<?php

declare(strict_types=1);

namespace App\Services\Artist;

use App\Exceptions\Artist\VersionMismatchException;
use App\Models\Performance;
use App\Models\Stage;
use Carbon\CarbonImmutable;
use Illuminate\Support\Facades\DB;

/**
 * RFC v0.2 D18 — single transactional move algorithm.
 *
 * Locks the dragged Performance row (FOR UPDATE), validates the
 * client's version number against the persisted one, and either parks
 * the row (stage_id → null) or places it on the target stage with a
 * cascade-bump of any time-overlapping rows on the same lane. Cascade
 * is single-level: every performance currently on
 * (target_stage, target_event, target_lane) whose [start, end] window
 * overlaps the new placement gets bumped to lane+1. Higher-lane chain
 * collisions resolve themselves in a subsequent move; the prototype
 * audit (§5) shows that single-level bumping is sufficient because
 * collisions are rare and the user is editing one block at a time.
 *
 * The PerformanceObserver auto-bumps `version` on save so we don't
 * need to set it explicitly here.
 */
final class LaneCascadeService
{
    /**
     * @return MoveResult moved + (other) cascaded performances
     *
     * @throws VersionMismatchException when client version is stale
     */
    public function move(
        Performance $performance,
        ?Stage $targetStage,
        ?CarbonImmutable $start,
        ?CarbonImmutable $end,
        ?int $targetLane,
        int $clientVersion,
    ): MoveResult {
        return DB::transaction(function () use (
            $performance,
            $targetStage,
            $start,
            $end,
            $targetLane,
            $clientVersion,
        ): MoveResult {
            /** @var Performance $locked */
            $locked = Performance::query()
                ->whereKey($performance->id)
                ->lockForUpdate()
                ->firstOrFail();

            if ((int) $locked->version !== $clientVersion) {
                throw new VersionMismatchException(
                    currentVersion: (int) $locked->version,
                    clientVersion: $clientVersion,
                );
            }

            if ($targetStage === null) {
                $locked->stage_id = null;
                if ($start !== null) {
                    $locked->start_at = $start;
                }
                if ($end !== null) {
                    $locked->end_at = $end;
                }
                if ($targetLane !== null) {
                    $locked->lane = $targetLane;
                }
                $locked->save();

                return new MoveResult($locked->refresh(), []);
            }

            if ($start === null || $end === null || $targetLane === null) {
                throw new \InvalidArgumentException(
                    'targetStage non-null requires start, end and targetLane.',
                );
            }

            $newEventId = $this->resolveEventIdForStageAndStart($targetStage, $start)
                ?? $performance->event_id;

            // Lock all peer performances on (stage, event, lane) for the
            // duration of the cascade. The first iteration locks the
            // already-existing rows; new placements that follow this
            // transaction wait on these row locks until commit.
            $existingOnLane = Performance::query()
                ->where('stage_id', $targetStage->id)
                ->where('event_id', $newEventId)
                ->where('lane', $targetLane)
                ->where('id', '!=', $locked->id)
                ->lockForUpdate()
                ->get();

            $cascaded = [];
            foreach ($existingOnLane as $other) {
                if ($this->overlaps($start, $end, $other->start_at, $other->end_at)) {
                    $other->lane = (int) $other->lane + 1;
                    $other->save();
                    $cascaded[] = $other->refresh();
                }
            }

            $locked->stage_id = $targetStage->id;
            $locked->event_id = $newEventId;
            $locked->start_at = $start;
            $locked->end_at = $end;
            $locked->lane = $targetLane;
            $locked->save();

            return new MoveResult($locked->refresh(), $cascaded);
        });
    }

    /**
     * For festivals, `target_start_at` falls inside one of the stage's
     * sub-events; for flat events, the stage's own event_id is the
     * answer. Returns null if no stage_day matches — caller falls back
     * to keeping the performance's existing event_id (validation rules
     * in the FormRequest already guarantee a valid stage_day exists).
     */
    private function resolveEventIdForStageAndStart(Stage $stage, CarbonImmutable $start): ?string
    {
        return $stage->stageDays()
            ->join('events', 'events.id', '=', 'stage_days.event_id')
            ->where('events.start_at', '<=', $start)
            ->where('events.end_at', '>=', $start)
            ->orderBy('events.start_at', 'desc')
            ->limit(1)
            ->value('stage_days.event_id');
    }

    private function overlaps(
        CarbonImmutable $aStart,
        CarbonImmutable $aEnd,
        \DateTimeInterface $bStart,
        \DateTimeInterface $bEnd,
    ): bool {
        return $aStart < CarbonImmutable::instance($bEnd)
            && CarbonImmutable::instance($bStart) < $aEnd;
    }
}