nouritsu/src/main/java/nouritsu/Nouritsu.java

package nouritsu;

import fi.iki.elonen.NanoHTTPD;
import fi.iki.elonen.NanoHTTPD.Response.Status;
import io.vavr.Tuple;
import io.vavr.Tuple2;
import io.vavr.collection.HashMap;
import io.vavr.collection.HashSet;
import io.vavr.collection.Map;
import io.vavr.control.Either;
import io.vavr.control.Try;
import nouritsu.types.ListWithRest;
import nouritsu.types.Resp;
import nouritsu.types.ShoppingItem;
import nouritsu.types.Store;

import java.io.IOException;
import java.util.function.BiFunction;
import java.util.function.Function;

public class Nouritsu extends NanoHTTPD {
  private final Dao dao;

  public Nouritsu(int port, Dao dao) throws IOException {
    super(port);
    start(10_000, false);
    this.dao = dao;
  }

  @Override
  public Response serve(IHTTPSession session) {
    return Try.of(() -> processRequest(session, dao))
      .fold(
        // This won’t be exposed to the outside world, so returning the error message is fine
        err -> {
          err.printStackTrace();
          return newFixedLengthResponse(Status.INTERNAL_ERROR, MIME_PLAINTEXT, err.getMessage());
        },
        response -> newFixedLengthResponse(response.status(), "application/json", response.text())
      );
  }

  private static final String HELP = """
    Valid endpoints are (not case sensitive):
    /addStore/?name=storeName&sections=first,second,third
    /addItem/?name=itemName&section=itemSection
    /addToList/?user=userId&item=itemName
    /getList/?user=userId
    /getListForStore/?user=userId&store=StoreName
    /clearList/?user=userId
    """;

  private static Resp processRequest(IHTTPSession session, Dao dao) {
    // Type inference can’t handle this :feelsBadMan:
    final BiFunction<Map<String, String>, Dao, Resp> handler =
      switch (session.getUri().replaceAll("/", "").toLowerCase()) {
        case "addstore" -> Nouritsu::addStore;
        case "additem" -> Nouritsu::addItem;
        case "addtolist" -> Nouritsu::addToList;
        case "getlist" -> Nouritsu::getList;
        case "getlistforstore" -> Nouritsu::getListOrdered;
        case "clearlist" -> Nouritsu::clearList;
        default -> (_1, _2) -> Resp.NOT_FOUND.apply(HELP);
      };
    return handler.apply(extractParams(session), dao);
  }

  // Repack the parameters into a vavr Map<Key, Head(Value)>.
  // Each parameter should only occur once per request.
  private static Map<String, String> extractParams(IHTTPSession session) {
    return session.getParameters()
      .entrySet()
      .stream()
      .map(Tuple::fromEntry)
      .collect(HashMap.collector())
      // guaranteed to be non-empty
      .mapValues(list -> list.get(0));
  }

  private static Resp clearList(Map<String, String> params, Dao dao) {
    return getFromParams(params, "user")
      .map(dao::clearList)
      .fold(Function.identity(), Resp.CREATED);
  }

  private static Resp getList(Map<String, String> params, Dao dao) {
    return getFromParams(params, "user")
      .flatMap(dao::getList)
      .map(Serde::serialize)
      .fold(Function.identity(), Resp.OK);
  }

  private static Resp getListOrdered(Map<String, String> params, Dao dao) {
    return getAsTuple(params, "user", "store")
      .flatMap(t -> t.apply((userId, storeName) ->
        zip(dao.getList(userId), dao.getStore(storeName))
      ))
      .map(t -> t.apply(Nouritsu::sortBySection))
      .map(Serde::serialize)
      .fold(Function.identity(), Resp.OK);
  }

  // Return a Tuple2 containing the values of two eithers if both were Either.Right
  private static <L, R1, R2> Either<L, Tuple2<R1, R2>> zip(Either<L, R1> first, Either<L, R2> second) {
    return first.flatMap(f -> second.map(s -> Tuple.of(f, s)));
  }

  // This kind of breaks the style,
  // but I can’t come up with a more elegant solution,
  // so lots of locals it is.
  private static ListWithRest sortBySection(HashSet<ShoppingItem> items, Store store) {
    final var itemsBySection = items.groupBy(ShoppingItem::section);
    final var sorted = store.sections().flatMap(s -> itemsBySection.get(s).getOrElse(HashSet.empty()));
    return new ListWithRest(sorted, items.removeAll(sorted));
  }

  private static Resp addToList(Map<String, String> params, Dao dao) {
    return getAsTuple(params, "user", "item")
      // This is where Kotlin-like destructuring would be really nice to have:
      // flatMap(id, item -> ...)
      .flatMap(t -> t.apply(dao::addToList))
      .fold(Function.identity(), Resp.CREATED);
  }

  private static Resp addItem(Map<String, String> params, Dao dao) {
    return getAsTuple(params, "name", "section")
      .flatMap(t -> t.apply(ShoppingItem::tryParse))
      .map(dao::addItem)
      .fold(Function.identity(), Resp.CREATED);
  }

  private static Resp addStore(Map<String, String> params, Dao dao) {
    return getAsTuple(params, "name", "sections")
      .flatMap(t -> t.apply(Store::tryParse))
      .map(dao::addStore)
      .fold(Function.identity(), Resp.CREATED);
  }

  private static Either<Resp, String> getFromParams(Map<String, String> params, String key) {
    return params.get(key).toEither(Resp.NOT_FOUND.apply("Parameter “%s” not found".formatted(key)));
  }

  // Get k1 and k2 as a tuple in Either.Right or Either.Left if k1 or k2 is/are not in the map.
  private static Either<Resp, Tuple2<String, String>> getAsTuple(Map<String, String> params, String k1, String k2) {
    return zip(getFromParams(params, k1), getFromParams(params, k2));
  }

  public static void main(String[] args) throws IOException {
    new Nouritsu(14523, new Dao());
  }
}