Support MultiBid feature

Detailed prebid/prebid-server#1715

Refactor winningBid logic.
Added IT test.

src/main/java/org/prebid/server/auction/ExchangeService.java

@@ -49,6 +49,7 @@
 import org.prebid.server.proto.openrtb.ext.request.ExtRequestPrebidBidderConfig;
 import org.prebid.server.proto.openrtb.ext.request.ExtRequestPrebidCache;
 import org.prebid.server.proto.openrtb.ext.request.ExtRequestPrebidData;
+import org.prebid.server.proto.openrtb.ext.request.ExtRequestPrebidMultiBid;
 import org.prebid.server.proto.openrtb.ext.request.ExtRequestPrebidSchain;
 import org.prebid.server.proto.openrtb.ext.request.ExtRequestPrebidSchainSchain;
 import org.prebid.server.proto.openrtb.ext.request.ExtRequestTargeting;
@@ -69,6 +70,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 
 /**
@@ -148,6 +150,7 @@ public Future<BidResponse> holdAuction(AuctionContext context) {
         final String publisherId = account.getId();
         final BidRequestCacheInfo cacheInfo = bidRequestCacheInfo(bidRequest);
         final boolean debugEnabled = isDebugEnabled(bidRequest);
+        final Map<String, ExtRequestPrebidMultiBid> bidderToMultiBids = bidderToMultiBids(bidRequest);
 
         return storedResponseProcessor.getStoredResponseResult(bidRequest.getImp(), aliases, timeout)
                 .map(storedResponseResult -> populateStoredResponse(storedResponseResult, storedResponse))
@@ -173,6 +176,7 @@ public Future<BidResponse> holdAuction(AuctionContext context) {
                         bidderResponses,
                         context,
                         cacheInfo,
+                        bidderToMultiBids,
                         debugEnabled))
                 .compose(bidResponse -> bidResponsePostProcessor.postProcess(
                         context.getRoutingContext(), uidsCookie, bidRequest, bidResponse, account));
@@ -249,6 +253,17 @@ private static ExtRequestPrebid extRequestPrebid(BidRequest bidRequest) {
         return requestExt != null ? requestExt.getPrebid() : null;
     }
 
+    private static Map<String, ExtRequestPrebidMultiBid> bidderToMultiBids(BidRequest bidRequest) {
+        final ExtRequestPrebid extRequestPrebid = extRequestPrebid(bidRequest);
+        final List<ExtRequestPrebidMultiBid> multiBids = extRequestPrebid != null
+                ? extRequestPrebid.getMultibid()
+                : null;
+        return multiBids == null
+                ? Collections.emptyMap()
+                : multiBids.stream()
+                        .collect(Collectors.toMap(ExtRequestPrebidMultiBid::getBidder, Function.identity()));
+    }
+
     /**
      * Populates storedResponse parameter with stored {@link List<SeatBid>} and returns {@link List<Imp>} for which
      * request to bidders should be performed.
@@ -529,8 +544,7 @@ private List<BidderRequest> getBidderRequests(List<BidderPrivacyResult> bidderPr
      * Extracts a map of bidders to their arguments from {@link ObjectNode} prebid.bidders.
      */
     private static Map<String, JsonNode> bidderToPrebidBidders(BidRequest bidRequest) {
-        final ExtRequest requestExt = bidRequest.getExt();
-        final ExtRequestPrebid prebid = requestExt == null ? null : requestExt.getPrebid();
+        final ExtRequestPrebid prebid = extRequestPrebid(bidRequest);
         final ObjectNode bidders = prebid == null ? null : prebid.getBidders();
 
         if (bidders == null || bidders.isNull()) {

src/main/java/org/prebid/server/auction/WinningBidComparator.java

@@ -0,0 +1,28 @@
+package org.prebid.server.auction;
+
+import com.iab.openrtb.request.Imp;
+import org.prebid.server.auction.model.BidInfo;
+
+import java.util.Comparator;
+import java.util.Objects;
+
+/**
+ * Re
+ */
+public class WinningBidComparator implements Comparator<BidInfo> {
+
+    private final Comparator<BidInfo> priceComparator = Comparator.comparing(o -> o.getBid().getPrice());
+
+    @Override
+    public int compare(BidInfo bidInfo1, BidInfo bidInfo2) {
+        final Imp imp = bidInfo1.getCorrespondingImp();
+        // this should never happen
+        if (!Objects.equals(imp, bidInfo2.getCorrespondingImp())) {
+            throw new IllegalStateException(
+                    String.format("Error while determining winning bid: "
+                            + "Multiple bids for was found for impId: %s", imp.getId()));
+        }
+
+        return priceComparator.compare(bidInfo1, bidInfo2);
+    }
+}

src/main/java/org/prebid/server/auction/model/TargetingBidInfo.java

@@ -0,0 +1,21 @@
+package org.prebid.server.auction.model;
+
+import lombok.Builder;
+import lombok.Value;
+
+@Builder(toBuilder = true)
+@Value
+public class TargetingBidInfo {
+
+    BidInfo bidInfo;
+
+    String bidderCode;
+
+    boolean isTargetingEnabled;
+
+    boolean isWinningBid;
+
+    boolean isBidderWinningBid;
+
+    boolean isAddTargetBidderCode;
+}

src/main/java/org/prebid/server/proto/openrtb/ext/request/ExtRequestPrebid.java

@@ -109,4 +109,9 @@ public class ExtRequestPrebid {
      * Defines the contract for bidrequest.ext.prebid.channel
      */
     ExtRequestPrebidChannel channel;
+
+    /**
+     * Defines the contract for bidrequest.ext.prebid.multibid
+     */
+    List<ExtRequestPrebidMultiBid> multibid;
 }

src/main/java/org/prebid/server/proto/openrtb/ext/request/ExtRequestPrebidMultiBid.java

@@ -0,0 +1,28 @@
+package org.prebid.server.proto.openrtb.ext.request;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import lombok.Value;
+
+/**
+ * Defines the contract for bidrequest.ext.prebid.targeting
+ */
+@Value(staticConstructor = "of")
+public class ExtRequestPrebidMultiBid {
+
+    /**
+     * Defines the contract for bidrequest.ext.prebid.multibid.bidder
+     */
+    String bidder;
+
+    /**
+     * Defines the contract for bidrequest.ext.prebid.multibid.maxbids
+     */
+    @JsonProperty("maxbids")
+    Integer maxBids;
+
+    /**
+     * Defines the contract for bidrequest.ext.prebid.multibid.targetbiddercodeprefix
+     */
+    @JsonProperty("targetbiddercodeprefix")
+    String targetBidderCodePrefix;
+}

src/main/java/org/prebid/server/proto/openrtb/ext/response/ExtBidPrebid.java

@@ -20,6 +20,9 @@ public class ExtBidPrebid {
 
     Map<String, String> targeting;
 
+    @JsonProperty("targetbiddercode")
+    String targetBidderCode;
+
     ExtResponseCache cache;
 
     @JsonProperty("storedrequestattributes")

src/main/java/org/prebid/server/spring/config/ServiceConfiguration.java

@@ -11,7 +11,6 @@
 import org.prebid.server.auction.AuctionRequestFactory;
 import org.prebid.server.auction.BidResponseCreator;
 import org.prebid.server.auction.BidResponsePostProcessor;
-import org.prebid.server.auction.BidResponseReducer;
 import org.prebid.server.auction.ExchangeService;
 import org.prebid.server.auction.FpdResolver;
 import org.prebid.server.auction.ImplicitParametersExtractor;
@@ -26,6 +25,7 @@
 import org.prebid.server.auction.VideoRequestFactory;
 import org.prebid.server.auction.VideoResponseFactory;
 import org.prebid.server.auction.VideoStoredRequestProcessor;
+import org.prebid.server.auction.WinningBidComparator;
 import org.prebid.server.bidder.BidderCatalog;
 import org.prebid.server.bidder.BidderDeps;
 import org.prebid.server.bidder.BidderErrorNotifier;
@@ -457,7 +457,7 @@ BidResponseCreator bidResponseCreator(
             VastModifier vastModifier,
             EventsService eventsService,
             StoredRequestProcessor storedRequestProcessor,
-            BidResponseReducer bidResponseReducer,
+            WinningBidComparator winningBidComparator,
             IdGenerator bidIdGenerator,
             @Value("${settings.targeting.truncate-attr-chars}") int truncateAttrChars,
             Clock clock,
@@ -469,7 +469,7 @@ BidResponseCreator bidResponseCreator(
                 vastModifier,
                 eventsService,
                 storedRequestProcessor,
-                bidResponseReducer,
+                winningBidComparator,
                 bidIdGenerator,
                 truncateAttrChars,
                 clock,
@@ -533,8 +533,8 @@ StoredRequestProcessor storedRequestProcessor(
     }
 
     @Bean
-    BidResponseReducer bidResponseReducer() {
-        return new BidResponseReducer();
+    WinningBidComparator winningBidComparator() {
+        return new WinningBidComparator();
     }
 
     @Bean