2024 Day 9

Author: premun

src/2024/09/Program.cs

@@ -1,6 +1,118 @@
 using AdventOfCode.Common;
 
-var lines = Resources.GetInputFileLines();
+int[] diskMap = Resources.GetInputFileLines().First()
+    .Select(c => c - '0')
+    .ToArray();
 
-Console.WriteLine($"Part 1: {""}");
+var forwardStream = new BlockStream(diskMap);
+var backwardStream = new ReverseBlockStream(diskMap);
+
+ulong checksum = 0UL;
+var next = forwardStream.GetNext()!;
+var last = backwardStream.GetNext()!;
+int position = 0;
+
+var debug = true;
+
+while (next.Id < last.Id)
+{
+    while (next is FileBlock)
+    {
+        checksum += (ulong)(next.Id * position);
+        position++;
+        //Console.WriteLine($"{next.Id} * {position}");
+        if (debug) Console.Write((char)(next.Id + '0'));
+        next = forwardStream.GetNext();
+    }
+
+    while (last is FreeBlock)
+    {
+        last = backwardStream.GetNext();
+    }
+
+    checksum += (ulong)(last.Id * position);
+    position++;
+    //Console.WriteLine($"{last.Id} * {position}");
+    if (debug) Console.Write((char)(last.Id + '0'));
+
+    next = forwardStream.GetNext();
+    last = backwardStream.GetNext();
+}
+
+if (debug) Console.Write('.');
+
+// We ended up somewhere in a single file block, we need to process it
+if (next is FileBlock)
+{
+    // TODO: This could be better - we could make the pointers meet inside the block
+    var sum = 0UL;
+    for (int i = 0; i < diskMap.Length; i += 2)
+        sum += (ulong)diskMap[i];
+
+    for (ulong i = (ulong)position; i < sum; ++i)
+    {
+        checksum += (ulong)(last.Id * position);
+        position++;
+        if (debug) Console.Write((char)(last.Id + '0'));
+    }
+}
+
+Console.WriteLine();
+
+Console.WriteLine($"Part 1: {checksum}");
 Console.WriteLine($"Part 2: {""}");
+
+file class BlockStream
+{
+    private readonly IEnumerator<int> _diskMap;
+    protected int _id;
+    private int _bit;
+    private bool _currentBlockIsFile = true;
+
+    public BlockStream(IEnumerable<int> diskMap)
+    {
+        _diskMap = diskMap.GetEnumerator();
+        _id = 0;
+        _diskMap.MoveNext();
+        _bit = _diskMap.Current;
+    }
+
+    public Block GetNext()
+    {
+        while (_bit == 0)
+        {
+            if (!_diskMap.MoveNext())
+            {
+                return new FreeBlock(Id, int.MaxValue);
+            }
+
+            _currentBlockIsFile = !_currentBlockIsFile;
+            _bit = _diskMap.Current;
+
+            if (_currentBlockIsFile)
+            {
+                _id++;
+            }
+        }
+
+        _bit--;
+        return _currentBlockIsFile
+            ? new FileBlock(Id, _diskMap.Current)
+            : new FreeBlock(Id, _diskMap.Current);
+    }
+
+    protected virtual int Id => _id;
+}
+
+file class ReverseBlockStream(int[] diskMap)
+    : BlockStream(diskMap.Reverse())
+{
+    private readonly int _maxId = diskMap.Length / 2;
+
+    protected override int Id => _maxId - _id;
+    public override int Rest => _bit;
+}
+
+abstract file record Block(int Id, int Size);
+file record FileBlock(int Id, int Size) : Block(Id, Size);
+file record FreeBlock(int Id, int Size) : Block(Id, Size);