don't go to final-round with 1 candidate

BrightSpots · Jun 5, 2023 · 6c41305 · 6c41305
1 parent eb5a352
commit 6c41305
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Showing 3 changed files with 16 additions and 27 deletions.
diff --git a/src/main/java/network/brightspots/rcv/Tabulator.java b/src/main/java/network/brightspots/rcv/Tabulator.java
@@ -493,19 +493,30 @@ private List<String> identifyWinners(
     } else {
       // We should only look for more winners if we haven't already filled all the seats.
       if (winnerToRound.size() < config.getNumberOfWinners()) {
-        // If the number of continuing candidates equals the number of seats to fill, everyone wins.
         if (currentRoundCandidateToTally.size()
             == config.getNumberOfWinners() - winnerToRound.size()) {
+          // If the number of continuing candidates equals the number of seats to fill, everyone wins.
           selectedWinners.addAll(currentRoundCandidateToTally.keySet());
+        } else if (config.isFirstRoundDeterminesThresholdEnabled() &&
+                currentRoundCandidateToTally.size() - 1 == config.getNumberOfWinners()) {
+          // Edge case: if nobody meets the threshold, but we're on the penultimate round when
+          // isFirstRoundDeterminesThresholdEnabled is true, select the max vote getters as the winners.
+          BigDecimal maxVotes = currentRoundTallyToCandidates.lastKey();
+          selectedWinners = currentRoundTallyToCandidates.get(maxVotes);
         } else if (!config.isMultiSeatBottomsUpUntilNWinnersEnabled()) {
+          // Otherwise, select all winners above the threshold
           selectWinners(currentRoundTallyToCandidates, selectedWinners);
         }
       }
 
       // Edge case: if we've identified multiple winners in this round, but we're only supposed to
-      // elect one winner per round, pick the top vote-getter and defer the others to subsequent
-      // rounds.
-      if (config.isMultiSeatAllowOnlyOneWinnerPerRoundEnabled() && selectedWinners.size() > 1) {
+      // elect one winner per round, pick the top vote-getter. If this is a multi-winner election,
+      // defer the others to subsequent rounds. If this is a single-winner election where its possible
+      // for no candidate to reach the threshold (isFirstRoundDeterminesThresholdEnabled=true), the tiebreaker
+      // will choose the only winner.
+      boolean useTiebreakerIfNeeded = config.isMultiSeatAllowOnlyOneWinnerPerRoundEnabled()
+          || config.isFirstRoundDeterminesThresholdEnabled();
+      if (useTiebreakerIfNeeded && selectedWinners.size() > 1) {
         // currentRoundTallyToCandidates is sorted from low to high, so just look at the last key
         BigDecimal maxVotes = currentRoundTallyToCandidates.lastKey();
         selectedWinners = currentRoundTallyToCandidates.get(maxVotes);

diff --git a/...und_determines_threshold_test/first_round_determines_threshold_test_expected_summary.json b/...und_determines_threshold_test/first_round_determines_threshold_test_expected_summary.json
@@ -25,17 +25,6 @@
       "Carmona, Ralph C." : "5",
       "Vail, Christopher L." : "6"
     },
-    "tallyResults" : [ {
-      "eliminated" : "Carmona, Ralph C.",
-      "transfers" : {
-        "exhausted" : "5"
-      }
-    } ]
-  }, {
-    "round" : 3,
-    "tally" : {
-      "Vail, Christopher L." : "6"
-    },
     "tallyResults" : [ {
       "elected" : "Vail, Christopher L.",
       "transfers" : { }

diff --git a/...ld_tiebreaker_test/first_round_determines_threshold_tiebreaker_test_expected_summary.json b/...ld_tiebreaker_test/first_round_determines_threshold_tiebreaker_test_expected_summary.json
@@ -26,18 +26,7 @@
       "Vail, Christopher L." : "6"
     },
     "tallyResults" : [ {
-      "eliminated" : "Vail, Christopher L.",
-      "transfers" : {
-        "exhausted" : "6"
-      }
-    } ]
-  }, {
-    "round" : 3,
-    "tally" : {
-      "Carmona, Ralph C." : "6"
-    },
-    "tallyResults" : [ {
-      "elected" : "Carmona, Ralph C.",
+      "elected" : "Vail, Christopher L.",
       "transfers" : { }
     } ]
   } ]