<?php
namespace App\Services;

use App\Models\Participant;
use Illuminate\Support\Collection;

class MatcherService
{
    public function match(): void
    {
        $participants = Participant::all();

        if ($participants->count() < 2) {
            return;
        }

        // desperate denotes a user A that is supposed to give to a user B that dropped out
        // whilst user A has a user C giving to them, or vice versa
        $desperate     = $participants->where('desperate', true)->values();
        $nonDesperate  = $participants->where('desperate', false)->values();

        $desperate     = $desperate->shuffle();
        $nonDesperate  = $nonDesperate->shuffle();

        $assignments = [];

        /*
        |--------------------------------------------------------------------------
        | 1. Match DESPERATE participants
        |--------------------------------------------------------------------------
        | A desperate user should match:
        |   - First to another desperate participant
        |   - If none left, match to a non-desperate participant
        |--------------------------------------------------------------------------
        */

        foreach ($desperate as $p) {
            // Exclude self from possible options
            $pool = $desperate->filter(fn($d) => $d->id !== $p->id);

            if ($pool->isEmpty()) {
                $pool = $nonDesperate;
            }

            if ($pool->isEmpty()) {
                continue;
            }

            $candidate = $this->pickValidTarget($p, $pool, $assignments);
            if ($candidate) {
                $assignments[$p->id] = $candidate->id;
            }
        }

        /*
        |--------------------------------------------------------------------------
        | 2. Match NON-DESPERATE participants
        |--------------------------------------------------------------------------
        | A non-desperate participant always matches to the non-desperate pool.
        |--------------------------------------------------------------------------
        */

        foreach ($nonDesperate as $p) {
            // Exclude self
            $pool = $nonDesperate->filter(fn($d) => $d->id !== $p->id);

            if ($pool->isEmpty()) {
                continue;
            }

            $candidate = $this->pickValidTarget($p, $pool, $assignments);
            if ($candidate) {
                $assignments[$p->id] = $candidate->id;
            }
        }

        /*
        |--------------------------------------------------------------------------
        | 3. Write all results back to DB
        |--------------------------------------------------------------------------
        */

        foreach ($assignments as $id => $givingId) {
            Participant::where('id', $id)->update([
                'giving_id' => $givingId,
            ]);
        }
    }

    private function pickValidTarget(Participant $giver, Collection $pool, array $assignments): ?Participant
    {
        $shuffled = $pool->shuffle();

        // Avoid duplicates
        foreach ($shuffled as $candidate) {
            if (($assignments[$candidate->id] ?? null) === $giver->id) {
                continue;
            }

            return $candidate;
        }

        return null;
    }
}