River bed padding and feature search rings

- surface.bed: falling blocks under and beside river water are replaced by a padding palette unless allowed
- Ring order helper for bounded feature searches

core/src/test/java/art/arcane/iris/core/pack/PackHydrologyValidatorTest.java

@@ -113,6 +113,49 @@ public class PackHydrologyValidatorTest {
         assertTrue(result.errors().toString(), result.errors().isEmpty());
     }
 
+    @Test
+    public void bedRulesRejectBadPaddingAndFluidPaddingBlocks() throws Exception {
+        File pack = pack(riversDimension("""
+                {
+                  "enabled": true,
+                  "routing": {"tileSize": 1024, "sampleSpacing": 64, "oceanOutlets": true},
+                  "surface": {
+                    "bed": {
+                      "allowGravityBlocks": false,
+                      "padding": 9,
+                      "paddingPalette": {"palette": [{"block": "minecraft:water"}]}
+                    }
+                  }
+                }
+                """, ""));
+
+        PackHydrologyValidator.Validation result = validate(pack);
+
+        assertContains(result.errors(), "padding must be at most 8");
+        assertContains(result.errors(), "paddingPalette.palette[0].block must not be a fluid");
+    }
+
+    @Test
+    public void bedRulesAcceptSolidPaddingBlocks() throws Exception {
+        File pack = pack(riversDimension("""
+                {
+                  "enabled": true,
+                  "routing": {"tileSize": 1024, "sampleSpacing": 64, "oceanOutlets": true},
+                  "surface": {
+                    "bed": {
+                      "allowGravityBlocks": true,
+                      "padding": 0,
+                      "paddingPalette": {"palette": [{"block": "minecraft:clay"}, {"block": "minecraft:dirt", "weight": 2}]}
+                    }
+                  }
+                }
+                """, ""));
+
+        PackHydrologyValidator.Validation result = validate(pack);
+
+        assertTrue(result.errors().toString(), result.errors().stream().noneMatch((String error) -> error.contains(".bed")));
+    }
+
     @Test
     public void rejectsMouthLevelingDistanceBeyondTheRouteLength() throws Exception {
         File pack = pack(riversDimension("""

core/src/test/java/art/arcane/iris/engine/hydrology/runtime/HydrologyFeatureSearchTest.java

@@ -0,0 +1,47 @@
+package art.arcane.iris.engine.hydrology.runtime;
+
+import art.arcane.iris.engine.hydrology.HydrologyTileKey;
+import org.junit.Test;
+
+import java.util.HashSet;
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class HydrologyFeatureSearchTest {
+    @Test
+    public void ringZeroIsTheOriginTile() {
+        assertEquals(List.of(new HydrologyTileKey(3, -2)), HydrologyFeatureSearch.ring(3, -2, 0));
+    }
+
+    @Test
+    public void ringsCoverEveryTileAtExactlyTheirChebyshevDistance() {
+        HashSet<HydrologyTileKey> seen = new HashSet<>();
+        for (int radius = 0; radius <= 3; radius++) {
+            List<HydrologyTileKey> ring = HydrologyFeatureSearch.ring(1, 1, radius);
+            assertEquals(radius == 0 ? 1 : radius * 8, ring.size());
+            for (HydrologyTileKey key : ring) {
+                int distance = Math.max(Math.abs(key.tileX() - 1), Math.abs(key.tileZ() - 1));
+                assertEquals(radius, distance);
+                assertTrue(seen.add(key));
+            }
+        }
+        assertEquals(49, seen.size());
+    }
+
+    @Test
+    public void ringLimitCoversTheRequestedDistancePlusPublication() {
+        assertEquals(9, HydrologyFeatureSearch.ringLimit(8192, 1024, 358));
+        assertEquals(1, HydrologyFeatureSearch.ringLimit(0, 1024, 358));
+        assertEquals(0, HydrologyFeatureSearch.ringLimit(0, 1024, 0));
+    }
+
+    @Test
+    public void lowerBoundGrowsWithTheRingAndNeverGoesNegative() {
+        assertEquals(0L, HydrologyFeatureSearch.lowerBound(0, 1024, 358));
+        assertEquals(0L, HydrologyFeatureSearch.lowerBound(1, 1024, 358));
+        assertEquals(666L, HydrologyFeatureSearch.lowerBound(2, 1024, 358));
+        assertEquals(1690L, HydrologyFeatureSearch.lowerBound(3, 1024, 358));
+    }
+}