<?php
namespace Relay\Database;

final class RowReader
{
    public const PAGE = 100;

    private Db $db;

    public function __construct(Db $db)
    {
        $this->db = $db;
    }

    /** @return array{rows: object[], cursor: array, done: bool} */
    public function page(TableSchema $s, array $cursor, int $limit = self::PAGE): array
    {
        $limit = max(1, min(self::PAGE, $limit));
        $table = Db::ident($s->name);
        // Explicit column list: SELECT * skips INVISIBLE columns, and generated columns are left out on purpose.
        $select = implode(',', array_map([Db::class, 'ident'], $s->columns));
        foreach (array_keys($s->bins) as $col) {
            $select .= ', HEX(' . Db::ident($col) . ') AS ' . Db::ident(strtolower($col) . '__hex');
        }
        foreach (array_keys($s->bits) as $col) {
            $select .= ', ' . Db::ident($col) . '+0 AS ' . Db::ident(strtolower($col) . '__bit');
        }

        if ($s->key !== []) {
            $where = !empty($cursor['after']) ? ' WHERE ' . $this->after_clause($s->key, $cursor['after']) : '';
            $order = implode(',', array_map([Db::class, 'ident'], $s->key));
            $sql = "SELECT {$select} FROM {$table}{$where} ORDER BY {$order} LIMIT {$limit}";
        } else {
            $offset = (int) ($cursor['offset'] ?? 0);
            $order = implode(',', range(1, count($s->columns)));
            $sql = "SELECT {$select} FROM {$table} ORDER BY {$order} LIMIT {$offset}, {$limit}";
        }

        $rows = $this->db->results($sql);
        if ($rows === []) {
            return ['rows' => [], 'cursor' => $cursor, 'done' => true];
        }
        $last = $rows[count($rows) - 1];
        if ($s->key !== []) {
            $after = [];
            foreach ($s->key as $col) {
                $after[$col] = $last->$col;
            }
            $next = ['after' => $after];
        } else {
            $next = ['offset' => (int) ($cursor['offset'] ?? 0) + count($rows)];
        }
        return ['rows' => $rows, 'cursor' => $next, 'done' => count($rows) < $limit];
    }

    // Ported from WP Migrate 2.7.11 (GPL) Table::build_select_query compound-key clause:
    // (a = 1 AND b = 2 AND c > 3) OR (a = 1 AND b > 2) OR (a > 1)
    public function after_clause(array $key, array $after): string
    {
        $wpdb = $this->db->wpdb();
        $clauses = [];
        $cols = array_values($key);
        while ($cols !== []) {
            $parts = [];
            $last = count($cols) - 1;
            foreach ($cols as $i => $col) {
                $parts[] = Db::ident($col) . ($i === $last ? ' > ' : ' = ') . $wpdb->prepare('%s', (string) $after[$col]);
            }
            $clauses[] = '(' . implode(' AND ', $parts) . ')';
            array_pop($cols);
        }
        return '(' . implode(' OR ', $clauses) . ')';
    }
}