Centralize replacement context resolution

app.js

@@ -341,8 +341,7 @@ function updateAnalysis() {
 }
 
 function renderReplacementComparison(totals, currentDps, currentRegen, pieceScores) {
-  const targetPiece = getReplacementTargetPiece(state.newItem.target, pieceScores);
-  const candidate = createNewItemCandidate(state.newItem.target);
+  const { targetPiece, candidate } = getReplacementContext(pieceScores);
   const replacementTotals = replacePieceInTotals(totals, targetPiece, candidate);
   const replacementDps = calculateDps(replacementTotals, state.combatStyle);
   const replacementRegen = calculateRegenDetails(replacementTotals, replacementDps).total;
@@ -415,6 +414,17 @@ function getFixedReplacementTarget(target) {
   return match ? FIXED_ITEMS[Number(match[1])] : null;
 }
 
+function getReplacementContext(pieceScores) {
+  const target = state.newItem.target;
+  const targetPiece = getReplacementTargetPiece(target, pieceScores);
+  return {
+    target,
+    targetPiece,
+    targetIndex: targetPiece ? state.pieces.indexOf(targetPiece) : -1,
+    candidate: createNewItemCandidate(target),
+  };
+}
+
 function getReplacementTargetPiece(target, pieceScores) {
   if (state.newItem.targetPieceId) {
     const pinned = state.pieces.find((piece) => piece.id === state.newItem.targetPieceId);
@@ -477,13 +487,11 @@ function swapInNewItem() {
   const profile = PROFILES[state.profile];
   const totals = calculateTotals();
   const pieceScores = state.pieces.map((piece) => scorePiece(piece, profile, totals));
-  const targetPiece = getReplacementTargetPiece(state.newItem.target, pieceScores);
-  if (!targetPiece) return;
-  const index = state.pieces.indexOf(targetPiece);
-  const candidate = createNewItemCandidate(state.newItem.target);
+  const { targetPiece, targetIndex, candidate } = getReplacementContext(pieceScores);
+  if (!targetPiece || targetIndex < 0) return;
   const previousPiece = clonePiece(targetPiece);
   const previousNewItem = cloneNewItem(state.newItem);
-  state.pieces[index] = createPieceFromCandidate(targetPiece, candidate);
+  state.pieces[targetIndex] = createPieceFromCandidate(targetPiece, candidate);
   state.newItem = createNewItemFromPiece(previousPiece, previousNewItem.target, targetPiece.id);
   render();
 }