lilo-stitcher/LiloStitcher.cs

using Microsoft.Extensions.Caching.Memory;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp.Formats.Png;

namespace LiloStitcher;

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 c in rowPart)
            row = row * 26 + (c - '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<Rgba32>? Get(string key) => cache.TryGetValue(key, out Image<Rgba32>? img) ? img : null;

    public void Set(string key, Image<Rgba32> img) =>
        cache.Set(key, img, new MemoryCacheEntryOptions
        {
            Size = TileBytes,
            SlidingExpiration = TimeSpan.FromMinutes(20)
        });
}

public class TileLoader(TileCache cache, string assetDir)
{
    public async Task<Image<Rgba32>> LoadAsync(string name, CancellationToken ct)
    {
        if (cache.Get(name) is { } hit)
            return hit;

        var path = Path.Combine(assetDir, $"{name}.png");
        await using var fs = File.OpenRead(path);
        var image = await Image.LoadAsync<Rgba32>(fs, ct).ConfigureAwait(false);
        cache.Set(name, image);
        return image;
    }
}

public class LiloStitcher(TileLoader loader)
{
    private const int TileSize = 720;

    public async Task<byte[]> CreateImageAsync(GenerateRequest req, CancellationToken ct)
    {
        var parts = req.CanvasRect.ToUpperInvariant()
            .Split(':', StringSplitOptions.TrimEntries | 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);

        int rows = rowMax - rowMin + 1;
        int cols = colMax - colMin + 1;

        var names = Enumerable.Range(rowMin, rows)
            .SelectMany(r => Enumerable.Range(colMin, cols).Select(c => $"{RowName(r)}{c}"))
            .ToArray();

        var bitmaps = await Task.WhenAll(names.Select(n => loader.LoadAsync(n, ct))).ConfigureAwait(false);
        var stitched = new Image<Rgba32>(cols * TileSize, rows * TileSize);

        int idx = 0;
        for (int r = 0; r < rows; r++)
        {
            for (int c = 0; c < cols; c++, idx++)
            {
                stitched.Mutate(ctx => ctx.DrawImage(bitmaps[idx], new Point(c * TileSize, r * TileSize), 1f));
            }
        }

        Validate(req.CropOffset, 2, nameof(req.CropOffset), inclusiveUpper: true);
        Validate(req.CropSize, 2, nameof(req.CropSize), inclusiveUpper: true);

        int offsetX = (int)(req.CropOffset[0] * stitched.Width);
        int offsetY = (int)(req.CropOffset[1] * stitched.Height);
        int cropWidth = (int)(req.CropSize[0] * stitched.Width);
        int cropHeight = (int)(req.CropSize[1] * stitched.Height);

        if (offsetX + cropWidth > stitched.Width) cropWidth = stitched.Width - offsetX;
        if (offsetY + cropHeight > stitched.Height) cropHeight = stitched.Height - offsetY;

        var cropRect = new Rectangle(offsetX, offsetY, cropWidth, cropHeight);

        using var cropped = stitched.Clone(ctx => ctx.Crop(cropRect));

        double scale = Math.Clamp(req.OutputScale, 0.0, 1.0);
        Image<Rgba32> output;
        if (scale < 1.0)
        {
            int width = Math.Max(1, (int)Math.Truncate(cropWidth * scale));
            int height = Math.Max(1, (int)Math.Truncate(cropHeight * scale));
            output = cropped.Clone(ctx => ctx.Resize(width, height));
        }
        else
        {
            output = cropped.Clone();
        }

        using var memStream = new MemoryStream();
        await output.SaveAsync(memStream, new PngEncoder
        {
        }, ct).ConfigureAwait(false);
        return memStream.ToArray();

        static void Validate(double[] arr, int len, string name, bool inclusiveUpper)
        {
            if (arr is null || arr.Length < len)
                throw new ArgumentException($"{name} must have length {len}");
            double upper = inclusiveUpper ? 1.0 : 1.0 - double.Epsilon;
            for (int i = 0; i < len; i++)
            {
                var v = arr[i];
                if (v < 0 || v > upper)
                    throw new ArgumentOutOfRangeException($"{name}[{i}]= {v} outside [0,{upper}]");
            }
        }

        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();
        }
    }
}