lilo-stitcher/LiloStitcher.cs

using Microsoft.Extensions.Caching.Memory;
using NetVips;

namespace lilo_stitcher_console;

public record GenerateRequest(
  string CanvasRect,
  double[] CropOffset,
  double[] CropSize,
  double OutputScale
);

public readonly record struct PlateCoordinate(int Row, int Col)
{
  public static PlateCoordinate Parse(string token)
  {
    var rowPart = new string(token.TakeWhile(char.IsLetter).ToArray()).ToUpperInvariant();
    var colPart = new string(token.SkipWhile(char.IsLetter).ToArray());

    int row = 0;
    foreach (var character in rowPart)
      row = row * 26 + (character - 'A' + 1);

    int.TryParse(colPart, out int col);
    return new PlateCoordinate(row, col);
  }
}

public class TileCache(IMemoryCache cache)
{
  private const long TileBytes = 720L * 720 * 3;

  public Image? Get(string key) => cache.TryGetValue(key, out Image? img) ? img : null;

  public void Set(string key, Image img) =>
    cache.Set(key, img, new MemoryCacheEntryOptions
    {
      Size = TileBytes,
      SlidingExpiration = TimeSpan.FromMinutes(20)
    });
}

public class TileLoader(TileCache cache, string assetDir)
{
  public async Task<Image> LoadAsync(string name, CancellationToken ct)
  {
    if (cache.Get(name) is { } hit)
      return hit;
    
    var image = await Task.Run(() => 
      Image.NewFromFile(Path.Combine(assetDir, $"{name}.png"), access: Enums.Access.Sequential), ct);

    cache.Set(name, image);
    return image;
  }
}

public class LiloStitcher(TileLoader loader)
{
    public async Task<string> CreateImageAsync(GenerateRequest req, CancellationToken ct)
    {
        (int rowMin, int colMin, int rows, int cols) = ParseCanvas(req.CanvasRect);

        Validate(req.CropOffset, nameof(req.CropOffset));
        Validate(req.CropSize,   nameof(req.CropSize));

        double scale = req.OutputScale;
        if (scale <= 0 || scale > 1)
            throw new ArgumentOutOfRangeException(nameof(req.OutputScale));

        var tiles = new List<Image>(rows * cols);
        for( int row = 0; row < rows; row++ )
        {
          for( int col = 0; col < cols; col++ )
          {
            string id = $"{RowName( rowMin + row )}{colMin + col}";
            var tile = await loader.LoadAsync( id, ct );
            if( scale < 1 ) tile = tile.Resize( scale );
            tiles.Add( tile );
          }
        }

        var mosaic = Image.Arrayjoin(tiles.ToArray(), across: cols);

        int offsetX = (int)Math.Truncate(req.CropOffset[0] * mosaic.Width);
        int offsetY = (int)Math.Truncate(req.CropOffset[1] * mosaic.Height);

        int restWidth = mosaic.Width - offsetX;
        int restHeight = mosaic.Height - offsetY;

        int cropWidth = Math.Max(1, (int)Math.Truncate(req.CropSize[0] * restWidth));
        int cropHeight = Math.Max(1, (int)Math.Truncate(req.CropSize[1] * restHeight));

        int cropX = (int)Math.Truncate(offsetX / 2.0 + (restWidth - cropWidth) / 2.0);
        int cropY = (int)Math.Truncate(offsetY / 2.0 + (restHeight - cropHeight) / 2.0);

        var cropRect = mosaic.Crop(cropX, cropY, cropWidth, cropHeight);

        string tmpPath = Path.Combine(Path.GetTempPath(), $"mosaic-{Guid.NewGuid():N}.png");
        cropRect.WriteToFile(tmpPath); 

        return tmpPath;
    }

    private static (int rowMin, int colMin, int rows, int cols) ParseCanvas(string rect)
    {
        var parts = rect.ToUpperInvariant().Split(':', StringSplitOptions.RemoveEmptyEntries);
        var part1 = PlateCoordinate.Parse(parts[0]);
        var part2 = PlateCoordinate.Parse(parts[1]);
        int rowMin = Math.Min(part1.Row, part2.Row); 
        int rowMax = Math.Max(part1.Row, part2.Row);
        int colMin = Math.Min(part1.Col, part2.Col); 
        int colMax = Math.Max(part1.Col, part2.Col);
        return (rowMin, colMin, rowMax - rowMin + 1, colMax - colMin + 1);
    }

    private static void Validate(double[] arr, string name)
    {
        if (arr is null || arr.Length < 2) 
          throw new ArgumentException($"{name} length");
        if (arr.Any(x => x < 0 || x > 1)) 
          throw new ArgumentOutOfRangeException(name);
    }

    static string RowName(int row)
    {
      var stringBuilder = new System.Text.StringBuilder();
      while (row > 0)
      {
        row--;
        stringBuilder.Insert(0, (char)('A' + row % 26));
        row /= 26;
      }
      return stringBuilder.ToString();
    }
}