<?php
/**
 * Automatic markdowns, the pure part: how far an item's price has come down after so
 * many days for sale. No WordPress here.
 *
 * The steps due are worked out from the days, not counted up run by run: running twice
 * in a day changes nothing the second time, and missed days are caught up. At most
 * `per_run` steps are taken at once, so an old item does not drop several steps in one
 * night unseen. The floor -- a minimum price, and the purchase price plus a margin --
 * is never crossed: an item at its floor is left for the shop to decide on.
 */

if (!defined('ABSPATH')) {
    exit;
}

/** The price after `$k` steps of `$pct` % of the starting price, in whole units. */
function usedkit_markdown_price($base, $pct, $k) {
    if ($k <= 0) {
        return (float) $base;
    }
    return (float) floor($base * (1 - $pct * $k / 100) + 0.00001);
}

/**
 * [steps, price, blocked]: the steps to have taken by now and the price they make, and
 * whether the floor is holding the price above where the schedule would have it.
 * `$cfg`: every (days), pct, per_run, min_price, floor_pct (over the purchase price).
 */
function usedkit_markdown_plan($base, $days, $done, $cfg, $cost = null) {
    $every = max(1, (int) $cfg['every']);
    $due = (int) floor($days / $every);
    $target = min($due, $done + max(1, (int) $cfg['per_run']));
    $floor = (float) $cfg['min_price'];
    if ($cost !== null && $cost > 0) {
        $floor = max($floor, $cost * (1 + (float) $cfg['floor_pct'] / 100));
    }
    $k = max($done, $target);
    while ($k > $done && usedkit_markdown_price($base, $cfg['pct'], $k) < $floor) {
        $k--;
    }
    $blocked = $k < $due && usedkit_markdown_price($base, $cfg['pct'], $k + 1) < $floor;
    return array($k, usedkit_markdown_price($base, $cfg['pct'], $k), $blocked);
}

/** Days until the next step, from the days for sale so far. */
function usedkit_markdown_next_in($days, $done, $every) {
    return max(0, ($done + 1) * max(1, (int) $every) - (int) $days);
}