Prefer longer routes when ranking river sources

routing.lengthPreference adds route length to the source score so outlets take their longest legal course.

core/src/main/java/art/arcane/iris/core/pack/PackHydrologyValidator.java

@@ -278,6 +278,7 @@ final class PackHydrologyValidator {
         PackJsonFieldChecks.validateOptionalDoubleRange(path, routing, "uphillPenalty", 0D, 128D, errors);
         PackJsonFieldChecks.validateOptionalDoubleRange(path, routing, "slopePenalty", 0D, 16D, errors);
         PackJsonFieldChecks.validateOptionalDoubleRange(path, routing, "confluenceAttraction", 0D, 1D, errors);
+        PackJsonFieldChecks.validateOptionalDoubleRange(path, routing, "lengthPreference", 0D, 8D, errors);
 
         int tileSize = integerValue(routing, "tileSize", 2048);
         int sampleSpacing = integerValue(routing, "sampleSpacing", 64);

core/src/main/java/art/arcane/iris/engine/hydrology/HydrologyPlanner.java

@@ -2113,9 +2113,10 @@ public final class HydrologyPlanner {
                 addSourceDiagnostic(node, surface, stable, HydrologyCandidateRejection.COURSE_TOO_SHORT, diagnostics);
                 continue;
             }
+            // Elevation and route length share the same scale: 1000 route blocks count as 100 blocks of height.
             double score = (required ? 1.0E15D : 0D)
                     + weight * 1.0E9D
-                    + terrain.naturalHeight() * 1.0E5D
+                    + (terrain.naturalHeight() + routeLength * 0.1D * settings.routing().lengthPreference()) * 1.0E5D
                     + routing.potential()[node.index()]
                     + HydrologyHash.unit(stable);
             candidates.add(new SourceCandidate(node.index(), stable, score, required));

core/src/main/java/art/arcane/iris/engine/hydrology/HydrologyPlannerSettings.java

@@ -35,7 +35,7 @@ public record HydrologyPlannerSettings(
         Source undergroundSources = new Source(true, 0.25D, Integer.MIN_VALUE, 0, 1, 512);
         return new HydrologyPlannerSettings(
                 63,
-                new Routing(2048, 64, 8192, 8192, 384, 192, 1.5D, 24D, 2D, 0.2D),
+                new Routing(2048, 64, 8192, 8192, 384, 192, 1.5D, 24D, 2D, 0.2D, 1D),
                 new Surface(true, surfaceSources, 4, 8, 2, 4, 10, 1.5D, Banks.defaults()),
                 new Hydraulics(8),
                 new Underground(true, undergroundSources, -48, 72, 3, 8, 1, 3, 6, 14, true),
@@ -151,7 +151,8 @@ public record HydrologyPlannerSettings(
             double valleyPreference,
             double uphillPenalty,
             double slopePenalty,
-            double confluenceAttraction
+            double confluenceAttraction,
+            double lengthPreference
     ) {
         public Routing {
             if (tileSize < 32 || sampleSpacing < 4 || tileSize % sampleSpacing != 0) {
@@ -177,6 +178,7 @@ public record HydrologyPlannerSettings(
             requireFiniteNonNegative(uphillPenalty, "uphillPenalty");
             requireFiniteNonNegative(slopePenalty, "slopePenalty");
             requireFiniteNonNegative(confluenceAttraction, "confluenceAttraction");
+            requireFiniteNonNegative(lengthPreference, "lengthPreference");
         }
 
         // Route refinement is derived from the lattice, never authored.

core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRuntime.java

@@ -671,7 +671,8 @@ public final class IrisHydrologyRuntime implements AutoCloseable {
                 routing.getValleyPreference(),
                 routing.getUphillPenalty(),
                 routing.getSlopePenalty(),
-                routing.getConfluenceAttraction()
+                routing.getConfluenceAttraction(),
+                routing.getLengthPreference()
         );
         HydrologyPlannerSettings.Source plannerSurfaceSources = new HydrologyPlannerSettings.Source(
                 surfaceEnabled,

core/src/main/java/art/arcane/iris/engine/object/IrisRiverRoutingConfig.java

@@ -70,4 +70,9 @@ public class IrisRiverRoutingConfig {
     @MaxNumber(1)
     @Desc("How strongly routes are drawn toward existing drainage.")
     private double confluenceAttraction = 0.2D;
+
+    @MinNumber(0)
+    @MaxNumber(8)
+    @Desc("How strongly longer source-to-outlet routes are preferred when choosing river sources; 0 ranks by elevation alone.")
+    private double lengthPreference = 1D;
 }