av-parser/tests/AvParser.Infrastructure.Tests/Collecting/MediaSignatureTests.cs

using System.Buffers.Binary;
using System.Text;
using AvParser.Core.Collecting;
using AvParser.Infrastructure.Collecting;

namespace AvParser.Infrastructure.Tests.Collecting;

/// <summary>Builds the smallest byte sequences that are still legitimately each format.</summary>
internal static class Samples
{
    private static readonly byte[] PngSignature = [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A];

    public static byte[] Jpeg() => [0xFF, 0xD8, 0xFF, 0xE0, 0x00, 0x10, .. "JFIF\0"u8, .. new byte[24]];

    public static byte[] Png(int width = 640, int height = 480)
    {
        var bytes = new List<byte>(PngSignature);
        bytes.AddRange(Chunk("IHDR", Ihdr(width, height)));
        bytes.AddRange(Chunk("IDAT", new byte[8]));

        return [.. bytes];
    }

    public static byte[] Apng(int width = 64, int height = 64)
    {
        var bytes = new List<byte>(PngSignature);
        bytes.AddRange(Chunk("IHDR", Ihdr(width, height)));
        // acTL must precede IDAT to count; a decoder ignores it afterwards and so do we.
        bytes.AddRange(Chunk("acTL", [0, 0, 0, 2, 0, 0, 0, 0]));
        bytes.AddRange(Chunk("IDAT", new byte[8]));

        return [.. bytes];
    }

    /// <summary>A PNG carrying acTL only after IDAT — a still image with junk appended.</summary>
    public static byte[] PngWithLateAnimationChunk()
    {
        var bytes = new List<byte>(PngSignature);
        bytes.AddRange(Chunk("IHDR", Ihdr(8, 8)));
        bytes.AddRange(Chunk("IDAT", new byte[8]));
        bytes.AddRange(Chunk("acTL", [0, 0, 0, 2, 0, 0, 0, 0]));

        return [.. bytes];
    }

    public static byte[] Gif(int frames = 1, int width = 12, int height = 34, bool modern = true)
    {
        var bytes = new List<byte>(Encoding.ASCII.GetBytes(modern ? "GIF89a" : "GIF87a"));

        bytes.AddRange(LittleEndian16(width));
        bytes.AddRange(LittleEndian16(height));
        bytes.Add(0x00); // no global colour table, which keeps the offsets simple
        bytes.Add(0x00);
        bytes.Add(0x00);

        for (var frame = 0; frame < frames; frame++)
        {
            bytes.Add(0x2C);
            bytes.AddRange(LittleEndian16(0));
            bytes.AddRange(LittleEndian16(0));
            bytes.AddRange(LittleEndian16(width));
            bytes.AddRange(LittleEndian16(height));
            bytes.Add(0x00); // no local colour table
            bytes.Add(0x02); // LZW minimum code size
            bytes.Add(0x03); // one sub-block of three bytes
            bytes.AddRange([0x01, 0x02, 0x03]);
            bytes.Add(0x00); // sub-block terminator
        }

        bytes.Add(0x3B);

        return [.. bytes];
    }

    /// <summary>An animated GIF whose frames are separated by graphic control extensions.</summary>
    public static byte[] GifWithExtensions()
    {
        var bytes = new List<byte>(Encoding.ASCII.GetBytes("GIF89a"));
        bytes.AddRange(LittleEndian16(4));
        bytes.AddRange(LittleEndian16(4));
        bytes.Add(0x80 | 0x01); // global colour table, 2^(1+1) = 4 entries
        bytes.Add(0x00);
        bytes.Add(0x00);
        bytes.AddRange(new byte[3 * 4]);

        for (var frame = 0; frame < 2; frame++)
        {
            bytes.Add(0x21);
            bytes.Add(0xF9);
            bytes.Add(0x04);
            bytes.AddRange([0x00, 0x0A, 0x00, 0x00]);
            bytes.Add(0x00);

            bytes.Add(0x2C);
            bytes.AddRange(LittleEndian16(0));
            bytes.AddRange(LittleEndian16(0));
            bytes.AddRange(LittleEndian16(4));
            bytes.AddRange(LittleEndian16(4));
            bytes.Add(0x00);
            bytes.Add(0x02);
            bytes.Add(0x02);
            bytes.AddRange([0x01, 0x02]);
            bytes.Add(0x00);
        }

        bytes.Add(0x3B);

        return [.. bytes];
    }

    public static byte[] WebpStill()
    {
        var body = new List<byte>("WEBP"u8.ToArray());
        body.AddRange("VP8 "u8.ToArray());
        body.AddRange(LittleEndian32(16));
        body.AddRange(new byte[16]);

        return Riff(body);
    }

    public static byte[] WebpLossless()
    {
        var body = new List<byte>("WEBP"u8.ToArray());
        body.AddRange("VP8L"u8.ToArray());
        body.AddRange(LittleEndian32(16));
        body.AddRange(new byte[16]);

        return Riff(body);
    }

    public static byte[] WebpExtended(bool animated, int width = 100, int height = 50)
    {
        var body = new List<byte>("WEBP"u8.ToArray());
        body.AddRange("VP8X"u8.ToArray());
        body.AddRange(LittleEndian32(10));
        body.Add((byte)(animated ? 0x02 : 0x10)); // ANIM, versus a plain alpha flag
        body.AddRange([0x00, 0x00, 0x00]);
        body.AddRange(LittleEndian24(width - 1));
        body.AddRange(LittleEndian24(height - 1));
        body.AddRange(new byte[8]);

        return Riff(body);
    }

    public static byte[] Mp4(string brand = "isom") =>
        [0x00, 0x00, 0x00, 0x20, .. "ftyp"u8, .. Encoding.ASCII.GetBytes(brand), .. new byte[20]];

    public static byte[] Avif() => Mp4("avif");

    public static byte[] WebM() => Ebml("webm");

    public static byte[] Matroska() => Ebml("matroska");

    public static byte[] Html(string body = "<!DOCTYPE html><html><body>Image not found</body></html>") =>
        Encoding.UTF8.GetBytes(body);

    private static byte[] Ebml(string docType)
    {
        var bytes = new List<byte> { 0x1A, 0x45, 0xDF, 0xA3 };
        bytes.AddRange([0x9F, 0x42, 0x86, 0x81, 0x01]); // a plausible-looking EBML header preamble
        bytes.Add(0x42);
        bytes.Add(0x82);
        bytes.Add((byte)docType.Length);
        bytes.AddRange(Encoding.ASCII.GetBytes(docType));
        bytes.AddRange(new byte[16]);

        return [.. bytes];
    }

    private static byte[] Riff(List<byte> body)
    {
        var bytes = new List<byte>("RIFF"u8.ToArray());
        bytes.AddRange(LittleEndian32(body.Count));
        bytes.AddRange(body);

        return [.. bytes];
    }

    private static byte[] Ihdr(int width, int height)
    {
        var data = new byte[13];
        BinaryPrimitives.WriteUInt32BigEndian(data.AsSpan(0, 4), (uint)width);
        BinaryPrimitives.WriteUInt32BigEndian(data.AsSpan(4, 4), (uint)height);
        data[8] = 8;

        return data;
    }

    private static byte[] Chunk(string type, byte[] data)
    {
        var bytes = new List<byte>();
        var length = new byte[4];
        BinaryPrimitives.WriteUInt32BigEndian(length, (uint)data.Length);

        bytes.AddRange(length);
        bytes.AddRange(Encoding.ASCII.GetBytes(type));
        bytes.AddRange(data);
        bytes.AddRange(new byte[4]); // CRC, never checked here

        return [.. bytes];
    }

    private static byte[] LittleEndian16(int value) => [(byte)(value & 0xFF), (byte)((value >> 8) & 0xFF)];

    private static byte[] LittleEndian24(int value) =>
        [(byte)(value & 0xFF), (byte)((value >> 8) & 0xFF), (byte)((value >> 16) & 0xFF)];

    private static byte[] LittleEndian32(int value) =>
        [(byte)(value & 0xFF), (byte)((value >> 8) & 0xFF), (byte)((value >> 16) & 0xFF), (byte)((value >> 24) & 0xFF)];
}

public class MediaSignatureTests
{
    [Fact]
    public void Jpeg_is_recognised() => MediaSignatures.Detect(Samples.Jpeg()).ShouldBe(MediaKind.Jpeg);

    [Fact]
    public void Png_is_recognised() => MediaSignatures.Detect(Samples.Png()).ShouldBe(MediaKind.Png);

    [Theory]
    [InlineData(true)]
    [InlineData(false)]
    public void Both_gif_versions_are_recognised(bool modern) =>
        MediaSignatures.Detect(Samples.Gif(modern: modern)).ShouldBe(MediaKind.Gif);

    [Fact]
    public void Webp_is_recognised() => MediaSignatures.Detect(Samples.WebpStill()).ShouldBe(MediaKind.WebP);

    [Theory]
    [InlineData("isom")]
    [InlineData("mp42")]
    [InlineData("avc1")]
    public void Mp4_brands_are_recognised(string brand) =>
        MediaSignatures.Detect(Samples.Mp4(brand)).ShouldBe(MediaKind.Mp4);

    [Fact]
    public void Avif_is_told_apart_from_mp4() => MediaSignatures.Detect(Samples.Avif()).ShouldBe(MediaKind.Avif);

    [Fact]
    public void WebM_is_recognised() => MediaSignatures.Detect(Samples.WebM()).ShouldBe(MediaKind.WebM);

    [Fact]
    public void Matroska_is_not_mistaken_for_webm()
    {
        // Both open with the same EBML magic; only the DocType separates them.
        MediaSignatures.Detect(Samples.Matroska()).ShouldBe(MediaKind.Unknown);
    }

    [Fact]
    public void Anything_unrecognised_is_unknown()
    {
        MediaSignatures.Detect(Samples.Html()).ShouldBe(MediaKind.Unknown);
        MediaSignatures.Detect(new byte[32]).ShouldBe(MediaKind.Unknown);
        MediaSignatures.Detect([]).ShouldBe(MediaKind.Unknown);
        MediaSignatures.Detect([0xFF]).ShouldBe(MediaKind.Unknown);
    }

    [Fact]
    public void A_truncated_prefix_never_throws()
    {
        var png = Samples.Png();

        for (var length = 0; length < png.Length; length++)
        {
            _ = MediaSignatures.Detect(png.AsSpan(0, length));
            _ = MediaSignatures.DetectAnimation(MediaKind.Png, png.AsSpan(0, length));
            _ = MediaSignatures.ReadDimensions(MediaKind.Png, png.AsSpan(0, length));
        }
    }

    [Theory]
    [InlineData("<!DOCTYPE html><html>")]
    [InlineData("<html><body>gone</body></html>")]
    [InlineData("   \n<!doctype HTML>")]
    [InlineData("<?xml version=\"1.0\"?>")]
    public void An_error_page_behind_a_200_is_spotted(string body) =>
        MediaSignatures.LooksLikeHtml(Samples.Html(body)).ShouldBeTrue();

    [Fact]
    public void Real_media_is_not_mistaken_for_html()
    {
        MediaSignatures.LooksLikeHtml(Samples.Png()).ShouldBeFalse();
        MediaSignatures.LooksLikeHtml(Samples.Gif()).ShouldBeFalse();
        MediaSignatures.LooksLikeHtml(Samples.Jpeg()).ShouldBeFalse();
    }

    [Fact]
    public void A_single_frame_gif_does_not_animate() =>
        MediaSignatures.DetectAnimation(MediaKind.Gif, Samples.Gif(frames: 1)).ShouldBeFalse();

    [Fact]
    public void A_two_frame_gif_animates() =>
        MediaSignatures.DetectAnimation(MediaKind.Gif, Samples.Gif(frames: 2)).ShouldBeTrue();

    [Fact]
    public void A_gif_with_control_extensions_between_frames_animates()
    {
        // Real animated GIFs interleave graphic control extensions and use a global colour table;
        // walking past both is where a naive scanner goes wrong.
        MediaSignatures.DetectAnimation(MediaKind.Gif, Samples.GifWithExtensions()).ShouldBeTrue();
    }

    [Fact]
    public void A_plain_png_does_not_animate() =>
        MediaSignatures.DetectAnimation(MediaKind.Png, Samples.Png()).ShouldBeFalse();

    [Fact]
    public void An_apng_animates() => MediaSignatures.DetectAnimation(MediaKind.Png, Samples.Apng()).ShouldBeTrue();

    [Fact]
    public void An_animation_chunk_after_the_first_idat_does_not_count()
    {
        // Decoders ignore acTL once IDAT has started, so claiming animation here would be a lie.
        MediaSignatures.DetectAnimation(MediaKind.Png, Samples.PngWithLateAnimationChunk()).ShouldBeFalse();
    }

    [Fact]
    public void A_plain_webp_does_not_animate()
    {
        MediaSignatures.DetectAnimation(MediaKind.WebP, Samples.WebpStill()).ShouldBeFalse();
        MediaSignatures.DetectAnimation(MediaKind.WebP, Samples.WebpLossless()).ShouldBeFalse();
        MediaSignatures.DetectAnimation(MediaKind.WebP, Samples.WebpExtended(animated: false)).ShouldBeFalse();
    }

    [Fact]
    public void An_extended_webp_with_the_anim_flag_animates() =>
        MediaSignatures.DetectAnimation(MediaKind.WebP, Samples.WebpExtended(animated: true)).ShouldBeTrue();

    [Fact]
    public void Png_dimensions_are_read()
    {
        var (width, height) = MediaSignatures.ReadDimensions(MediaKind.Png, Samples.Png(1920, 1080));

        width.ShouldBe(1920);
        height.ShouldBe(1080);
    }

    [Fact]
    public void Gif_dimensions_are_read()
    {
        var (width, height) = MediaSignatures.ReadDimensions(MediaKind.Gif, Samples.Gif(width: 320, height: 240));

        width.ShouldBe(320);
        height.ShouldBe(240);
    }

    [Fact]
    public void Extended_webp_dimensions_are_read()
    {
        // Stored as canvas-minus-one, which is the kind of off-by-one that ships unnoticed.
        var (width, height) = MediaSignatures.ReadDimensions(
            MediaKind.WebP,
            Samples.WebpExtended(animated: true, width: 800, height: 600)
        );

        width.ShouldBe(800);
        height.ShouldBe(600);
    }

    [Fact]
    public void Dimensions_are_absent_rather_than_guessed()
    {
        MediaSignatures.ReadDimensions(MediaKind.Jpeg, Samples.Jpeg()).ShouldBe((null, null));
        MediaSignatures.ReadDimensions(MediaKind.WebP, Samples.WebpStill()).ShouldBe((null, null));
    }

    [Fact]
    public void Garbage_never_throws()
    {
        var random = new Random(20260813);

        for (var attempt = 0; attempt < 500; attempt++)
        {
            var bytes = new byte[random.Next(0, 200)];
            random.NextBytes(bytes);

            var kind = MediaSignatures.Detect(bytes);
            _ = MediaSignatures.DetectAnimation(kind, bytes);
            _ = MediaSignatures.ReadDimensions(kind, bytes);
            _ = MediaSignatures.LooksLikeHtml(bytes);
        }
    }
}