aoc-2023-day14-state-compressor

aoc-2023-day14-state-compressor.cs

enum RockType { None, Round, Cube }

interface IPlatformState
{
	int Width { get; }
	int Height { get; }
	RockType GetRockAt(int row, int col);
}

interface IBitstreamWriter
{
	void Write(uint value, int bitLength);
}

/// <summary>Compress a platform state after an eastward tilt.</summary>
void CompressPlatformState(IPlatformState state, IBitstreamWriter writer)
{
	var h = state.Height;
	var w = state.Width;
	for (int r = 0; r < h; r++)
	{
		uint count = 0;
		uint size = 0;
		uint size_limit = 0;
		int size_bits = 0;
		bool empty = false;
		for (int c = w - 1; c >= 0; --c)
		{
			var rock = state.GetRockAt(r, c);
			switch (rock)
			{
				case RockType.Cube:
					if (size_bits > 0)
					{
						// two horizontally adjacent cubes: ##
						writer.Write(count, size_bits);
						size = size_limit = count = 0; size_bits = 0;
						empty = false; // empty can't be true if size==0
					}
					break;
				case RockType.Round:
					// this cannot happen after a tilt
					if (empty) throw new Exception("invalid platform state");
					++count;
					goto round_or_dot;
				case RockType.None:
					empty = true;
				round_or_dot:
					++size;
					if (size > size_limit)
					{
						if (size == 1) { size_limit = 1; size_bits = 1; }
						else { size_limit <<= 1; size_bits++; }
					}
					break;
			}
		}
		if (size_bits > 0) { writer.Write(count, size_bits); }
	}
}