<?php
namespace OCA\Chores\Service;

use \DateInterval;
use \DateTimeImmutable;
use \Exception;

/**
 * A schedule-string is composed of 3 parts separated by colons
 *
 * unit:interval:scontraints
 *
 * unit:
 *   d - days
 *   w - week
 *   m - months
 *   s - single occurence, do not repeat, delete after completion
 *   o - on demand, no due date, do not delete
 *
 * interval:
 *   an integer specifying how many units until next due date
 *
 * constraint:
 *   if unit is | constraint can be
 *   -----------+----------------------------------
 *   d,s,o      | '-' (no constraint)
 *   -----------+----------------------------------
 *   w          | '-' (no constraint)
 *              | 0..6 weekday, starting with 0=sunday
 *              |   + ',even', or ',odd' (NOT IMPLEMENTED)
 *   -----------+----------------------------------
 *   m          | '-' (no constraint)
 *              | 0 .. 31 (to always occur on this day of the month)
 *              |         (when the month has less days, the last day of the
 *              |          month is chosen)
 */
class Schedule {
  const SINGLE = 's';
  const DAYS = 'd';
  const WEEKS = 'w';
  const MONTHS = 'm';
  const ON_DEMAND = 'o';

  protected /* int */ $interval;
  protected /* ?int */ $constraint;
  protected /* string(unit) */ $unit;

  private $durationString;

  protected function __construct(int $interval, string $unit, string $constraint) {
    $this->interval = $interval;
    $this->unit = $unit;
    $this->constraint = Schedule::validateConstraint($unit, $constraint);

    $this->durationString = "P" . $this->interval . Schedule::getDurationUnit($this->unit);
  }

  private function baseAdvance(): DateInterval {
    return new DateInterval($this->durationString);
  }

  private function getNudge(DateTimeImmutable $naiveNextDate): DateInterval {
    switch ($this->unit) {
    case Schedule::WEEKS:
      $isWeekday = intval($naiveNextDate->format("w"));
      $weekdayDiff = $this->constraint - $isWeekday;
      $nudge = posmod($weekdayDiff, 7);
      if ($nudge >= 4) {
        $nudge = $nudge - 7;
      }
      return makeDaysInterval($nudge);
    default: throw new Exception("impossible");
    }
  }

  public function nextDueDate(\DateTimeImmutable $completedDate): \DateTimeImmutable {
    if ($this->unit == Schedule::ON_DEMAND) {
      // for on-demand chores, we reuse the due field to store "last done date"
      return $completedDate;
    }
    $naiveNextDate = $completedDate->add($this->baseAdvance());
    if ($this->constraint !== null) {
      $nudge = $this->getNudge($naiveNextDate);
      return $naiveNextDate->add($nudge);
    } else {
      return $naiveNextDate;
    }
  }

  public static function parseSchedule(string $scheduleDesc): Schedule {
    $parts = explode(":", $scheduleDesc, 3);

    return new Schedule(validateInt($parts[1], 0, 60), $parts[0], $parts[2]);
  }

  private static function getDurationUnit(string $unit): string {
    switch ($unit) {
    case Schedule::DAYS:
      return 'D';
    case Schedule::WEEKS:
      return 'W';
    case Schedule::MONTHS:
      return 'M';
    case Schedule::ON_DEMAND:
      return '///ON_DEMAND';
    default:
      throw new Exception("invalid unit: $unit");
    }
  }

  private static function validateConstraint(string $unit, string $constraint): ?int {
    if ($constraint == "-") {
      return null;
    }
    switch($unit) {
    case Schedule::DAYS: throw new Exception("invalid constraint for " . $unit . ": " . $constraint);
    case Schedule::WEEKS: // fallthrough
      return validateInt($constraint, 0, 6);
    case Schedule::MONTHS: //
      return validateInt($constraint, 1, 31);
    }
    throw new Exception("invalid constraint for " . $unit . ": " . $constraint);
  }
}

function isint(string $s): bool {
  return !!preg_match('/^\d+$/', $s);
}
function validateInt(string $s, int $min, int $max): int {
  if (!isint($s)) {
    throw new Exception("invalid int: " . $s);
  }
  $i = intval($s);
  if ($i < $min || $i > $max) {
    throw new Exception("value out of range, valid range: " . $min . "..." . $max);
  }
  return $i;
}

function posmod(int $a, int $b): int {
  return ($b + ($a % $b)) % $b;
}

function makeDaysInterval(int $days): DateInterval {
  $i = new DateInterval("P" . abs($days) . "D");
  if ($days < 0) {
    $i->invert = 1;
  }
  return $i;
}