Sample program for YAML file patching abilities

src/main/java/io/stargate/sgv2/jsonapi/util/YamlMerger.java

@@ -0,0 +1,105 @@
+package io.stargate.sgv2.jsonapi.util;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import com.fasterxml.jackson.databind.node.ValueNode;
+import com.fasterxml.jackson.dataformat.yaml.YAMLMapper;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Iterator;
+import java.util.Map;
+
+/**
+ * Utility for deep-merging YAML documents with simple, predictable semantics:
+ *
+ * <ul>
+ *   <li>Objects: recursively merged; fields from patch override or extend base
+ *   <li>Arrays: replaced entirely when present in patch
+ *   <li>Scalars (string/number/boolean/null): replaced by patch value
+ * </ul>
+ */
+public final class YamlMerger {
+
+  private final ObjectMapper yamlMapper;
+
+  public YamlMerger() {
+    this.yamlMapper = new YAMLMapper();
+  }
+
+  /** Merge two YAML strings and return the merged YAML string. */
+  public String mergeYamlStrings(String baseYaml, String patchYaml) {
+    try {
+      JsonNode base = yamlMapper.readTree(baseYaml);
+      JsonNode patch = yamlMapper.readTree(patchYaml);
+      JsonNode merged = mergeNodes(base, patch);
+      return yamlMapper.writeValueAsString(merged);
+    } catch (IOException e) {
+      throw new IllegalArgumentException("Failed to merge YAML", e);
+    }
+  }
+
+  /** Merge two YAML input streams and return the merged YAML string. */
+  public String mergeYamlStreams(InputStream baseYaml, InputStream patchYaml) {
+    try {
+      JsonNode base = yamlMapper.readTree(baseYaml);
+      JsonNode patch = yamlMapper.readTree(patchYaml);
+      JsonNode merged = mergeNodes(base, patch);
+      return yamlMapper.writeValueAsString(merged);
+    } catch (IOException e) {
+      throw new IllegalArgumentException("Failed to merge YAML streams", e);
+    }
+  }
+
+  /** Core merge logic following the documented semantics. */
+  public JsonNode mergeNodes(JsonNode base, JsonNode patch) {
+    if (base == null || base.isNull()) {
+      return deepCopy(patch);
+    }
+    if (patch == null) {
+      return deepCopy(base);
+    }
+
+    // If both are objects, merge field-by-field
+    if (base.isObject() && patch.isObject()) {
+      ObjectNode result = base.deepCopy();
+      Iterator<Map.Entry<String, JsonNode>> fields = patch.fields();
+      while (fields.hasNext()) {
+        Map.Entry<String, JsonNode> entry = fields.next();
+        String fieldName = entry.getKey();
+        JsonNode patchValue = entry.getValue();
+        JsonNode baseValue = result.get(fieldName);
+        if (baseValue != null) {
+          JsonNode mergedChild = mergeNodes(baseValue, patchValue);
+          result.set(fieldName, mergedChild);
+        } else {
+          result.set(fieldName, deepCopy(patchValue));
+        }
+      }
+      return result;
+    }
+
+    // If both are arrays, replace entirely with patch (no element-wise merging)
+    if (base.isArray() && patch.isArray()) {
+      return ((ArrayNode) patch).deepCopy();
+    }
+
+    // Otherwise scalars or differing types: patch overrides
+    return deepCopy(patch);
+  }
+
+  private JsonNode deepCopy(JsonNode node) {
+    if (node == null) {
+      return null;
+    }
+    if (node.isObject() || node.isArray()) {
+      return node.deepCopy();
+    }
+    if (node instanceof ValueNode) {
+      return node;
+    }
+    // Default path
+    return node.deepCopy();
+  }
+}