this is a really slow way to do this, instead just find the adjacent columns and update accordingly
allMaps = allObs |> map getObsMap;
loopyMaps = allMaps
|> builtins.filter (m: (getFastpath {obsMap = m;}).looped);
part2faster = builtins.length loopyMaps;
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}