#30 Ignore fields in JsonDiff

README.md

@@ -132,6 +132,17 @@ final JsonPatch patch = JsonDiff.asJsonPatch(source, target);
 final JsonNode patchNode = JsonDiff.asJson(source, target);
 ```
 
+It's possible to ignore fields in Json Diff. List of ignored fields should be specified as JsonPointer 
+or JsonPath paths. If ignored field does not exist in target or source object, it's ignored.
+
+```java
+final List<String> fieldsToIgnore = new ArrayList<>();
+fieldsToIgnore.add("/id");
+fieldsToIgnore.add("$.cars[-1:]");
+final JsonPatch patch = JsonDiff.asJsonPatch(source, target, fieldsToIgnore);
+final JsonNode patchNode = JsonDiff.asJson(source, target, fieldsToIgnore);
+```
+
 **Important note**: the API offers **no guarantee at all** about patch "reuse";
 that is, the generated patch is only guaranteed to safely transform the given
 source to the given target. Do not expect it to give the result you expect on

src/main/java/com/gravity9/jsonpatch/diff/JsonDiff.java

@@ -30,16 +30,23 @@
 import com.github.fge.msgsimple.bundle.MessageBundle;
 import com.github.fge.msgsimple.load.MessageBundles;
 import com.gravity9.jsonpatch.JsonPatch;
+import com.gravity9.jsonpatch.JsonPatchException;
 import com.gravity9.jsonpatch.JsonPatchMessages;
+import com.gravity9.jsonpatch.JsonPatchOperation;
+import com.gravity9.jsonpatch.RemoveOperation;
+import com.jayway.jsonpath.PathNotFoundException;
+
+import javax.annotation.ParametersAreNonnullByDefault;
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;
-import javax.annotation.ParametersAreNonnullByDefault;
 
 /**
  * JSON "diff" implementation
@@ -96,6 +103,57 @@ public static JsonPatch asJsonPatch(final JsonNode source,
 		return processor.getPatch();
 	}
 
+	/**
+	 * Generate a JSON patch for transforming the source node into the target
+	 * node ignoring given fields
+	 *
+	 * @param source the node to be patched
+	 * @param target the expected result after applying the patch
+	 * @param fieldsToIgnore list of JsonPath or JsonPointer paths which should be ignored when generating diff. Non-existing fields are ignored.
+	 * @return the patch as a {@link JsonPatch}
+	 * @throws JsonPatchException if fieldsToIgnored not in valid JsonPath or JsonPointer format
+	 * @since 2.0.0
+	 */
+	public static JsonPatch asJsonPatchIgnoringFields(final JsonNode source,
+										final JsonNode target, final List<String> fieldsToIgnore) throws JsonPatchException {
+		BUNDLE.checkNotNull(source, "common.nullArgument");
+		BUNDLE.checkNotNull(target, "common.nullArgument");
+		final List<JsonPatchOperation> ignoredFieldsRemoveOperations = getJsonPatchRemoveOperationsForIgnoredFields(fieldsToIgnore);
+
+		JsonNode sourceWithoutIgnoredFields = removeIgnoredFields(source, ignoredFieldsRemoveOperations);
+		JsonNode targetWithoutIgnoredFields = removeIgnoredFields(target, ignoredFieldsRemoveOperations);
+
+		final Map<JsonPointer, JsonNode> unchanged
+				= getUnchangedValues(sourceWithoutIgnoredFields, targetWithoutIgnoredFields);
+		final DiffProcessor processor = new DiffProcessor(unchanged);
+
+		generateDiffs(processor, JsonPointer.empty(), sourceWithoutIgnoredFields, targetWithoutIgnoredFields);
+		return processor.getPatch();
+	}
+
+	private static JsonNode removeIgnoredFields(JsonNode node, List<JsonPatchOperation> ignoredFieldsRemoveOperations) throws JsonPatchException {
+		JsonNode nodeWithoutIgnoredFields = node;
+		for (JsonPatchOperation operation : ignoredFieldsRemoveOperations) {
+			nodeWithoutIgnoredFields = removeIgnoredFieldOrIgnore(nodeWithoutIgnoredFields, operation);
+		}
+		return nodeWithoutIgnoredFields;
+	}
+
+	private static JsonNode removeIgnoredFieldOrIgnore(JsonNode nodeWithoutIgnoredFields, JsonPatchOperation operation) throws JsonPatchException {
+		try {
+			List<JsonPatchOperation> operationsList = new ArrayList<>();
+			operationsList.add(operation);
+			return new JsonPatch(operationsList).apply(nodeWithoutIgnoredFields);
+		} catch (JsonPatchException e) {
+			// If remove for specific path throws PathNotFound, it means that node does not contain specific field which should be ignored.
+			// See more `empty patch if object does not contain ignored field` in diff.json file.
+			if (e.getCause() instanceof PathNotFoundException) {
+				return nodeWithoutIgnoredFields;
+			}
+			throw e;
+		}
+	}
+
 	/**
 	 * Generate a JSON patch for transforming the source node into the target
 	 * node
@@ -114,6 +172,27 @@ public static JsonNode asJson(final JsonNode source, final JsonNode target) {
 		}
 	}
 
+	/**
+	 * Generate a JSON patch for transforming the source node into the target
+	 * node ignoring given fields
+	 *
+	 * @param source the node to be patched
+	 * @param target the expected result after applying the patch
+	 * @param fieldsToIgnore list of JsonPath or JsonPointer paths which should be ignored when generating diff. Non-existing fields are ignored.
+	 * @return the patch as a {@link JsonNode}
+	 * @throws JsonPatchException if fieldsToIgnored not in valid JsonPath or JsonPointer format
+	 * @since 2.0.0
+	 */
+	public static JsonNode asJsonIgnoringFields(final JsonNode source, final JsonNode target, List<String> fieldsToIgnore) throws JsonPatchException {
+		final String s;
+		try {
+			s = MAPPER.writeValueAsString(asJsonPatchIgnoringFields(source, target, fieldsToIgnore));
+			return MAPPER.readTree(s);
+		} catch (IOException e) {
+			throw new RuntimeException("cannot generate JSON diff", e);
+		}
+	}
+
 	private static void generateDiffs(final DiffProcessor processor,
 									  final JsonPointer pointer, final JsonNode source, final JsonNode target) {
 		if (EQUIVALENCE.equivalent(source, target))
@@ -157,24 +236,24 @@ private static void generateObjectDiffs(final DiffProcessor processor,
 											final JsonPointer pointer, final ObjectNode source,
 											final ObjectNode target) {
 		final Set<String> firstFields
-			= collect(source.fieldNames(), new TreeSet<String>());
+			= collect(source.fieldNames(), new TreeSet<>());
 		final Set<String> secondFields
-			= collect(target.fieldNames(), new TreeSet<String>());
+			= collect(target.fieldNames(), new TreeSet<>());
 
-		final Set<String> copy1 = new HashSet<String>(firstFields);
+		final Set<String> copy1 = new HashSet<>(firstFields);
 		copy1.removeAll(secondFields);
 
 		for (final String field : Collections.unmodifiableSet(copy1))
 			processor.valueRemoved(pointer.append(field), source.get(field));
 
-		final Set<String> copy2 = new HashSet<String>(secondFields);
+		final Set<String> copy2 = new HashSet<>(secondFields);
 		copy2.removeAll(firstFields);
 
 
 		for (final String field : Collections.unmodifiableSet(copy2))
 			processor.valueAdded(pointer.append(field), target.get(field));
 
-		final Set<String> intersection = new HashSet<String>(firstFields);
+		final Set<String> intersection = new HashSet<>(firstFields);
 		intersection.retainAll(secondFields);
 
 		for (final String field : intersection)
@@ -222,7 +301,7 @@ private static void generateArrayDiffs(final DiffProcessor processor,
 
 	static Map<JsonPointer, JsonNode> getUnchangedValues(final JsonNode source,
 														 final JsonNode target) {
-		final Map<JsonPointer, JsonNode> ret = new HashMap<JsonPointer, JsonNode>();
+		final Map<JsonPointer, JsonNode> ret = new HashMap<>();
 		computeUnchanged(ret, JsonPointer.empty(), source, target);
 		return ret;
 	}
@@ -278,4 +357,13 @@ private static void computeArray(final Map<JsonPointer, JsonNode> ret,
 			computeUnchanged(ret, pointer.append(i), source.get(i),
 				target.get(i));
 	}
+
+	private static List<JsonPatchOperation> getJsonPatchRemoveOperationsForIgnoredFields(List<String> fieldsToIgnore) {
+		final List<JsonPatchOperation> ignoredFieldsRemoveOperations = new ArrayList<>();
+		for (String fieldToIgnore : fieldsToIgnore) {
+			ignoredFieldsRemoveOperations.add(new RemoveOperation(fieldToIgnore));
+		}
+		return ignoredFieldsRemoveOperations;
+	}
+
 }

src/test/java/com/gravity9/jsonpatch/diff/JsonDiffTest.java

@@ -19,19 +19,23 @@
 
 package com.gravity9.jsonpatch.diff;
 
+import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
 import com.github.fge.jackson.JsonLoader;
 import com.github.fge.jackson.JsonNumEquals;
 import com.google.common.collect.Lists;
 import com.gravity9.jsonpatch.JsonPatch;
 import com.gravity9.jsonpatch.JsonPatchException;
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
 import org.testng.annotations.DataProvider;
 import org.testng.annotations.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 public final class JsonDiffTest {
 
@@ -50,7 +54,9 @@ public Iterator<Object[]> getPatchesOnly() {
 		final List<Object[]> list = Lists.newArrayList();
 
 		for (final JsonNode node : testData)
-			list.add(new Object[]{node.get("first"), node.get("second")});
+			if (!node.has("ignoreFields")) {
+				list.add(new Object[]{node.get("first"), node.get("second")});
+			}
 
 		return list.iterator();
 	}
@@ -73,7 +79,7 @@ public Iterator<Object[]> getLiteralPatches() {
 		final List<Object[]> list = Lists.newArrayList();
 
 		for (final JsonNode node : testData) {
-			if (!node.has("patch"))
+			if (!node.has("patch") || node.has("ignoreFields"))
 				continue;
 			list.add(new Object[]{
 				node.get("message").textValue(), node.get("first"),
@@ -97,4 +103,73 @@ public void generatedPatchesAreWhatIsExpected(final String message,
 				+ "expected: %s\nactual: %s\n", message, expected, actual
 		).isTrue();
 	}
+
+	@DataProvider
+	public Iterator<Object[]> getDiffsWithIgnoredFields() {
+		final List<Object[]> list = Lists.newArrayList();
+
+		for (final JsonNode node : testData) {
+			if (node.has("ignoreFields")) {
+				list.add(new Object[]{
+						node.get("message").textValue(), node.get("first"),
+						node.get("second"), node.get("patch"), node.get("ignoreFields")
+				});
+			}
+		}
+
+		return list.iterator();
+	}
+
+	@Test(
+			dataProvider = "getDiffsWithIgnoredFields"
+	)
+	public void generatedPatchesIgnoreFields(final String message,
+											 final JsonNode first, final JsonNode second, final JsonNode expected,
+											 final JsonNode ignoreFields) throws JsonPatchException {
+
+		final List<String> ignoreFieldsList = new ArrayList<>();
+		final Iterator<JsonNode> ignoreFieldsIterator = ignoreFields.elements();
+		while (ignoreFieldsIterator.hasNext()) {
+			ignoreFieldsList.add(ignoreFieldsIterator.next().textValue());
+		}
+
+		final JsonNode actual = JsonDiff.asJsonIgnoringFields(first, second, ignoreFieldsList);
+
+		assertThat(EQUIVALENCE.equivalent(expected, actual)).overridingErrorMessage(
+				"patch is not what was expected\nscenario: %s\n"
+						+ "expected: %s\nactual: %s\n", message, expected, actual
+		).isTrue();
+	}
+
+	@DataProvider
+	public Iterator<Object[]> getInvalidIgnoreFieldsExpressions() {
+		final List<Object[]> list = Lists.newArrayList();
+		list.add(new Object[]{
+				"$.a[(@.length-1)]", "Could not parse token starting at position 3. Expected ?, ', 0-9, * "
+		});
+		list.add(new Object[]{
+				"/a/?", "Invalid path, `?` are not allowed in JsonPointer expressions."
+		});
+		return list.iterator();
+	}
+
+	@Test(
+			dataProvider = "getInvalidIgnoreFieldsExpressions"
+	)
+	public void shouldNotPerformDiffWhenIgnoreFieldsContainsInvalidExpression(String ignoreFieldsExpression, String expectedExceptionMessage) throws JsonProcessingException {
+		// given
+		JsonNode source = new ObjectMapper().readTree("{\"a\": \"1\"}");
+		JsonNode target = new ObjectMapper().readTree("{\"a\": \"1\"}");
+		List<String> ignoreFields = new ArrayList<>();
+		ignoreFields.add(ignoreFieldsExpression);
+
+		// when
+		assertThatThrownBy(() -> JsonDiff.asJsonIgnoringFields(source, target, ignoreFields))
+				.isExactlyInstanceOf(JsonPatchException.class)
+				.hasMessageStartingWith(expectedExceptionMessage);
+
+		assertThatThrownBy(() -> JsonDiff.asJsonPatchIgnoringFields(source, target, ignoreFields))
+				.isExactlyInstanceOf(JsonPatchException.class)
+				.hasMessageStartingWith(expectedExceptionMessage);
+	}
 }