satellite-provider/SatelliteProvider.Services.TileDownloader/UavTileQualityGate.cs

using Microsoft.Extensions.Options;
using SatelliteProvider.Common.Configs;
using SatelliteProvider.Common.DTO;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;
using SixLabors.ImageSharp.Processing;

namespace SatelliteProvider.Services.TileDownloader;

public interface IUavTileQualityGate
{
    UavTileQualityResult Validate(ReadOnlyMemory<byte> imageBytes, string? contentType, UavTileMetadata metadata);
}

public sealed record UavTileQualityResult(bool Accepted, string? Reason, string? Details)
{
    public static UavTileQualityResult Pass() => new(true, null, null);
    public static UavTileQualityResult Fail(string reason, string? details = null) => new(false, reason, details);
}

public sealed class UavTileQualityGate : IUavTileQualityGate
{
    private static readonly byte[] JpegMagicBytes = { 0xFF, 0xD8, 0xFF };

    private readonly UavQualityConfig _qualityConfig;
    private readonly MapConfig _mapConfig;
    private readonly TimeProvider _timeProvider;

    public UavTileQualityGate(
        IOptions<UavQualityConfig> qualityConfig,
        IOptions<MapConfig> mapConfig,
        TimeProvider? timeProvider = null)
    {
        ArgumentNullException.ThrowIfNull(qualityConfig);
        ArgumentNullException.ThrowIfNull(mapConfig);

        _qualityConfig = qualityConfig.Value;
        _mapConfig = mapConfig.Value;
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    public UavTileQualityResult Validate(ReadOnlyMemory<byte> imageBytes, string? contentType, UavTileMetadata metadata)
    {
        ArgumentNullException.ThrowIfNull(metadata);

        // Rule 1 (Format): content-type AND magic bytes both indicate JPEG.
        if (!HasJpegContentType(contentType) || !HasJpegMagicBytes(imageBytes.Span))
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.InvalidFormat);
        }

        // Rule 2 (Size band): configurable lower/upper bounds.
        if (imageBytes.Length < _qualityConfig.MinBytes || imageBytes.Length > _qualityConfig.MaxBytes)
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.SizeOutOfBand);
        }

        // Rule 3 (Dimensions): strict equality with MapConfig.TileSizePixels.
        // ImageSharp.Image.Identify reads only the JPEG header — no full decode cost.
        ImageInfo? info;
        try
        {
            using var identifyStream = new ReadOnlyMemoryStream(imageBytes);
            info = Image.Identify(identifyStream);
        }
        catch (UnknownImageFormatException)
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.InvalidFormat);
        }
        catch (InvalidImageContentException)
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.InvalidFormat);
        }

        if (info is null || info.Width != _mapConfig.TileSizePixels || info.Height != _mapConfig.TileSizePixels)
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.WrongDimensions);
        }

        // Rule 4 (Captured-at age): forbid future timestamps beyond clock skew and
        // reject anything older than the configured max age.
        var now = _timeProvider.GetUtcNow().UtcDateTime;
        var capturedAt = metadata.CapturedAt.UtcDateTime;

        if (capturedAt > now.AddSeconds(_qualityConfig.CapturedAtFutureSkewSeconds))
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.CapturedAtFuture);
        }

        if (capturedAt < now.AddDays(-_qualityConfig.MaxAgeDays))
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.CapturedAtTooOld);
        }

        // Rule 5 (Blank/uniform): pixel-luminance variance on a downsampled image.
        // Runs last because it requires the full decode pass.
        try
        {
            var variance = ComputeLuminanceVariance(imageBytes);
            if (variance < _qualityConfig.MinLuminanceVariance)
            {
                return UavTileQualityResult.Fail(UavTileRejectReasons.ImageTooUniform);
            }
        }
        catch (UnknownImageFormatException)
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.InvalidFormat);
        }
        catch (InvalidImageContentException)
        {
            return UavTileQualityResult.Fail(UavTileRejectReasons.InvalidFormat);
        }

        return UavTileQualityResult.Pass();
    }

    private static bool HasJpegContentType(string? contentType)
    {
        if (string.IsNullOrWhiteSpace(contentType))
        {
            return false;
        }

        // Allow trailing parameters such as "image/jpeg; charset=binary".
        var separator = contentType.IndexOf(';');
        var mediaType = separator >= 0 ? contentType[..separator].Trim() : contentType.Trim();
        return string.Equals(mediaType, "image/jpeg", StringComparison.OrdinalIgnoreCase);
    }

    private static bool HasJpegMagicBytes(ReadOnlySpan<byte> bytes)
    {
        if (bytes.Length < JpegMagicBytes.Length)
        {
            return false;
        }

        for (var i = 0; i < JpegMagicBytes.Length; i++)
        {
            if (bytes[i] != JpegMagicBytes[i])
            {
                return false;
            }
        }

        return true;
    }

    private double ComputeLuminanceVariance(ReadOnlyMemory<byte> imageBytes)
    {
        using var stream = new ReadOnlyMemoryStream(imageBytes);
        using var image = Image.Load<L8>(stream);

        var sampleSize = Math.Clamp(_qualityConfig.LuminanceSampleSize, 4, _mapConfig.TileSizePixels);
        if (image.Width != sampleSize || image.Height != sampleSize)
        {
            image.Mutate(ctx => ctx.Resize(sampleSize, sampleSize));
        }

        double mean = 0.0;
        double m2 = 0.0;
        long n = 0;

        image.ProcessPixelRows(accessor =>
        {
            for (var y = 0; y < accessor.Height; y++)
            {
                var row = accessor.GetRowSpan(y);
                for (var x = 0; x < row.Length; x++)
                {
                    n++;
                    double value = row[x].PackedValue;
                    var delta = value - mean;
                    mean += delta / n;
                    m2 += delta * (value - mean);
                }
            }
        });

        return n > 1 ? m2 / (n - 1) : 0.0;
    }
}

internal sealed class ReadOnlyMemoryStream : Stream
{
    private readonly ReadOnlyMemory<byte> _memory;
    private int _position;

    public ReadOnlyMemoryStream(ReadOnlyMemory<byte> memory)
    {
        _memory = memory;
    }

    public override bool CanRead => true;
    public override bool CanSeek => true;
    public override bool CanWrite => false;
    public override long Length => _memory.Length;

    public override long Position
    {
        get => _position;
        set => _position = checked((int)value);
    }

    public override void Flush() { }

    public override int Read(byte[] buffer, int offset, int count)
    {
        var span = _memory.Span;
        var remaining = span.Length - _position;
        if (remaining <= 0)
        {
            return 0;
        }

        var toCopy = Math.Min(remaining, count);
        span.Slice(_position, toCopy).CopyTo(buffer.AsSpan(offset, toCopy));
        _position += toCopy;
        return toCopy;
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        _position = origin switch
        {
            SeekOrigin.Begin => checked((int)offset),
            SeekOrigin.Current => checked(_position + (int)offset),
            SeekOrigin.End => checked(_memory.Length + (int)offset),
            _ => throw new ArgumentOutOfRangeException(nameof(origin)),
        };
        return _position;
    }

    public override void SetLength(long value) => throw new NotSupportedException();
    public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
}