patch: allow for adding values to arrays

README.md

@@ -1225,6 +1225,15 @@ Example:
 }
 ```
 
+#### `$add`
+
+The `$add` operation allows for adding a value to an array.
+
+- `$add`
+    - `path` : The [JSON path](https://github.com/json-path/JsonPath#readme) to the array
+    - `value` : The value to add. If the given value is a string, variable placeholders of the form `${...}` will be replaced from the environment variables and the resulting string can be converted by setting value-type.
+    - `value-type` : **optional** [see below](#valuetype)
+
 ### ValueType
 
 One of the following identifiers or can be prefixed with `list of ` to indicate a list of the identified type:

src/main/java/me/itzg/helpers/patch/PatchSetProcessor.java

@@ -17,6 +17,7 @@
 import me.itzg.helpers.env.Interpolator;
 import me.itzg.helpers.env.Interpolator.Result;
 import me.itzg.helpers.env.MissingVariablesException;
+import me.itzg.helpers.patch.model.PatchAddOperation;
 import me.itzg.helpers.patch.model.PatchDefinition;
 import me.itzg.helpers.patch.model.PatchOperation;
 import me.itzg.helpers.patch.model.PatchPutOperation;
@@ -151,6 +152,13 @@ private void applyOps(Map<String, Object> data, List<PatchOperation> ops) throws
                         "put", putOp + " at " + putOp.getPath(),
                         obj -> doc.put(putOp.getPath(), putOp.getKey(), obj)
                 );
+            } else if (op instanceof PatchAddOperation) {
+                final PatchAddOperation addOp = (PatchAddOperation) op;
+                processValueType(
+                        addOp.getValue(), addOp.getValueType(),
+                        "add", addOp + " at " + addOp.getPath(),
+                        obj -> doc.add(addOp.getPath(), obj)
+                );
             }
         }
     }

src/main/java/me/itzg/helpers/patch/model/PatchAddOperation.java

@@ -0,0 +1,25 @@
+package me.itzg.helpers.patch.model;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.annotation.JsonPropertyDescription;
+import com.fasterxml.jackson.databind.JsonNode;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+@EqualsAndHashCode(callSuper = true)
+@Data
+public class PatchAddOperation extends PatchOperation {
+    @JsonProperty(required = true)
+    @JsonPropertyDescription("The JSON path to the array to add to.")
+    String path;
+
+    @JsonProperty(required = true)
+    @JsonPropertyDescription(VALUE_DESCRIPTION)
+    JsonNode value;
+
+    @JsonProperty("value-type")
+    @JsonPropertyDescription(VALUE_TYPE_DESCRIPTION)
+    @JsonInclude(JsonInclude.Include.NON_NULL)
+    String valueType;
+}

src/main/java/me/itzg/helpers/patch/model/PatchOperation.java

@@ -6,7 +6,8 @@
 @JsonTypeInfo(include = JsonTypeInfo.As.WRAPPER_OBJECT, use = JsonTypeInfo.Id.NAME)
 @JsonSubTypes({
         @JsonSubTypes.Type(value = PatchSetOperation.class, name = "$set"),
-        @JsonSubTypes.Type(value = PatchPutOperation.class, name = "$put")
+        @JsonSubTypes.Type(value = PatchPutOperation.class, name = "$put"),
+        @JsonSubTypes.Type(value = PatchAddOperation.class, name = "$add"),
 })
 public abstract class PatchOperation {
     protected static final String VALUE_DESCRIPTION = "The value to set." +

src/test/java/me/itzg/helpers/patch/PatchSetProcessorTest.java

@@ -3,6 +3,7 @@
 import static java.util.Collections.singletonList;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.Mockito.*;
+import static uk.org.webcompere.modelassert.json.JsonAssertions.assertJson;
 import static uk.org.webcompere.modelassert.json.JsonAssertions.assertYaml;
 
 import com.fasterxml.jackson.databind.node.ArrayNode;
@@ -19,6 +20,7 @@
 import java.util.stream.Stream;
 import me.itzg.helpers.env.EnvironmentVariablesProvider;
 import me.itzg.helpers.env.Interpolator;
+import me.itzg.helpers.patch.model.PatchAddOperation;
 import me.itzg.helpers.patch.model.PatchDefinition;
 import me.itzg.helpers.patch.model.PatchPutOperation;
 import me.itzg.helpers.patch.model.PatchSet;
@@ -74,6 +76,31 @@ void setInJson(String file, boolean allowComments, @TempDir Path tempDir) throws
         );
     }
 
+    @Test
+    void addToArray(@TempDir Path tempDir) throws IOException {
+        final Path src = tempDir.resolve("testing.json");
+        Files.copy(Paths.get("src/test/resources/patch/testing-with-array.json"), src);
+
+        final PatchSetProcessor processor = new PatchSetProcessor(
+            new Interpolator(environmentVariablesProvider, "CFG_")
+        );
+
+        processor.process(new PatchSet()
+            .setPatches(singletonList(
+                new PatchDefinition()
+                    .setFile(src.toString())
+                    .setOps(singletonList(
+                        new PatchAddOperation()
+                            .setPath("$.outer.array")
+                            .setValue(new TextNode("new value"))
+                    ))
+            ))
+        );
+
+        assertJson(src)
+            .at("/outer/array/0").hasValue("new value");
+    }
+
     @Test
     void setInJson5(@TempDir Path tempDir) throws IOException {
         final Path src = tempDir.resolve("testing.json5");

src/test/resources/patch/testing-with-array.json

@@ -0,0 +1,5 @@
+{
+  "outer": {
+    "array": []
+  }
+}