Utility function to generate the patch between two objects.

cJSON_Utils.c

@@ -343,7 +343,8 @@ static cJSON *cJSONUtils_SortList(cJSON *list)
 
 void cJSONUtils_SortObject(cJSON *object)	{object->child=cJSONUtils_SortList(object->child);}
 
-cJSON* cJSONUtils_MergePatch(cJSON *target, cJSON *patch) {
+cJSON* cJSONUtils_MergePatch(cJSON *target, cJSON *patch)
+{
 	if (!patch || patch->type != cJSON_Object) {cJSON_Delete(target);return cJSON_Duplicate(patch,1);}
 	if (!target || target->type != cJSON_Object) {cJSON_Delete(target);target=cJSON_CreateObject();}
 
@@ -360,3 +361,35 @@ cJSON* cJSONUtils_MergePatch(cJSON *target, cJSON *patch) {
 	}
 	return target;
 }
+
+cJSON *cJSONUtils_GenerateMergePatch(cJSON *from,cJSON *to)
+{
+	cJSON *patch=0;
+	if (!to) return cJSON_CreateNull();
+	if (to->type!=cJSON_Object || !from || from->type!=cJSON_Object) return cJSON_Duplicate(to,1);
+	cJSONUtils_SortObject(from);
+	cJSONUtils_SortObject(to);
+	from=from->child;to=to->child;
+	patch=cJSON_CreateObject();
+	while (from || to)
+	{
+		int compare=from?(to?strcmp(from->string,to->string):-1):1;
+		if (compare<0)
+		{
+			cJSON_AddItemToObject(patch,from->string,cJSON_CreateNull());
+			from=from->next;
+		}
+		else if (compare>0)
+		{
+			cJSON_AddItemToObject(patch,to->string,cJSON_Duplicate(to,1));
+			to=to->next;
+		}
+		else
+		{
+			if (cJSONUtils_Compare(from,to)) cJSON_AddItemToObject(patch,to->string,cJSONUtils_GenerateMergePatch(from,to));
+			from=from->next;to=to->next;
+		}
+	}
+	if (!patch->child) {cJSON_Delete(patch);return 0;}
+	return patch;
+}

cJSON_Utils.h

@@ -23,6 +23,7 @@ int cJSONUtils_ApplyPatches(cJSON *object,cJSON *patches);	/* Returns 0 for succ
 
 /* Implement RFC7386 (https://tools.ietf.org/html/rfc7396) JSON Merge Patch spec. */
 cJSON* cJSONUtils_MergePatch(cJSON *target, cJSON *patch); /* target will be modified by patch. return value is new ptr for target. */
+cJSON *cJSONUtils_GenerateMergePatch(cJSON *from,cJSON *to); /* generates a patch to move from -> to */
 
 char *cJSONUtils_FindPointerFromObjectTo(cJSON *object,cJSON *target);	/* Given a root object and a target object, construct a pointer from one to the other.	*/
 

test_utils.c

@@ -148,5 +148,18 @@ int main()
 
 		free(before);free(patchtext);free(after);cJSON_Delete(object);cJSON_Delete(patch);
 	}
+	
+	/* Generate Merge tests: */
+	for (i=0;i<15;i++)
+	{
+		cJSON *from=cJSON_Parse(merges[i][0]);
+		cJSON *to=cJSON_Parse(merges[i][2]);
+		cJSON *patch=cJSONUtils_GenerateMergePatch(from,to);
+		from=cJSONUtils_MergePatch(from,patch);
+		char *patchtext=cJSON_PrintUnformatted(patch);
+		char *patchedtext=cJSON_PrintUnformatted(from);
+		printf("Patch [%s] vs [%s] = [%s] vs [%s] (%s)\n",patchtext,merges[i][1],patchedtext,merges[i][2],strcmp(patchedtext,merges[i][2])?"FAIL":"OK");
+		cJSON_Delete(from);cJSON_Delete(to);cJSON_Delete(patch);free(patchtext);free(patchedtext);
+	}
 
 }