Possible fix for clay generation.

src/main/java/net/minecraft/server/WorldGenClay.java

@@ -0,0 +1,63 @@
+package net.minecraft.server;
+
+import java.util.Random;
+
+public class WorldGenClay extends WorldGenerator {
+
+    private int a;
+    private int b;
+
+    public WorldGenClay(int i) {
+        this.a = Block.CLAY.id;
+        this.b = i;
+    }
+
+    public boolean a(World world, Random random, int i, int j, int k) {
+        if (world.getMaterial(i, j, k) != Material.WATER) {
+            return false;
+        } else {
+            float f = random.nextFloat() * 3.1415927F;
+            double d0 = (double) ((float) (i + 8) + MathHelper.sin(f) * (float) this.b / 8.0F);
+            double d1 = (double) ((float) (i + 8) - MathHelper.sin(f) * (float) this.b / 8.0F);
+            double d2 = (double) ((float) (k + 8) + MathHelper.cos(f) * (float) this.b / 8.0F);
+            double d3 = (double) ((float) (k + 8) - MathHelper.cos(f) * (float) this.b / 8.0F);
+            double d4 = (double) (j + random.nextInt(3) + 2);
+            double d5 = (double) (j + random.nextInt(3) + 2);
+
+            for (int l = 0; l <= this.b; ++l) {
+                double d6 = d0 + (d1 - d0) * (double) l / (double) this.b;
+                double d7 = d4 + (d5 - d4) * (double) l / (double) this.b;
+                double d8 = d2 + (d3 - d2) * (double) l / (double) this.b;
+                double d9 = random.nextDouble() * (double) this.b / 16.0D;
+                double d10 = (double) (MathHelper.sin((float) l * 3.1415927F / (float) this.b) + 1.0F) * d9 + 1.0D;
+                double d11 = (double) (MathHelper.sin((float) l * 3.1415927F / (float) this.b) + 1.0F) * d9 + 1.0D;
+                int i1 = MathHelper.floor(d6 - d10 / 2.0D);
+                int j1 = MathHelper.floor(d6 + d10 / 2.0D);
+                int k1 = MathHelper.floor(d7 - d11 / 2.0D);
+                int l1 = MathHelper.floor(d7 + d11 / 2.0D);
+                int i2 = MathHelper.floor(d8 - d10 / 2.0D); // CraftBukkit - d6 -> d8
+                int j2 = MathHelper.floor(d8 + d10 / 2.0D); // CraftBukkit - d6 -> d8
+
+                for (int k2 = i1; k2 <= j1; ++k2) {
+                    for (int l2 = k1; l2 <= l1; ++l2) {
+                        for (int i3 = i2; i3 <= j2; ++i3) {
+                            double d12 = ((double) k2 + 0.5D - d6) / (d10 / 2.0D);
+                            double d13 = ((double) l2 + 0.5D - d7) / (d11 / 2.0D);
+                            double d14 = ((double) i3 + 0.5D - d8) / (d10 / 2.0D);
+
+                            if (d12 * d12 + d13 * d13 + d14 * d14 < 1.0D) {
+                                int j3 = world.getTypeId(k2, l2, i3);
+
+                                if (j3 == Block.SAND.id) {
+                                    world.setRawTypeId(k2, l2, i3, this.a);
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+
+            return true;
+        }
+    }
+}