Harden simulation profile snapshots

src/main/java/com/livingworld/debug/SimulationProfileSnapshot.java

@@ -1,7 +1,9 @@
 package com.livingworld.debug;
 
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.StringJoiner;
 
 /**
  * Immutable snapshot of one simulation profiling cycle.
@@ -24,12 +26,26 @@ public record SimulationProfileSnapshot(
         if (eventsPublished < 0 || regionsUpdated < 0 || savesPerformed < 0) {
             throw new IllegalArgumentException("profile counts must not be negative");
         }
-        moduleTimings = Map.copyOf(new LinkedHashMap<>(moduleTimings));
+        LinkedHashMap<String, Long> timingCopy = new LinkedHashMap<>();
+        moduleTimings.forEach((moduleId, timingNanos) -> {
+            if (moduleId == null || moduleId.isBlank()) {
+                throw new IllegalArgumentException("Module IDs must not be blank");
+            }
+            if (timingNanos == null || timingNanos < 0) {
+                throw new IllegalArgumentException("Module timings must be non-negative");
+            }
+            timingCopy.put(moduleId, timingNanos);
+        });
+        moduleTimings = Collections.unmodifiableMap(timingCopy);
     }
 
     public String toHumanReadableString() {
+        StringJoiner formattedTimings = new StringJoiner(", ", "{", "}");
+        moduleTimings.forEach((moduleId, timingNanos) ->
+                formattedTimings.add(moduleId + "=" + timingNanos / 1_000_000.0 + "ms"));
+
         return "cycle=" + (totalCycleNanos / 1_000_000.0) + "ms"
-                + ", modules=" + moduleTimings
+                + ", modules=" + formattedTimings
                 + ", events=" + eventsPublished
                 + ", regions=" + regionsUpdated
                 + ", saves=" + savesPerformed

src/test/java/com/livingworld/debug/SimulationProfileSnapshotTest.java

@@ -0,0 +1,53 @@
+package com.livingworld.debug;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+
+class SimulationProfileSnapshotTest {
+
+    @Test
+    void defensivelyCopiesAndLocksModuleTimings() {
+        Map<String, Long> timings = new LinkedHashMap<>();
+        timings.put("ecology", 2_000_000L);
+
+        SimulationProfileSnapshot snapshot =
+                new SimulationProfileSnapshot(3_000_000L, timings, 2, 3, 1, false);
+        timings.put("settlements", 1_000_000L);
+
+        assertEquals(Map.of("ecology", 2_000_000L), snapshot.moduleTimings());
+        assertThrows(
+                UnsupportedOperationException.class,
+                () -> snapshot.moduleTimings().put("weather", 500_000L));
+    }
+
+    @Test
+    void rejectsInvalidValues() {
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new SimulationProfileSnapshot(-1, Map.of(), 0, 0, 0, false));
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new SimulationProfileSnapshot(0, Map.of("", 1L), 0, 0, 0, false));
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new SimulationProfileSnapshot(0, Map.of("test", -1L), 0, 0, 0, false));
+    }
+
+    @Test
+    void formatsMeasurementsForDebugOutput() {
+        SimulationProfileSnapshot snapshot = new SimulationProfileSnapshot(
+                3_000_000L, Map.of("test", 2_000_000L), 2, 3, 1, true);
+
+        String output = snapshot.toHumanReadableString();
+
+        assertTrue(output.contains("cycle=3.0ms"));
+        assertTrue(output.contains("test=2.0ms"));
+        assertTrue(output.contains("events=2"));
+        assertTrue(output.contains("budgetExceeded=true"));
+    }
+}