Bid placement with row lock

<?php

namespace App\Auctions;

use App\Models\Auction;
use App\Models\Bid;
use Illuminate\Support\Facades\DB;
use Illuminate\Validation\ValidationException;

final class BidService
{
    /** Increment grows with the price: [from_amount => step] in cents. */
    private const STEPS = [0 => 100, 5000 => 250, 25000 => 500, 100000 => 2500];

    public function place(int $auctionId, int $userId, int $amount): Bid
    {
        return DB::transaction(function () use ($auctionId, $userId, $amount) {
            // Every bidder queues on this lock, so each sees the true current price.
            $auction = Auction::whereKey($auctionId)->lockForUpdate()->firstOrFail();

            if ($auction->status !== 'live' || $auction->ends_at->isPast()) {
                $this->fail('This auction is closed.');
            }
            if ($auction->seller_id === $userId) {
                $this->fail('You cannot bid on your own listing.');
            }
            if ($auction->leader_id === $userId) {
                $this->fail('You are already the highest bidder.');
            }

            $min = $auction->bid_count === 0
                ? $auction->start_price
                : $auction->current_price + $this->step($auction->current_price);

            if ($amount < $min) {
                $this->fail('Minimum bid is '.number_format($min / 100, 2).'.');
            }

            $bid = Bid::create(['auction_id' => $auction->id, 'user_id' => $userId, 'amount' => $amount]);

            $auction->update([
                'current_price' => $amount,
                'leader_id' => $userId,
                'bid_count' => $auction->bid_count + 1,
            ]);

            return $bid;
        });
    }

    private function step(int $price): int
    {
        $step = self::STEPS[0];
        foreach (self::STEPS as $from => $s) {
            if ($price >= $from) {
                $step = $s;
            }
        }

        return $step;
    }

    private function fail(string $message): never
    {
        throw ValidationException::withMessages(['amount' => $message]);
    }
}