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Day09.cs
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Day09.cs
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using AdventOfCode.CSharp.Common;
using System;
using System.Numerics;
namespace AdventOfCode.CSharp.Y2021.Solvers;
public class Day09 : ISolver
{
public static void Solve(ReadOnlySpan<byte> input, Solution solution)
{
int width = input.IndexOf((byte)'\n');
int height = input.Length / (width + 1);
Span<int> rowBasins = stackalloc int[width];
Span<int> basins = stackalloc int[4096]; // assume a max of 4096 basins
int basinCount = 0;
int cursor = 0;
// first row
int prevBasin = -1;
for (int x = 0; x < width; x++)
{
int locationHeight = input[cursor++] - '0';
if (locationHeight == 9)
{
rowBasins[x] = -1;
prevBasin = -1;
continue;
}
if (prevBasin == -1)
{
prevBasin = basinCount++;
basins[prevBasin] = (1 << (locationHeight + 16)) | 1;
}
else
{
basins[prevBasin] = (1 << (locationHeight + 16)) | (basins[prevBasin] + 1);
}
rowBasins[x] = prevBasin;
}
cursor++; // skip newline
for (int y = 1; y < height; y++)
{
prevBasin = -1;
for (int x = 0; x < width; x++)
{
int locationHeight = input[cursor++] - '0';
if (locationHeight == 9)
{
rowBasins[x] = -1;
prevBasin = -1;
continue;
}
int prevRowBasin = rowBasins[x];
if (prevBasin == -1)
{
if (prevRowBasin == -1)
{
prevBasin = basinCount++;
basins[prevBasin] = (1 << (locationHeight + 16)) | 1;
}
else
{
int prevRowBasinCount;
while ((prevRowBasinCount = basins[prevRowBasin]) < 0)
prevRowBasin = -prevRowBasinCount;
basins[prevRowBasin] = (1 << (locationHeight + 16)) | (prevRowBasinCount + 1);
prevBasin = prevRowBasin;
}
}
else
{
if (prevRowBasin == -1)
{
basins[prevBasin] = (1 << (locationHeight + 16)) | (basins[prevBasin] + 1);
}
else
{
// merge scenario
int prevRowBasinCount;
while ((prevRowBasinCount = basins[prevRowBasin]) < 0)
prevRowBasin = -prevRowBasinCount;
// if they are different, then merge the basins
if (prevRowBasin != prevBasin)
{
var prevBasinCount = basins[prevBasin];
int basinCombinedHeights = (prevBasinCount | prevRowBasinCount | (1 << (locationHeight + 16))) & 0x7FFF0000;
int basinCombinedCounts = (prevBasinCount + prevRowBasinCount + 1) & 0xFFFF;
basins[prevBasin] = basinCombinedHeights | basinCombinedCounts;
basins[prevRowBasin] = -prevBasin;
}
else
{
basins[prevBasin] = (1 << (locationHeight + 16)) | (prevRowBasinCount + 1);
}
}
}
rowBasins[x] = prevBasin;
}
cursor++;
}
int riskLevelSum = 0;
int max1 = 0;
int max2 = 0;
int max3 = 0;
for (int i = 0; i < basinCount; i++)
{
var basin = basins[i];
if (basin > 0)
{
int basinLowestHeight = BitOperations.TrailingZeroCount(basin >> 16);
riskLevelSum += basinLowestHeight + 1;
int basinSize = basin & 0xFFFF;
if (basinSize < max3)
continue;
if (basinSize < max2)
{
max3 = basinSize;
continue;
}
max3 = max2;
if (basinSize < max1)
{
max2 = basinSize;
}
else
{
max2 = max1;
max1 = basinSize;
}
}
}
solution.SubmitPart1(riskLevelSum);
solution.SubmitPart2(max1 * max2 * max3);
}
}