30 (X) Reservations in 30 (Y) Days Algorithm

x_reservations_per_y_days.php

<?php

function checkUserBookingRules($bookingData, $username) 
{
	/* $bookingData is a new or modified booking request */
	$existingBooking = key_exists('code', $bookingData) ? $bookingData['code'] : null;
	/* ... other rule checks ... */
	$startWindow = /* $Y days before booking date */;
	$windowLength = /* (2 * $Y) + how far out the user can book + 2 extra days */;
	$spanDayBookings = $this->getUserBookings($username, $startWindow, $windowLength);
	if ($existingBooking) {
		/* remove existing booking being modified from $spanDayBookings */
	}
	$spanDayBookings = self::indexBookingsByDate(
		self::filterBookingsByStatus(
			$spanDayBookings, 
			[/*confirmed booking status IDs*/]
		)
	);
	$nextAllowed = $this->getNextAllowedDate($spanDayBookings, $bookingData);
	if ($nextAllowed != $bookingData['date']) {
		/*reservation is declined with helpful information including $nextAllowed if applicable*/
	}
	/* otherwise reservation passes X reservations in Y days check */
	/* ... */
}

function getNextAllowedDate($existingBookings, $newBooking) 
{
	/* ... */
	$bookingDate = /* validated version of $newBooking['date'] */;
	$scanResults = /* converts date indexed array of bookings to date indexed array of booking counts */;
	$scanTalley = [];
	$spanTotals = []; /* how many reservations are within $Y days either direction of the date */
	/* these blocks loops over each date, sum the reservations across spans of $Y within $scanTalley into $spanTotals, then handle the trailing $Y days at the end of $scanTalley */
	foreach($scanResults as $scanDate => $scanCount) {
		$scanTalley[$scanDate] = $scanCount;
		if (count($scanTalley) >= $Y) {
			reset($scanTalley);
			$completedTallyDate = key($scanTalley);
			$spanTotals[$completedTallyDate] = array_sum($scanTalley);
			unset($scanTalley[$completedTallyDate]);
		}
	}
	foreach($scanTalley as $scanDate => $unused) {
		$spanTotals[$scanDate] = array_sum($scanTalley);
		unset($scanTalley[$scanDate]);
	}
	/* loop through every span of $Y days in the totals, pick the largest number of bookings in that range, if that date is the booking date or later and the booking count less than the max allowed, return that as the next date allowed. this ensures adding the reservation doesn't violate the limit for any given span */
	$spanRange = [];
	foreach($spanTotals as $scanDate => $scanCount) {
		$spanRange[] = $scanCount;
		if (count($spanRange) > $Y) {
			array_shift($spanRange);
		}
		if ($spanDate < $bookingDate) {
			continue;
		}
		if(max($spanRange) < $X) {
	/* return the earliest date the user can book */
			return $scanDate;
		}
	}
	/* if we arrive here, no date in the user’s booking window will satisfy the booking limit */
	return null;
}