add sparse workload placement

backend/app/Schedule/EvenDistributionScheduler.php

@@ -18,7 +18,7 @@ class EvenDistributionScheduler
         $differenceInDays = $startDate->diff($targetDate)->days;
         $dayCount = $differenceInDays + 1;
 
-        if ($assignmentCount < $dayCount) {
+        if ($workloadPlacement === WorkloadPlacement::Spread) {
             return $this->sparseDates(
                 assignmentCount: $assignmentCount,
                 dayCount: $dayCount,
@@ -26,7 +26,7 @@ class EvenDistributionScheduler
             );
         }
 
-        return $this->dailyDates(
+        return $this->groupedDates(
             assignmentCount: $assignmentCount,
             dayCount: $dayCount,
             startDate: $startDate,
@@ -63,17 +63,17 @@ class EvenDistributionScheduler
     /**
      * @return list<DateTimeImmutable>
      */
-    private function dailyDates(
+    private function groupedDates(
         int $assignmentCount,
         int $dayCount,
         DateTimeImmutable $startDate,
         WorkloadPlacement $workloadPlacement,
     ): array {
         $assignmentsPerDay = intdiv($assignmentCount, $dayCount);
-        $heavierDayCount = $assignmentCount % $dayCount;
+        $remainderDayCount = $assignmentCount % $dayCount;
-        $heavierBlockStart = $this->heavierBlockStart(
+        $remainderBlockStart = $this->remainderBlockStart(
             dayCount: $dayCount,
-            heavierDayCount: $heavierDayCount,
+            remainderDayCount: $remainderDayCount,
             workloadPlacement: $workloadPlacement,
         );
         $dates = [];
@@ -81,8 +81,8 @@ class EvenDistributionScheduler
         for ($dayIndex = 0; $dayIndex < $dayCount; $dayIndex++) {
             $assignmentCountForDay = $assignmentsPerDay;
             if (
-                $dayIndex >= $heavierBlockStart
+                $dayIndex >= $remainderBlockStart
-                && $dayIndex < $heavierBlockStart + $heavierDayCount
+                && $dayIndex < $remainderBlockStart + $remainderDayCount
             ) {
                 $assignmentCountForDay++;
             }
@@ -96,9 +96,9 @@ class EvenDistributionScheduler
         return $dates;
     }
 
-    private function heavierBlockStart(
+    private function remainderBlockStart(
         int $dayCount,
-        int $heavierDayCount,
+        int $remainderDayCount,
         WorkloadPlacement $workloadPlacement,
     ): int {
         if ($workloadPlacement === WorkloadPlacement::Start) {
@@ -106,9 +106,9 @@ class EvenDistributionScheduler
         }
 
         if ($workloadPlacement === WorkloadPlacement::End) {
-            return $dayCount - $heavierDayCount;
+            return $dayCount - $remainderDayCount;
         }
 
-        return intdiv($dayCount - $heavierDayCount, 2);
+        return intdiv($dayCount - $remainderDayCount, 2);
     }
 }

backend/app/Schedule/UseCases/CreateSchedule/CreateSchedule.php

@@ -64,7 +64,7 @@ class CreateSchedule
                 'targetDate must not be before startDate',
             );
         }
-        $workloadPlacement = $this->workloadPlacement(
+        $requestedWorkloadPlacement = $this->workloadPlacement(
             $request->workloadPlacement,
         );
 
@@ -78,6 +78,13 @@ class CreateSchedule
             throw new BadRequestException('level has no elements');
         }
 
+        $workloadPlacement = $this->resolvedWorkloadPlacement(
+            requestedWorkloadPlacement: $requestedWorkloadPlacement,
+            assignmentCount: count($elements),
+            startDate: $startDate,
+            targetDate: $targetDate,
+        );
+
         $scheduledDates = $this->evenDistributionScheduler->scheduledDates(
             assignmentCount: count($elements),
             startDate: $startDate,
@@ -225,19 +232,50 @@ class CreateSchedule
     /**
      * @throws BadRequestException
      */
-    private function workloadPlacement(?string $value): WorkloadPlacement
+    private function workloadPlacement(?string $value): ?WorkloadPlacement
     {
         if ($value === null) {
-            return WorkloadPlacement::Middle;
+            return null;
         }
 
         $workloadPlacement = WorkloadPlacement::tryFrom($value);
         if ($workloadPlacement === null) {
             throw new BadRequestException(
-                'workloadPlacement must be start, middle, or end',
+                'workloadPlacement must be spread, start, middle, or end',
             );
         }
 
         return $workloadPlacement;
     }
+
+    /**
+     * @throws BadRequestException
+     */
+    private function resolvedWorkloadPlacement(
+        ?WorkloadPlacement $requestedWorkloadPlacement,
+        int $assignmentCount,
+        DateTimeImmutable $startDate,
+        DateTimeImmutable $targetDate,
+    ): WorkloadPlacement {
+        $differenceInDays = $startDate->diff($targetDate)->days;
+        $dayCount = $differenceInDays + 1;
+        $isSparse = $assignmentCount < $dayCount;
+
+        if ($requestedWorkloadPlacement === null) {
+            return $isSparse
+                ? WorkloadPlacement::Spread
+                : WorkloadPlacement::Middle;
+        }
+
+        if (
+            $requestedWorkloadPlacement === WorkloadPlacement::Spread
+            && ! $isSparse
+        ) {
+            throw new BadRequestException(
+                'spread placement requires fewer assignments than days',
+            );
+        }
+
+        return $requestedWorkloadPlacement;
+    }
 }

backend/app/Schedule/WorkloadPlacement.php

@@ -4,6 +4,7 @@ namespace App\Schedule;
 
 enum WorkloadPlacement: string
 {
+    case Spread = 'spread';
     case Start = 'start';
     case Middle = 'middle';
     case End = 'end';