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JSON Patch (RFC 6902) and JSON Merge Patch (RFC 7386) implementation in Java
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/*
 * Copyright (c) 2014, Francis Galiegue ([email protected])
 *
 * This software is dual-licensed under:
 *
 * - the Lesser General Public License (LGPL) version 3.0 or, at your option, any
 *   later version;
 * - the Apache Software License (ASL) version 2.0.
 *
 * The text of this file and of both licenses is available at the root of this
 * project or, if you have the jar distribution, in directory META-INF/, under
 * the names LGPL-3.0.txt and ASL-2.0.txt respectively.
 *
 * Direct link to the sources:
 *
 * - LGPL 3.0: https://www.gnu.org/licenses/lgpl-3.0.txt
 * - ASL 2.0: http://www.apache.org/licenses/LICENSE-2.0.txt
 */

package com.github.fge.jsonpatch.diff;

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.github.fge.jackson.JacksonUtils;
import com.github.fge.jackson.JsonNumEquals;
import com.github.fge.jackson.NodeType;
import com.github.fge.jackson.jsonpointer.JsonPointer;
import com.github.fge.jsonpatch.JsonPatch;
import com.github.fge.jsonpatch.JsonPatchMessages;
import com.github.fge.msgsimple.bundle.MessageBundle;
import com.github.fge.msgsimple.load.MessageBundles;

import javax.annotation.ParametersAreNonnullByDefault;
import java.io.IOException;
import java.util.*;

/**
 * JSON "diff" implementation
 *
 * This class generates a JSON Patch (as in, an RFC 6902 JSON Patch) given
 * two JSON values as inputs. The patch can be obtained directly as a {@link
 * JsonPatch} or as a {@link JsonNode}.
 *
 * Note: there is no guarantee about the usability of the generated
 * patch for any other source/target combination than the one used to generate
 * the patch.
 *
 * This class always performs operations in the following order: removals,
 * additions and replacements. It then factors removal/addition pairs into
 * move operations, or copy operations if a common element exists, at the same
 * {@link JsonPointer pointer}, in both the source and destination.
 *
 * You can obtain a diff either as a {@link JsonPatch} directly or, for
 * backwards compatibility, as a {@link JsonNode}.
 *
 * @since 1.2
 */
@ParametersAreNonnullByDefault
public final class JsonDiff
{
    private static final MessageBundle BUNDLE
        = MessageBundles.getBundle(JsonPatchMessages.class);
    private static final ObjectMapper MAPPER = JacksonUtils.newMapper();

    private static final JsonNumEquals EQUIVALENCE
        = JsonNumEquals.getInstance();

    private JsonDiff()
    {
    }

    /**
     * Generate a JSON patch for transforming the source node into the target
     * node
     *
     * @param source the node to be patched
     * @param target the expected result after applying the patch
     * @return the patch as a {@link JsonPatch}
     *
     * @since 1.9
     */
    public static JsonPatch asJsonPatch(final JsonNode source,
        final JsonNode target)
    {
        BUNDLE.checkNotNull(source, "common.nullArgument");
        BUNDLE.checkNotNull(target, "common.nullArgument");
        final Map unchanged
            = getUnchangedValues(source, target);
        final DiffProcessor processor = new DiffProcessor(unchanged);

        generateDiffs(processor, JsonPointer.empty(), source, target);
        return processor.getPatch();
    }

    /**
     * Generate a JSON patch for transforming the source node into the target
     * node
     *
     * @param source the node to be patched
     * @param target the expected result after applying the patch
     * @return the patch as a {@link JsonNode}
     */
    public static JsonNode asJson(final JsonNode source, final JsonNode target)
    {
        final String s;
        try {
            s = MAPPER.writeValueAsString(asJsonPatch(source, target));
            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))
            return;

        final NodeType firstType = NodeType.getNodeType(source);
        final NodeType secondType = NodeType.getNodeType(target);

        /*
         * Node types differ: generate a replacement operation.
         */
        if (firstType != secondType) {
            processor.valueReplaced(pointer, source, target);
            return;
        }

        /*
         * If we reach this point, it means that both nodes are the same type,
         * but are not equivalent.
         *
         * If this is not a container, generate a replace operation.
         */
        if (!source.isContainerNode()) {
            processor.valueReplaced(pointer, source, target);
            return;
        }

        /*
         * If we reach this point, both nodes are either objects or arrays;
         * delegate.
         */
        if (firstType == NodeType.OBJECT)
            generateObjectDiffs(processor, pointer, (ObjectNode) source,
                (ObjectNode) target);
        else // array
            generateArrayDiffs(processor, pointer, (ArrayNode) source,
                (ArrayNode) target);
    }

    private static void generateObjectDiffs(final DiffProcessor processor,
        final JsonPointer pointer, final ObjectNode source,
        final ObjectNode target)
    {
        final Set firstFields
                = collect(source.fieldNames(), new TreeSet());
        final Set secondFields
                = collect(target.fieldNames(), new TreeSet());

        final Set copy1 = new HashSet(firstFields);
        copy1.removeAll(secondFields);

        for (final String field: Collections.unmodifiableSet(copy1))
            processor.valueRemoved(pointer.append(field), source.get(field));

        final Set copy2 = new HashSet(secondFields);
        copy2.removeAll(firstFields);


        for (final String field: Collections.unmodifiableSet(copy2))
            processor.valueAdded(pointer.append(field), target.get(field));

        final Set intersection = new HashSet(firstFields);
        intersection.retainAll(secondFields);

        for (final String field: intersection)
            generateDiffs(processor, pointer.append(field), source.get(field),
                target.get(field));
    }

    private static  Set collect(Iterator from, Set to) {
        if (from == null) {
            throw new NullPointerException();
        }
        if (to == null) {
            throw new NullPointerException();
        }
        while (from.hasNext()) {
            to.add(from.next());
        }
        return Collections.unmodifiableSet(to);
    }



    private static void generateArrayDiffs(final DiffProcessor processor,
        final JsonPointer pointer, final ArrayNode source,
        final ArrayNode target)
    {
        final int firstSize = source.size();
        final int secondSize = target.size();
        final int size = Math.min(firstSize, secondSize);

        /*
         * Source array is larger; in this case, elements are removed from the
         * target; the index of removal is always the original arrays's length.
         */
        for (int index = size; index < firstSize; index++)
            processor.valueRemoved(pointer.append(size), source.get(index));

        for (int index = 0; index < size; index++)
            generateDiffs(processor, pointer.append(index), source.get(index),
                target.get(index));

        // Deal with the destination array being larger...
        for (int index = size; index < secondSize; index++)
            processor.valueAdded(pointer.append("-"), target.get(index));
    }


    static Map getUnchangedValues(final JsonNode source,
        final JsonNode target)
    {
        final Map ret = new HashMap();
        computeUnchanged(ret, JsonPointer.empty(), source, target);
        return ret;
    }

    private static void computeUnchanged(final Map ret,
        final JsonPointer pointer, final JsonNode first, final JsonNode second)
    {
        if (EQUIVALENCE.equivalent(first, second)) {
            ret.put(pointer, second);
            return;
        }

        final NodeType firstType = NodeType.getNodeType(first);
        final NodeType secondType = NodeType.getNodeType(second);

        if (firstType != secondType)
            return; // nothing in common

        // We know they are both the same type, so...

        switch (firstType) {
            case OBJECT:
                computeObject(ret, pointer, first, second);
                break;
            case ARRAY:
                computeArray(ret, pointer, first, second);
                break;
            default:
                /* nothing */
        }
    }

    private static void computeObject(final Map ret,
        final JsonPointer pointer, final JsonNode source,
        final JsonNode target)
    {
        final Iterator firstFields = source.fieldNames();

        String name;

        while (firstFields.hasNext()) {
            name = firstFields.next();
            if (!target.has(name))
                continue;
            computeUnchanged(ret, pointer.append(name), source.get(name),
                target.get(name));
        }
    }

    private static void computeArray(final Map ret,
        final JsonPointer pointer, final JsonNode source, final JsonNode target)
    {
        final int size = Math.min(source.size(), target.size());

        for (int i = 0; i < size; i++)
            computeUnchanged(ret, pointer.append(i), source.get(i),
                target.get(i));
    }
}