Spring pool tests and single source marker

- Only the centerline cell of the first station carries the source feature
- Headwater and styled geometry tests expect the spring pool
- Transect table starts with the first 24 stations

core/src/main/java/art/arcane/iris/engine/hydrology/surface/SurfaceFootprintCompiler.java

@@ -82,7 +82,8 @@ public final class SurfaceFootprintCompiler {
             HydraulicSegment segment = exposed.get(stations.segmentIndex()[column.station()]);
             int flowX = (int) StrictMath.round(centerline.tangentX()[column.station()]);
             int flowZ = (int) StrictMath.round(centerline.tangentZ()[column.station()]);
-            boolean source = column.station() == 0 && column.role() == SurfaceRole.CHANNEL && !column.apron();
+            boolean source = column.station() == 0 && column.role() == SurfaceRole.CHANNEL && !column.apron()
+                    && column.x() == centerline.x()[0] && column.z() == centerline.z()[0];
             int y = column.role() == SurfaceRole.CHANNEL ? column.headY() : column.height();
             HydrologyFeatureRef feature = features.feature(segment, column.role(), source, column.x(), y, column.z(), flowX, flowZ);
             columns.add(new SurfaceLayerColumn(

core/src/test/java/art/arcane/iris/engine/hydrology/HydrologyFootprintCompilerTest.java

@@ -1367,7 +1367,7 @@ public class HydrologyFootprintCompilerTest {
     }
 
     @Test
-    public void surfaceHeadwaterTapersUpToChannelWidthWithoutARadialBasin() {
+    public void surfaceHeadwaterOpensAsASpringPoolThatNarrowsToTheChannel() {
         HydraulicSegment segment = new HydraulicSegment(
                 521L,
                 520L,
@@ -1385,16 +1385,21 @@ public class HydrologyFootprintCompilerTest {
         HydrologyFootprintCompiler compiler = compiler(terrain);
         RiverFootprint footprint = compiler.compile(List.of(course));
 
-        assertTrue(surfaceFluidLayer(footprint, -4, 0) == null);
+        assertTrue(wetHalfWidth(footprint, 0) > wetHalfWidth(footprint, 32));
-        assertTrue(wetHalfWidth(footprint, 0) < wetHalfWidth(footprint, 32));
+        assertTrue(wetHalfWidth(footprint, 0) >= 2 * wetHalfWidth(footprint, 56));
-        assertTrue(wetHalfWidth(footprint, 32) <= wetHalfWidth(footprint, 56));
+        assertTrue(Math.abs(wetHalfWidth(footprint, 32) - wetHalfWidth(footprint, 56)) <= 1);
+        for (int x = 1; x <= 24; x++) {
+            assertTrue(wetHalfWidth(footprint, x) <= wetHalfWidth(footprint, x - 1) + 1);
+        }
         HydrologyColumnLayer source = surfaceFluidLayer(footprint, 0, 0);
         HydrologyColumnLayer cruise = surfaceFluidLayer(footprint, 56, 0);
         assertTrue(source != null);
         assertTrue(cruise != null);
+        assertTrue(source.feature().source());
+        assertFalse(surfaceFluidLayer(footprint, 0, 3).feature().source());
         int sourceDepth = source.fluidHeadY() - source.bedY();
         int cruiseDepth = cruise.fluidHeadY() - cruise.bedY();
-        assertTrue(Math.abs(sourceDepth - cruiseDepth) <= 1);
+        assertTrue(sourceDepth >= cruiseDepth + 1);
     }
 
     @Test

core/src/test/java/art/arcane/iris/engine/hydrology/HydrologyStyledGeometryTest.java

@@ -74,9 +74,11 @@ public class HydrologyStyledGeometryTest {
         RiverCourse surface = courses(first, RiverCourseType.SURFACE).getFirst();
         DrainageNode surfaceSource = first.node(surface.sourceNodeId().orElseThrow()).orElseThrow();
         HydraulicSegment surfaceStart = surface.segments().getFirst();
-        int maximumSurfaceWidth = surfaceSettings.surface().maximumWidth()
+        // The first segment carries the spring pool: wider by the spring ratio and one block deeper.
+        int maximumSurfaceWidth = (int) Math.ceil(surfaceSettings.surface().maximumWidth()
+                * surfaceSettings.surface().banks().springWidthRatio())
                 + (surfaceStart.receivingPool() ? 2 : 0);
-        int maximumSurfaceDepth = surfaceSettings.surface().maximumDepth()
+        int maximumSurfaceDepth = surfaceSettings.surface().maximumDepth() + 1
                 + (surfaceStart.receivingPool() ? 1 : 0);
         assertTrue(surfaceStart.width() >= surfaceSettings.surface().minimumWidth());
         assertTrue(surfaceStart.width() <= maximumSurfaceWidth);

probe/src/main/java/art/arcane/iris/probe/RiverTransectProbe.java

@@ -47,6 +47,7 @@ public final class RiverTransectProbe {
     private static final int MAXIMUM_PLAN_COLUMNS = 6_000_000;
     private static final int MAXIMUM_DETAILS = 16;
     private static final int TABLE_HALF_SECTION = 16;
+    private static final int[] HEADWATER_SECTIONS = {0, 6, 12, 18, 24};
 
     private RiverTransectProbe() {
     }
@@ -495,10 +496,14 @@ public final class RiverTransectProbe {
             IrisComplex complex,
             IrisHydrologyRuntime runtime
     ) throws IOException {
-        int[] stations = sectionStations(centerline.size());
+        int[] spaced = sectionStations(centerline.size());
+        int[] stations = new int[HEADWATER_SECTIONS.length + spaced.length];
+        for (int index = 0; index < HEADWATER_SECTIONS.length; index++) {
+            stations[index] = Math.min(centerline.size() - 1, HEADWATER_SECTIONS[index]);
+        }
+        System.arraycopy(spaced, 0, stations, HEADWATER_SECTIONS.length, spaced.length);
         StringBuilder text = new StringBuilder();
-        for (int section = 0; section < SECTIONS; section++) {
+        for (int station : stations) {
-            int station = stations[section];
             text.append("station ").append(station)
                     .append(" at ").append(centerline.x()[station]).append(',').append(centerline.z()[station])
                     .append(" head=").append(column(complex, runtime, centerline.x()[station], centerline.z()[station]).water())