I am a big fan of leet code so when we decided to have a code challenge in our team I thought it wouldn’t be a bad idea to build one which should include pretty basic functionalities like: Ability to register and sign in to the application Present the problem (questions) Provide an interface for the end-users to write, compile and execute code. Overall, it was an amazing learning experience and I am happy to walk through how I did it. Guidelines Before I start working on this I decided to come up with some guidelines so I don't over-engineer this and end up building a Frankenstein out of it, here are few of those guidelines: The whole system should be implemented in 3 to 4 days Reuse as much as you can when it comes to technology or service. build anything new unless until it doesn't exist DON'T standing cost for the infrastructure. Pay for what you use and pay as you can ZERO ONLY MINIMAL Technologies & Runtime Services + JQuery (UI) Bootstrap AWS Lambda AWS API Gateway AWS Dynamodb AWS Dynamodb streams AWS Simple email service (SES) AWS S3 (For static site hosting) High-level Architecture The system consists of four keys functionalities based on which these modules are designed: Registration & sign-in Compile & Execute Score Calculator Winner Predictor Registration & Sign-in Flow We have a single lambda function that takes care of registering a new user and authenticating an existing user in the application. Part of registration flow user enters “FirstName”, “LastName” & “Email address” which is passed in as “QueryParam” to API gateway. Gateway invokes a Lambda function which takes care of saving the user details in DynamoDb, generating a random registration code (UUID based) and sending it in an email to the registered email address The sign-in flow will authenticate the user with the passed in email address and UUID using the Lambda function Compile & Execute When the user submits the code, the web application will call the API gateway with the following parameters: QuestionsId User types code Registration code API gateway will invoke the lambda function which will get information about the question/problem statement from the “Questions” table in Dynamodb (data looks like below). From the table data, we take the “GeneratedCode” attribute value and replace various tokens like — name — , — codeworking — & — testcases — with the class name (generated), the user typed code, and test cases (from columns “Test_1”… “Test_N”). Here is how the replaced code will look: { { { count= ; index= ; ( ){ {
                  s.charAt(index++);
                  count++;
              } (Exception e){ ;
              }
          } count;
      }
   } { actual= ; expected = ;
        String input = ; input = ;
        expected = ;
        actual = Solution.calculate(input); actual == expected :  String.format( , input, expected, actual);
    }
} // --name-- public class f_sdsds_a11223_dd // --codeworking-- static class Solution public static int calculate (String s) int 0 int 0 while true try catch break return public static void main (String args[]) int 0 int 3 null // --testcases-- "Hi1" 3 assert "--helper-- Input %s -> Expected = %s but got = %s" Since Lambda containers don't have JDK installed we have to package “tools.jar” inside the deployment package so we can run the compilation inside the containers, here is the code snippet on how we run compile inside lambda: (StringInputStream inputStream = StringInputStream(data);
     ByteArrayOutputStream outputStream = ByteArrayOutputStream();
     ByteArrayOutputStream errorOutputStream = ByteArrayOutputStream()) {
    Class<?> javacTool = Class.forName( );
    Method create = javacTool.getMethod( );
    JavaCompiler compiler = (JavaCompiler) create.invoke( );
    compiler.run(inputStream, outputStream, errorOutputStream, path.toFile().getAbsolutePath()); ProcessResponse(replaceNewLine(outputStream), replaceNewLine(errorOutputStream));
} (Exception e) { ProcessResponse( , + e.getMessage());
} try new new new "com.sun.tools.javac.api.JavacTool" "create" null return new catch return new null "Compilation error --> " If compilation is successful, we run the generated class which contains the test cases. java -ea -cp /tmp/f_sdsds_a11223_dd If execution is successful without any assertion failures, execution time is stored in “UserSubmission” table. Score Calculator We utilize DynamoDB streams to update the total points and total execution time for any given user. A lambda function listens to all updates done on “UserSubmission” table and performs these calculations once the submission is successful. Winner Predictor To find the winners, we run a simple query to get the top 3 rows in “UserStastics” table generated by sorting “TotalPoints” column in descending order and “TotalExecution” column in ascending Demo Here is the demo of the application: I know this doesn't solve all the use cases that a popular code challenge websites like “leetcode” or “topcoder” solves. But this is a good start & foundation which we can keep adding on. If you like this post then leave a comment. That way, I know you have made it all the way to the end 😀 . Thanks again for reading. Previously published at https://medium.com/javarevisited/build-code-challenge-application-23aa40d183fa

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