perf(algorithm): precompute history count, drop per-candidate sort (#9)

- generatePlan computes a histCount Map once; scoreCandidate now does an O(1)
  lookup instead of filtering the whole history per candidate per day.
- minGap eligibility finds the latest prior date in a single pass instead of
  spreading history+monthAssignments into a new array and sorting it.

Behaviour is unchanged (all algorithm tests pass).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/algorithm.js

@@ -47,22 +47,23 @@ function isEligible(employee, isoDate, monthAssignments, allHistory) {
   }
 
   if (c.minGapDays > 0) {
-    const allEntries = [...allHistory, ...monthAssignments];
-    const lastDate = allEntries
-      .filter(a => a.employeeId === employee.id && a.date < isoDate)
-      .map(a => a.date)
-      .sort()
-      .at(-1);
+    // Latest prior assignment date for this employee — single pass, no sort/alloc
+    let lastDate = null;
+    const isLater = a => a.employeeId === employee.id && a.date < isoDate &&
+      (lastDate === null || a.date > lastDate);
+    for (const a of allHistory) if (isLater(a)) lastDate = a.date;
+    for (const a of monthAssignments) if (isLater(a)) lastDate = a.date;
     if (lastDate && daysBetween(lastDate, isoDate) < c.minGapDays) return false;
   }
 
   return true;
 }
 
-// randomFn injectable so tests can pass () => 0.5 for deterministic output
-function scoreCandidate(employee, isoDate, allHistory, randomFn) {
+// randomFn injectable so tests can pass () => 0.5 for deterministic output.
+// histCount is a precomputed Map<employeeId, number> of past assignments.
+function scoreCandidate(employee, isoDate, histCount, randomFn) {
   const wd = getWeekday(isoDate);
-  const myTotal = allHistory.filter(h => h.employeeId === employee.id).length;
+  const myTotal = histCount.get(employee.id) || 0;
 
   // Fewer past assignments → higher (less negative) score
   let score = -myTotal;
@@ -87,6 +88,10 @@ export function generatePlan(workdays, employees, history, randomFn = Math.rando
   const generated = [];
   const skippedDates = [];
 
+  // Precompute each employee's history count once (used for fairness scoring)
+  const histCount = new Map();
+  for (const h of history) histCount.set(h.employeeId, (histCount.get(h.employeeId) || 0) + 1);
+
   for (const date of workdays) {
     const taken = monthAssignments.filter(a => a.date === date);
     const needed = slotsPerDay - taken.length;
@@ -103,7 +108,7 @@ export function generatePlan(workdays, employees, history, randomFn = Math.rando
     }
 
     const scored = candidates
-      .map(e => ({ employee: e, score: scoreCandidate(e, date, history, randomFn) }))
+      .map(e => ({ employee: e, score: scoreCandidate(e, date, histCount, randomFn) }))
       .sort((a, b) => b.score - a.score);
 
     for (let i = 0; i < needed; i++) {