Fix noise's bug when coord is negative (#37)

Author: sunnycase

src/MineCase.Algorithm/Noise/PerlinNoise.cs

@@ -193,7 +193,10 @@ public void AddNoise(float[,,] noise, Vector3 offset, Vector3 scale, float noise
         private static (int integer, float remainder) Split(float value)
         {
             var integer = (int)value;
-            return (integer % 256, value - integer);
+            var remainder = value - integer;
+            integer %= 256;
+            if (integer < 0) integer += 256;
+            return (integer, remainder);
         }
 
         private static float Fade(float t)

tests/UnitTest/NoiseTest.cs

@@ -59,7 +59,7 @@ public void TestOctavedPerlinNoise3D()
             {
                 var noise = new OctavedNoise<PerlinNoise>(new PerlinNoise(100), 8, 1);
                 var noiseValue = new float[xExtent, yExtent, 1];
-                noise.Noise(noiseValue, Vector3.Zero, new Vector3(0.1f, 0.1f, 0));
+                noise.Noise(noiseValue, new Vector3(-10, 10, -10), new Vector3(0.1f, 0.1f, 0));
                 for (int x = 0; x < xExtent; x++)
                 {
                     for (int y = 0; y < yExtent; y++)