add: go back button in rankings

assets/js/rankings.js

@@ -6,6 +6,7 @@
   const REMOTE_RANKINGS_URL = "/api/rankings";
   const REMOTE_RANKINGS_VOTE_URL = "/api/rankings/vote";
   const REMOTE_RANKINGS_RESET_URL = "/api/rankings/reset";
+  const REMOTE_RANKINGS_UNDO_URL = "/api/rankings/undo";
 
   function $(id) {
     return document.getElementById(id);
@@ -33,6 +34,10 @@
     return [leftId, rightId].sort().join(":");
   }
 
+  function isValidMealId(id) {
+    return typeof id === "string" && /^\d+$/.test(id);
+  }
+
   function createDefaultEntry(id, defaultRating) {
     return {
       id,
@@ -189,7 +194,7 @@
 
         return state;
       },
-      async submitVote(seedData, state, winnerId, loserId, pairKey) {
+      async submitVote(seedData, state, winnerId, loserId, pairKey, leftId, rightId) {
         if (mode === "server" && typeof fetch === "function") {
           try {
             const remoteState = await requestJson(REMOTE_RANKINGS_VOTE_URL, {
@@ -198,6 +203,8 @@
                 winnerId,
                 loserId,
                 pairKey,
+                leftId,
+                rightId,
               }),
             });
 
@@ -212,7 +219,41 @@
           }
         }
 
-        const nextState = applyVote(seedData, state, winnerId, loserId, pairKey);
+        const nextState = applyVote(
+          seedData,
+          state,
+          winnerId,
+          loserId,
+          pairKey,
+          leftId,
+          rightId
+        );
+
+        if (mode === "local") {
+          local.save(nextState);
+        }
+
+        return nextState;
+      },
+      async undo(seedData, state) {
+        if (mode === "server" && typeof fetch === "function") {
+          try {
+            const remoteState = await requestJson(REMOTE_RANKINGS_UNDO_URL, {
+              method: "POST",
+            });
+
+            return syncStoredState(seedData, remoteState);
+          } catch (error) {
+            setMode(
+              getFallbackMode(),
+              local.available
+                ? "Server sync failed, so go back now applies only in this browser."
+                : "Server sync failed, so go back lasts only until you reload."
+            );
+          }
+        }
+
+        const nextState = undoLastVote(seedData, state);
 
         if (mode === "local") {
           local.save(nextState);
@@ -281,6 +322,7 @@
       voteCount: 0,
       lastPairKey: null,
       updatedAt: null,
+      undo: null,
       elo: {
         defaultRating: seedData.elo.defaultRating,
         kFactor: seedData.elo.kFactor,
@@ -295,7 +337,7 @@
     };
   }
 
-  function syncStoredState(seedData, storedState) {
+  function syncStateCore(seedData, storedState) {
     if (!storedState || typeof storedState !== "object" || Array.isArray(storedState)) {
       return createSeedState(seedData);
     }
@@ -327,6 +369,7 @@
           : derivedVoteCount,
       lastPairKey: typeof storedState.lastPairKey === "string" ? storedState.lastPairKey : null,
       updatedAt: typeof storedState.updatedAt === "string" ? storedState.updatedAt : null,
+      undo: null,
       elo: {
         defaultRating: seedData.elo.defaultRating,
         kFactor: seedData.elo.kFactor,
@@ -335,6 +378,89 @@
     };
   }
 
+  function createUndoSnapshot(state) {
+    return {
+      voteCount: state.voteCount,
+      lastPairKey: state.lastPairKey,
+      updatedAt: state.updatedAt,
+      elo: {
+        defaultRating: state.elo.defaultRating,
+        kFactor: state.elo.kFactor,
+        entries: state.elo.entries.map(cloneEntry),
+      },
+    };
+  }
+
+  function resolveUndoPairIds(winnerId, loserId, leftId, rightId) {
+    const validPair =
+      isValidMealId(leftId) &&
+      isValidMealId(rightId) &&
+      leftId !== rightId &&
+      [leftId, rightId].includes(winnerId) &&
+      [leftId, rightId].includes(loserId);
+
+    if (validPair) {
+      return { leftId, rightId };
+    }
+
+    return { leftId: winnerId, rightId: loserId };
+  }
+
+  function syncUndo(seedData, storedUndo) {
+    if (!storedUndo || typeof storedUndo !== "object" || Array.isArray(storedUndo)) {
+      return null;
+    }
+
+    const { leftId, rightId, winnerId, loserId, snapshot } = storedUndo;
+
+    if (
+      !isValidMealId(leftId) ||
+      !isValidMealId(rightId) ||
+      !isValidMealId(winnerId) ||
+      !isValidMealId(loserId) ||
+      leftId === rightId ||
+      winnerId === loserId
+    ) {
+      return null;
+    }
+
+    const pairIds = new Set([leftId, rightId]);
+    const mealIds = new Set(seedData.meals.map((meal) => meal.id));
+
+    if (
+      !pairIds.has(winnerId) ||
+      !pairIds.has(loserId) ||
+      ![leftId, rightId, winnerId, loserId].every((id) => mealIds.has(id))
+    ) {
+      return null;
+    }
+
+    return {
+      pairKey: createPairKey(leftId, rightId),
+      leftId,
+      rightId,
+      winnerId,
+      loserId,
+      snapshot: createUndoSnapshot(syncStateCore(seedData, snapshot)),
+    };
+  }
+
+  function syncStoredState(seedData, storedState) {
+    const nextState = syncStateCore(seedData, storedState);
+
+    return {
+      ...nextState,
+      undo: syncUndo(seedData, storedState?.undo),
+    };
+  }
+
+  function restoreUndoSnapshot(seedData, snapshot) {
+    return {
+      ...syncStateCore(seedData, snapshot),
+      undo: null,
+    };
+  }
+
   function compareRankedMeals(left, right) {
     const leftMatches = left.wins + left.losses;
     const rightMatches = right.wins + right.losses;
@@ -385,12 +511,17 @@
     return 1 / (1 + Math.pow(10, (opponentRating - rating) / 400));
   }
 
-  function applyVote(seedData, state, winnerId, loserId, pairKey) {
+  function applyVote(seedData, state, winnerId, loserId, pairKey, leftId, rightId) {
     const entryById = new Map(
       state.elo.entries.map((entry) => [entry.id, cloneEntry(entry)])
     );
     const winner = entryById.get(winnerId);
     const loser = entryById.get(loserId);
+    const undoPair = resolveUndoPairIds(winnerId, loserId, leftId, rightId);
+    const resolvedPairKey =
+      typeof pairKey === "string" && pairKey.length > 0
+        ? pairKey
+        : createPairKey(undoPair.leftId, undoPair.rightId);
 
     if (!winner || !loser) {
       return state;
@@ -407,8 +538,16 @@
     return {
       ...state,
       voteCount: state.voteCount + 1,
-      lastPairKey: pairKey,
+      lastPairKey: resolvedPairKey,
       updatedAt: new Date().toISOString(),
+      undo: {
+        pairKey: createPairKey(undoPair.leftId, undoPair.rightId),
+        leftId: undoPair.leftId,
+        rightId: undoPair.rightId,
+        winnerId,
+        loserId,
+        snapshot: createUndoSnapshot(state),
+      },
       elo: {
         ...state.elo,
         entries: seedData.meals.map((meal) => entryById.get(meal.id)),
@@ -416,6 +555,14 @@
     };
   }
 
+  function undoLastVote(seedData, state) {
+    if (!state.undo) {
+      return state;
+    }
+
+    return restoreUndoSnapshot(seedData, state.undo.snapshot);
+  }
+
   function choosePair(rankedMeals, avoidedPairKeys) {
     if (rankedMeals.length < 2) {
       return null;
@@ -543,6 +690,7 @@
       voteStatus: $("vote-status"),
       voteMessage: $("vote-message"),
       skipPair: $("skip-pair"),
+      undoVote: $("undo-vote"),
       resetRankings: $("reset-rankings"),
     };
 
@@ -553,6 +701,7 @@
       !elements.voteStatus ||
       !elements.voteMessage ||
       !elements.skipPair ||
+      !elements.undoVote ||
       !elements.resetRankings
     ) {
       return;
@@ -561,15 +710,31 @@
     elements.voteMessage.textContent = "Loading saved rankings...";
 
     const persistence = createPersistence();
+    const mealById = new Map(seedData.meals.map((meal) => [meal.id, meal]));
     let state = await persistence.load(seedData);
     let currentPair = null;
     let currentPairKey = null;
+    let pendingPairIds = null;
     let lastMessage = "Choose the better meal to start ranking.";
     let busy = false;
 
     await persistence.save(state);
 
     function queueNextPair(rankedMeals) {
+      if (pendingPairIds) {
+        const rankedMealById = new Map(rankedMeals.map((meal) => [meal.id, meal]));
+        const left = rankedMealById.get(pendingPairIds.leftId);
+        const right = rankedMealById.get(pendingPairIds.rightId);
+
+        pendingPairIds = null;
+
+        if (left && right && left.id !== right.id) {
+          currentPair = { left, right };
+          currentPairKey = createPairKey(left.id, right.id);
+          return;
+        }
+      }
+
       currentPair = choosePair(rankedMeals, [currentPairKey, state.lastPairKey]);
       currentPairKey = currentPair
         ? createPairKey(currentPair.left.id, currentPair.right.id)
@@ -594,6 +759,7 @@
       elements.voteStatus.textContent = getStatusText(persistence);
       elements.voteMessage.textContent = lastMessage;
       elements.rankingSummary.textContent = getSummaryText(seedData, state, persistence);
+      elements.undoVote.disabled = busy || !state.undo;
       elements.resetRankings.disabled = busy;
       elements.rankings.innerHTML = rankedMeals
         .map((meal, index) => renderRankingCard(meal, index + 1))
@@ -629,7 +795,15 @@
       render(`Saving ${winnerTitle} over ${loserTitle}...`);
 
       try {
-        state = await persistence.submitVote(seedData, state, winnerId, loserId, currentPairKey);
+        state = await persistence.submitVote(
+          seedData,
+          state,
+          winnerId,
+          loserId,
+          currentPairKey,
+          currentPair.left.id,
+          currentPair.right.id
+        );
         currentPair = null;
 
         const notice = persistence.consumeNotice();
@@ -646,6 +820,42 @@
       }
     }
 
+    async function handleUndo() {
+      if (!state.undo || busy) {
+        return;
+      }
+
+      const undoInfo = state.undo;
+      const winnerTitle = mealById.get(undoInfo.winnerId)?.title || "that meal";
+      const loserTitle = mealById.get(undoInfo.loserId)?.title || "the other meal";
+      let nextMessage = `Went back before ${winnerTitle} over ${loserTitle}. Pick again.`;
+
+      busy = true;
+      render(`Going back before ${winnerTitle} over ${loserTitle}...`);
+
+      try {
+        state = await persistence.undo(seedData, state);
+        currentPair = null;
+        currentPairKey = null;
+        pendingPairIds = {
+          leftId: undoInfo.leftId,
+          rightId: undoInfo.rightId,
+        };
+
+        const notice = persistence.consumeNotice();
+
+        if (notice) {
+          nextMessage = `${nextMessage} ${notice}`;
+        }
+      } catch (error) {
+        console.error(error);
+        nextMessage = "Failed to go back to the previous vote.";
+      } finally {
+        busy = false;
+        render(nextMessage);
+      }
+    }
+
     elements.duelCards.addEventListener("click", async (event) => {
       const button = event.target.closest("[data-meal-id]");
 
@@ -667,6 +877,10 @@
       render("Skipped that pair.");
     });
 
+    elements.undoVote.addEventListener("click", async () => {
+      await handleUndo();
+    });
+
     elements.resetRankings.addEventListener("click", async () => {
       if (busy) {
         return;
@@ -710,7 +924,17 @@
         return;
       }
 
-      if (!currentPair || busy) {
+      if (busy) {
+        return;
+      }
+
+      if (event.key.toLowerCase() === "z" && state.undo) {
+        event.preventDefault();
+        await handleUndo();
+        return;
+      }
+
+      if (!currentPair) {
         return;
       }