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Day 21
I solved part 1 the naive way and was reasonably happy with it. I was successfully using that solution with some negative-modulus junk to brute force part 2 (I could get the correct answer for the example data's smaller number of steps), but there was no way it was completing in a reasonable amount of time or without blowing all my RAM. I was at a complete loss for how to solve part 2 the "smart" way, so I learned from this video: https://www.youtube.com/watch?v=9UOMZSL0JTg I understand why this works now, but I was not going to be getting it on my own (nor did I really want to spend the time it would have taken me to attempt to arrive at a similar solution).
This commit is contained in:
@ -73,6 +73,8 @@
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<EmbeddedResource Include="inputs\20.txt" />
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<EmbeddedResource Include="inputs\20a.txt" />
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<EmbeddedResource Include="inputs\20b.txt" />
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<EmbeddedResource Include="inputs\21.txt" />
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<EmbeddedResource Include="inputs\21a.txt" />
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</ItemGroup>
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<ItemGroup>
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131
inputs/21.txt
Normal file
131
inputs/21.txt
Normal file
@ -0,0 +1,131 @@
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...................................................................................................................................
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.................................................................S.................................................................
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..#.#..#...........#.###...#.......#..........#...#.........#.#....#......#.........#...#....#...#.##....#....#..........##....#...
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......#.#...#............##.......#.......##...............#..................#.#....#..#..#..#..#...#...#...#.....................
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..##.....#..........#...#..#.....#....#.#.....................#...#.................#.....#..#...#.#................#.........##.#.
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.........#..#.##........#...####......#.......#................#...#............#..#.#...........#....#.....#.#....#...###.........
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...#........#....#.....#.#...........#.#.......#....#.............#................#.....#..#..#..#.#.......#...#.....#.....##.....
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............#.#...#.....#..#..........#.....#..........................###.#.......##..#......#...#......#.........##...###.....#..
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.#.#......#.#..............#............#..........#.#....................#......#..#...#....#..#.#......#...............#.#.......
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...................................................................................................................................
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11
inputs/21a.txt
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11
inputs/21a.txt
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@ -0,0 +1,11 @@
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...........
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.....###.#.
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.###.##..#.
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..#.#...#..
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....#.#....
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.##..S####.
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.##..#...#.
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.......##..
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.##.#.####.
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.##..##.##.
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...........
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133
src/21.cs
Normal file
133
src/21.cs
Normal file
@ -0,0 +1,133 @@
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using aoc2023.Util;
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namespace aoc2023;
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internal class Day21 : Day
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{
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private int width;
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private int height;
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private readonly HashSet<ivec2> garden = [];
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private ivec2 start = new(0, 0);
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internal override void Parse()
|
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{
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var lines = Parsing.ReadAllLines($"{GetDay()}").ToList();
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height = lines.Count;
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for (int y = 0; y < height; y++)
|
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{
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width = lines[y].Length;
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for (int x = 0; x < width; x++)
|
||||
{
|
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if (lines[y][x] == '.' || lines[y][x] == 'S')
|
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{
|
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var pt = new ivec2(x, y);
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garden.Add(pt);
|
||||
|
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if (lines[y][x] == 'S')
|
||||
{
|
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start = pt;
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
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private long FillGrid(long startY, long startX, long steps)
|
||||
{
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||||
HashSet<(long, long)> filled = [];
|
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HashSet<(long, long)> visited = [];
|
||||
Queue<(long y, long x, long step)> q = [];
|
||||
q.Enqueue((startY, startX, steps));
|
||||
|
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while (q.TryDequeue(out (long y, long x, long step) currLoc))
|
||||
{
|
||||
if (currLoc.step % 2 == 0)
|
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{
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filled.Add((currLoc.y, currLoc.x));
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}
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||||
|
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if (currLoc.step == 0)
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{
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continue;
|
||||
}
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||||
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foreach (var (newY, newX) in ((long, long c)[]) [(currLoc.y + 1, c: currLoc.x), (currLoc.y - 1, c: currLoc.x), (currLoc.y, currLoc.x + 1), (currLoc.y, currLoc.x - 1)])
|
||||
{
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||||
if (newY < 0 || newY >= height || newX < 0 || newX >= width || !garden.Contains(new ivec2(newY, newX)) ||
|
||||
!visited.Add((newY, newX)))
|
||||
{
|
||||
continue;
|
||||
}
|
||||
|
||||
q.Enqueue((newY, newX, currLoc.step - 1));
|
||||
}
|
||||
}
|
||||
|
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return filled.Count;
|
||||
}
|
||||
|
||||
internal override string Part1()
|
||||
{
|
||||
HashSet<ivec2> next = [start];
|
||||
HashSet<ivec2> reachableNow = [];
|
||||
const int numSteps = 64;
|
||||
for (int step = 0; step < numSteps; step++)
|
||||
{
|
||||
HashSet<ivec2> curr = [..next];
|
||||
next.Clear();
|
||||
foreach (var pos in curr)
|
||||
{
|
||||
foreach (var neighbor in pos.GetBoundedOrthogonalNeighbors(0, 0, width, height))
|
||||
{
|
||||
if (!garden.Contains(neighbor))
|
||||
{
|
||||
continue;
|
||||
}
|
||||
|
||||
if (step == numSteps - 1)
|
||||
{
|
||||
reachableNow.Add(neighbor);
|
||||
}
|
||||
|
||||
next.Add(neighbor);
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
long numVisited = reachableNow.Count;
|
||||
return $"<+white>{numVisited}";
|
||||
}
|
||||
|
||||
internal override string Part2()
|
||||
{
|
||||
const long steps = 26501365;
|
||||
long gridWidth = steps / width - 1;
|
||||
long odd = (long)System.Math.Pow(gridWidth / 2 * 2 + 1, 2);
|
||||
long even = (long)System.Math.Pow((gridWidth + 1) / 2 * 2, 2);
|
||||
|
||||
long oddPoints = FillGrid(start.y, start.x, width * 2 + 1);
|
||||
long eventPoints = FillGrid(start.y, start.x, width * 2);
|
||||
|
||||
long cornerTop = FillGrid(width - 1, start.x, width - 1);
|
||||
long cornerRight = FillGrid(start.y, 0, width - 1);
|
||||
long cornerBottom = FillGrid(0, start.x, width - 1);
|
||||
long cornerLeft = FillGrid(start.y, width - 1, width - 1);
|
||||
|
||||
long smallTopRight = FillGrid(width - 1, 0, width / 2 - 1);
|
||||
long smallTopLeft = FillGrid(width - 1, width - 1, width / 2 - 1);
|
||||
long smallBottomRight = FillGrid(0, 0, width / 2 - 1);
|
||||
long smallBottomLeft = FillGrid(0, width - 1, width / 2 - 1);
|
||||
|
||||
long largeTopRight = FillGrid(width - 1, 0, width * 3 / 2 - 1);
|
||||
long largeTopLeft = FillGrid(width - 1, width - 1, width * 3 / 2 - 1);
|
||||
long largeBottomRight = FillGrid(0, 0, width * 3 / 2 - 1);
|
||||
long largeBottomLeft = FillGrid(0, width - 1, width * 3 / 2 - 1);
|
||||
|
||||
long numVisited = odd * oddPoints +
|
||||
even * eventPoints +
|
||||
cornerTop + cornerRight + cornerBottom + cornerLeft +
|
||||
(gridWidth + 1) * (smallTopRight + smallTopLeft + smallBottomRight + smallBottomLeft) +
|
||||
gridWidth * (largeTopRight + largeTopLeft + largeBottomRight + largeBottomLeft);
|
||||
return $"<+white>{numVisited}";
|
||||
}
|
||||
}
|
||||
Reference in New Issue
Block a user