<?php

declare(strict_types=1);

namespace App\Database\Migrations;

use CodeIgniter\Database\Migration;
use RuntimeException;

final class EnsureWhatsappGroupLinksUniquePerSchoolYear extends Migration
{
    public function up(): void
    {
        if ($this->db->DBDriver !== 'MySQLi') {
            throw new RuntimeException('This migration requires MySQL/MariaDB through the MySQLi driver.');
        }

        if (! $this->db->tableExists('whatsapp_group_links')
            || ! $this->db->fieldExists('class_section_id', 'whatsapp_group_links')
            || ! $this->db->fieldExists('school_year', 'whatsapp_group_links')) {
            return;
        }

        $duplicates = $this->db->query(
            'SELECT class_section_id, school_year, COUNT(*) AS row_count
             FROM `whatsapp_group_links`
             GROUP BY class_section_id, school_year
             HAVING COUNT(*) > 1
             LIMIT 5'
        )->getResultArray();

        if ($duplicates !== []) {
            throw new RuntimeException(
                'Cannot add whatsapp_group_links unique key by school year because duplicate class_section_id/school_year rows exist: '
                . json_encode($duplicates)
            );
        }

        $this->dropIndexIfExists('whatsapp_group_links', 'uq_section_term');

        $this->db->query(
            'ALTER TABLE `whatsapp_group_links`
             ADD UNIQUE INDEX `uq_section_term` (`class_section_id`, `school_year`)'
        );
    }

    public function down(): void
    {
        throw new RuntimeException('This migration is intentionally irreversible because reverting can fail once multiple school years exist for the same section.');
    }

    private function dropIndexIfExists(string $table, string $index): void
    {
        $exists = $this->db->query(
            'SELECT COUNT(*) AS aggregate
             FROM information_schema.STATISTICS
             WHERE TABLE_SCHEMA = DATABASE()
               AND TABLE_NAME = ?
               AND INDEX_NAME = ?',
            [$table, $index]
        )->getRow();

        if ((int) ($exists->aggregate ?? 0) === 0) {
            return;
        }

        $this->db->query(sprintf(
            'ALTER TABLE `%s` DROP INDEX `%s`',
            str_replace('`', '``', $table),
            str_replace('`', '``', $index)
        ));
    }
}