<?php

namespace App\Schedule;

use DateTimeImmutable;

class EvenDistributionScheduler
{
    /**
     * @return list<DateTimeImmutable>
     */
    public function scheduledDates(
        int $assignmentCount,
        DateTimeImmutable $startDate,
        DateTimeImmutable $targetDate,
        WorkloadPlacement $workloadPlacement,
    ): array {
        $differenceInDays = $startDate->diff($targetDate)->days;
        $dayCount = $differenceInDays + 1;

        if ($assignmentCount < $dayCount) {
            return $this->sparseDates(
                assignmentCount: $assignmentCount,
                dayCount: $dayCount,
                startDate: $startDate,
            );
        }

        return $this->dailyDates(
            assignmentCount: $assignmentCount,
            dayCount: $dayCount,
            startDate: $startDate,
            workloadPlacement: $workloadPlacement,
        );
    }

    /**
     * @return list<DateTimeImmutable>
     */
    private function sparseDates(
        int $assignmentCount,
        int $dayCount,
        DateTimeImmutable $startDate,
    ): array {
        if ($assignmentCount === 1) {
            return [$startDate];
        }

        $dates = [];
        for ($assignmentIndex = 0;
            $assignmentIndex < $assignmentCount;
            $assignmentIndex++
        ) {
            $scaledIndex = $assignmentIndex * ($dayCount - 1)
                / ($assignmentCount - 1);
            $dayIndex = (int) floor($scaledIndex + 0.5);
            $dates[] = $startDate->modify("+{$dayIndex} days");
        }

        return $dates;
    }

    /**
     * @return list<DateTimeImmutable>
     */
    private function dailyDates(
        int $assignmentCount,
        int $dayCount,
        DateTimeImmutable $startDate,
        WorkloadPlacement $workloadPlacement,
    ): array {
        $assignmentsPerDay = intdiv($assignmentCount, $dayCount);
        $heavierDayCount = $assignmentCount % $dayCount;
        $heavierBlockStart = $this->heavierBlockStart(
            dayCount: $dayCount,
            heavierDayCount: $heavierDayCount,
            workloadPlacement: $workloadPlacement,
        );
        $dates = [];

        for ($dayIndex = 0; $dayIndex < $dayCount; $dayIndex++) {
            $assignmentCountForDay = $assignmentsPerDay;
            if (
                $dayIndex >= $heavierBlockStart
                && $dayIndex < $heavierBlockStart + $heavierDayCount
            ) {
                $assignmentCountForDay++;
            }

            $date = $startDate->modify("+{$dayIndex} days");
            for ($index = 0; $index < $assignmentCountForDay; $index++) {
                $dates[] = $date;
            }
        }

        return $dates;
    }

    private function heavierBlockStart(
        int $dayCount,
        int $heavierDayCount,
        WorkloadPlacement $workloadPlacement,
    ): int {
        if ($workloadPlacement === WorkloadPlacement::Start) {
            return 0;
        }

        if ($workloadPlacement === WorkloadPlacement::End) {
            return $dayCount - $heavierDayCount;
        }

        return intdiv($dayCount - $heavierDayCount, 2);
    }
}