public void Solve(ReadOnlySpan <byte> input, Solution solution)
{
bool[,] pixels = new bool[50, 6];
bool[] rowBuffer = new bool[50];
bool[] colBuffer = new bool[6];
foreach (ReadOnlySpan <byte> line in input.SplitLines())
{
if (line[1] == 'e') // rect
{
var reader = new SpanReader(line.Slice("rect ".Length));
int width = reader.ReadPosIntUntil('x');
int height = reader.ReadPosIntUntilEnd();
for (int y = 0; y < height; y++)
{
for (int x = 0; x < width; x++)
{
pixels[x, y] = true;
}
}
}
else if (line[7] == 'c') // rotate column
{
var reader = new SpanReader(line.Slice("rotate column x=".Length));
int column = reader.ReadPosIntUntil(' ');
reader.SkipLength("by ".Length);
int rotateAmount = reader.ReadPosIntUntilEnd();
for (int i = 0; i < 6; i++)
{
colBuffer[i] = pixels[column, i];
int target = i - rotateAmount;
if (target < 0)
{
target += 6;
}
// if the target has already been swapped, get it from the buffer
pixels[column, i] = target < i ? colBuffer[target] : pixels[column, target];
}
}
else // rotate row
{
var reader = new SpanReader(line.Slice("rotate row y=".Length));
int row = reader.Peek() - '0';
reader.SkipLength("0 by ".Length);
int rotateAmount = reader.ReadPosIntUntilEnd();
for (int i = 0; i < 50; i++)
{
rowBuffer[i] = pixels[i, row];
int target = i - rotateAmount;
if (target < 0)
{
target += 50;
}
// if the target has already been swapped, get it from the buffer
pixels[i, row] = target < i ? rowBuffer[target] : pixels[target, row];
}
}
}
int part1 = 0;
foreach (bool pixel in pixels)
{
if (pixel)
{
part1++;
}
}
Span <char> part2 = stackalloc char[10];
for (int i = 0; i < 10; i++)
{
int letterPixels = 0;
for (int row = 0; row < 6; row++)
{
for (int col = 0; col < 5; col++)
{
if (pixels[i * 5 + col, row])
{
letterPixels |= 1 << (29 - (row * 5 + col));
}
}
}
part2[i] = OCR.MaskToLetter(letterPixels);
}
solution.SubmitPart1(part1);
solution.SubmitPart2(part2);
}
public void Solve(ReadOnlySpan <byte> input, Solution solution)
{
int foldsIndex = input.IndexOf((byte)'f');
ReadOnlySpan <byte> foldsInput = input.Slice(foldsIndex);
ReadOnlySpan <byte> dotsInput = input.Slice(0, foldsIndex - 1);
Span <int> xFolds = stackalloc int[32];
Span <int> yFolds = stackalloc int[32];
ParseFolds(foldsInput, xFolds, yFolds, out int numXFolds, out int numYFolds, out bool firstFoldIsX);
var firstFoldAxis = firstFoldIsX ? xFolds[0] : yFolds[0];
int maxX = xFolds[0] * 2 + 1;
int maxY = yFolds[0] * 2 + 1;
Span <byte> finalXPositions = stackalloc byte[maxX + 1];
Span <byte> finalYPositions = stackalloc byte[maxY + 1];
for (byte x = 0; x < 40; x++)
{
finalXPositions[x] = x;
}
for (int i = numXFolds - 1; i >= 0; i--)
{
int fold = xFolds[i];
var dst = finalXPositions.Slice(fold + 1, fold);
finalXPositions.Slice(0, fold).CopyTo(dst);
dst.Reverse();
}
for (byte y = 0; y < 6; y++)
{
finalYPositions[y] = y;
}
for (int i = numYFolds - 1; i >= 0; i--)
{
int fold = yFolds[i];
var dst = finalYPositions.Slice(fold + 1, fold);
finalYPositions.Slice(0, fold).CopyTo(dst);
dst.Reverse();
}
Span <int> letterMasks = stackalloc int[8];
var dotsAfterOneFold = new HashSet <int>();
int dotsInputCursor = 0;
while (dotsInputCursor < dotsInput.Length)
{
ParseDot(dotsInput, ref dotsInputCursor, out int x, out int y);
if (firstFoldIsX)
{
if (x > firstFoldAxis)
{
x = 2 * firstFoldAxis - x;
}
}
else
{
if (y > firstFoldAxis)
{
y = 2 * firstFoldAxis - y;
}
}
if (dotsAfterOneFold.Add((x << 16) | y))
{
x = finalXPositions[x];
y = finalYPositions[y];
(int letter, int col) = Math.DivRem(x, 5);
letterMasks[letter] |= 1 << (29 - (y * 5 + col));
}
}
solution.SubmitPart1(dotsAfterOneFold.Count);
Span <char> letters = stackalloc char[8];
for (int i = 0; i < 8; i++)
{
letters[i] = OCR.MaskToLetter(letterMasks[i]);
}
solution.SubmitPart2(letters);
}
