Complete persistence metadata foundation

src/main/java/com/livingworld/data/saved/SaveMetadata.java

@@ -0,0 +1,44 @@
+package com.livingworld.data.saved;
+
+/**
+ * Immutable metadata for saved game data.
+ *
+ * @param schemaVersion the schema version number (must be > 0)
+ * @param modVersion    the mod version string (must not be blank)
+ * @param createdAt     the timestamp when the save was created
+ * @param updatedAt     the timestamp when the save was last updated
+ */
+public record SaveMetadata(
+        int schemaVersion,
+        String modVersion,
+        long createdAt,
+        long updatedAt
+) {
+
+    /**
+     * Constructs a {@link SaveMetadata} and validates all fields.
+     *
+     * @param schemaVersion the schema version number (must be > 0)
+     * @param modVersion    the mod version string (must not be blank)
+     * @param createdAt     the timestamp when the save was created
+     * @param updatedAt     the timestamp when the save was last updated
+     * @throws IllegalArgumentException if {@code schemaVersion} is not greater than 0
+     * @throws IllegalArgumentException if {@code modVersion} is blank
+     * @throws IllegalArgumentException if {@code updatedAt} is less than {@code createdAt}
+     */
+    public SaveMetadata {
+        if (schemaVersion <= 0) {
+            throw new IllegalArgumentException(
+                    "schemaVersion must be greater than 0, got: " + schemaVersion);
+        }
+
+        if (modVersion == null || modVersion.isBlank()) {
+            throw new IllegalArgumentException("modVersion must not be blank");
+        }
+
+        if (updatedAt < createdAt) {
+            throw new IllegalArgumentException(
+                    "updatedAt (" + updatedAt + ") must be >= createdAt (" + createdAt + ")");
+        }
+    }
+}

src/main/java/com/livingworld/data/serialization/PersistenceWriter.java

@@ -1,56 +1,47 @@
 package com.livingworld.data.serialization;
 
 /**
- * Abstraction for writing save data to a persistence target.
- *
- * <p>This interface hides the underlying storage format (JSON, NBT, etc.) from
- * modules. Modules write their data through this abstraction without knowing
- * how or where it is persisted.</p>
+ * Interface for writing data values with associated keys.
  */
 public interface PersistenceWriter {
 
     /**
-     * Writes a string value with the given key.
+     * Writes a string value.
      *
-     * @param key   the identifier for this value (must not be null or blank)
-     * @param value the string to write
-     * @throws IllegalArgumentException if key is null or blank
+     * @param key   the key identifier
+     * @param value the string value to write
      */
     void writeString(String key, String value);
 
     /**
-     * Writes an integer value with the given key.
+     * Writes an integer value.
      *
-     * @param key   the identifier for this value (must not be null or blank)
-     * @param value the integer to write
-     * @throws IllegalArgumentException if key is null or blank
+     * @param key   the key identifier
+     * @param value the integer value to write
      */
     void writeInt(String key, int value);
 
     /**
-     * Writes a long value with the given key.
+     * Writes a long value.
      *
-     * @param key   the identifier for this value (must not be null or blank)
-     * @param value the long to write
-     * @throws IllegalArgumentException if key is null or blank
+     * @param key   the key identifier
+     * @param value the long value to write
      */
     void writeLong(String key, long value);
 
     /**
-     * Writes a double value with the given key.
+     * Writes a double value.
      *
-     * @param key   the identifier for this value (must not be null or blank)
-     * @param value the double to write
-     * @throws IllegalArgumentException if key is null or blank
+     * @param key   the key identifier
+     * @param value the double value to write
      */
     void writeDouble(String key, double value);
 
     /**
-     * Writes a boolean value with the given key.
+     * Writes a boolean value.
      *
-     * @param key   the identifier for this value (must not be null or blank)
-     * @param value the boolean to write
-     * @throws IllegalArgumentException if key is null or blank
+     * @param key   the key identifier
+     * @param value the boolean value to write
      */
     void writeBoolean(String key, boolean value);
 }

src/test/java/com/livingworld/data/saved/SaveMetadataTest.java

@@ -0,0 +1,40 @@
+package com.livingworld.data.saved;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class SaveMetadataTest {
+
+    @Test
+    void acceptsVersionedChronologicalMetadata() {
+        SaveMetadata metadata = new SaveMetadata(1, "0.1.0", 100L, 200L);
+
+        assertEquals(1, metadata.schemaVersion());
+        assertEquals("0.1.0", metadata.modVersion());
+        assertEquals(100L, metadata.createdAt());
+        assertEquals(200L, metadata.updatedAt());
+    }
+
+    @Test
+    void rejectsInvalidSchemaVersion() {
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new SaveMetadata(0, "0.1.0", 100L, 100L));
+    }
+
+    @Test
+    void rejectsBlankModVersion() {
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new SaveMetadata(1, " ", 100L, 100L));
+    }
+
+    @Test
+    void rejectsUpdateBeforeCreation() {
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new SaveMetadata(1, "0.1.0", 200L, 100L));
+    }
+}