<?php


namespace App\Tables;

use App\Models\Row;
use MightyPork\Utils\Utils;

/**
 * Generates a sequence of string codes for internal naming of columns
 *
 * Produces database-unique lowercase identifiers for created or proposed columns.
 * Column identifiers are used in row objects to uniquely identify data even if the
 * revision header (columns object) is modified, such as reordering, changing name,
 * adding new column in concurrent proposals, etc.
 *
 * Thanks to this uniqueness, it could even be possible to compare or merge forks
 * of the same table.
 */
class ColumnNumerator
{
    const ALPHABET = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
                     'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'];

    /** @var int */
    private $next;
    /** @var int */
    private $last;

    /**
     * Create numerator for the given number of columns
     */
    public function __construct($capacity)
    {
        list($this->next, $this->last) = Row::allocateColIDs($capacity);
    }

    /**
     * Get next column name, incrementing the internal state
     *
     * @return string
     */
    public function next()
    {
        if ($this->next > $this->last)
            throw new \OutOfBoundsException("Column numerator has run out of allocated GCID slots");

        $key = Utils::alphabetEncode($this->next, self::ALPHABET);
        $this->next++;
        return $key;
    }
}