Astc tests

AssetRipper.TextureDecoder.Tests/ByteArrayDeviation.cs

@@ -0,0 +1,40 @@
+namespace AssetRipper.TextureDecoder.Tests;
+
+internal static class ByteArrayDeviation
+{
+	internal static void AssertMinimalDeviation(byte[] decoded, byte[] original, double maxMeanDeviation, double maxStandardDeviation)
+	{
+		if (decoded.Length != original.Length)
+		{
+			Assert.Fail($"Differing array lengths\nDecoded length {decoded.Length}\nOriginal length {original.Length}");
+			return;
+		}
+
+		long differenceSum = 0;
+
+		for (int i = 0; i < decoded.Length; i++)
+		{
+			differenceSum += decoded[i] - original[i];
+		}
+
+		double mean = differenceSum / (double)decoded.Length;
+
+		double sumOfSquaredDeviations = 0;
+
+		for (int i = 0; i < decoded.Length; i++)
+		{
+			double deviation = decoded[i] - original[i] - mean;
+			sumOfSquaredDeviations += deviation * deviation;
+		}
+
+		double standardDeviation = Math.Sqrt(sumOfSquaredDeviations / (decoded.Length - 1));
+		//Not sure if Bessel's correction is needed here, but it doesn't hurt, especially since length is around 1 million in the current use.
+
+		Assert.Multiple(() =>
+		{
+			Assert.That(mean, Is.LessThan(maxMeanDeviation), "Mean too far positive");
+			Assert.That(mean, Is.GreaterThan(-maxMeanDeviation), "Mean too far negative");
+			Assert.That(standardDeviation, Is.LessThan(maxStandardDeviation), "Standard deviation too large");
+		});
+	}
+}