<?php

/*
 * This file is part of the Fidry CPUCounter Config package.
 *
 * (c) Théo FIDRY <theo.fidry@gmail.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

declare(strict_types=1);

namespace Fidry\CpuCoreCounter;

use Fidry\CpuCoreCounter\Finder\CpuCoreFinder;
use Fidry\CpuCoreCounter\Finder\EnvVariableFinder;
use Fidry\CpuCoreCounter\Finder\FinderRegistry;
use InvalidArgumentException;
use function implode;
use function max;
use function sprintf;
use function sys_getloadavg;
use const PHP_EOL;

final class CpuCoreCounter
{
    /**
     * @var list<CpuCoreFinder>
     */
    private $finders;

    /**
     * @var positive-int|null
     */
    private $count;

    /**
     * @param list<CpuCoreFinder>|null $finders
     */
    public function __construct(?array $finders = null)
    {
        $this->finders = $finders ?? FinderRegistry::getDefaultLogicalFinders();
    }

    /**
     * @param positive-int|0    $reservedCpus      Number of CPUs to reserve. This is useful when you want
     *                                             to reserve some CPUs for other processes. If the main
     *                                             process is going to be busy still, you may want to set
     *                                             this value to 1.
     * @param non-zero-int|null $countLimit        The maximum number of CPUs to return. If not provided, it
     *                                             may look for a limit in the environment variables, e.g.
     *                                             KUBERNETES_CPU_LIMIT. If negative, the limit will be
     *                                             the total number of cores found minus the absolute value.
     *                                             For instance if the system has 10 cores and countLimit=-2,
     *                                             then the effective limit considered will be 8.
     * @param float|null        $loadLimit         Element of [0., 1.]. Percentage representing the
     *                                             amount of cores that should be used among the available
     *                                             resources. For instance, if set to 0.7, it will use 70%
     *                                             of the available cores, i.e. if 1 core is reserved, 11
     *                                             cores are available and 5 are busy, it will use 70%
     *                                             of (11-1-5)=5 cores, so 3 cores. Set this parameter to null
     *                                             to skip this check. Beware that 1 does not mean "no limit",
     *                                             but 100% of the _available_ resources, i.e. with the
     *                                             previous example, it will return 5 cores. How busy is
     *                                             the system is determined by the system load average
     *                                             (see $systemLoadAverage).
     * @param float|null        $systemLoadAverage The system load average. If passed, it will use
     *                                             this information to limit the available cores based
     *                                             on the _available_ resources. For instance, if there
     *                                             is 10 cores but 3 are busy, then only 7 cores will
     *                                             be considered for further calculation. If set to
     *                                             `null`, it will use `sys_getloadavg()` to check the
     *                                             load of the system in the past minute. You can
     *                                             otherwise pass an arbitrary value. Should be a
     *                                             positive float.
     *
     * @see https://php.net/manual/en/function.sys-getloadavg.php
     */
    public function getAvailableForParallelisation(
        int $reservedCpus = 0,
        ?int $countLimit = null,
        ?float $loadLimit = null,
        ?float $systemLoadAverage = 0.
    ): ParallelisationResult {
        self::checkCountLimit($countLimit);
        self::checkLoadLimit($loadLimit);
        self::checkSystemLoadAverage($systemLoadAverage);

        $totalCoreCount = $this->getCountWithFallback(1);
        $availableCores = max(1, $totalCoreCount - $reservedCpus);

        // Adjust available CPUs based on current load
        if (null !== $loadLimit) {
            // https://github.com/phpstan/phpstan/issues/13198
            /** @var float $correctedSystemLoadAverage */
            $correctedSystemLoadAverage = null === $systemLoadAverage
                ? sys_getloadavg()[0] ?? 0.
                : $systemLoadAverage;

            $availableCores = max(
                1,
                $loadLimit * ($availableCores - $correctedSystemLoadAverage)
            );
        }

        if (null === $countLimit) {
            $correctedCountLimit = self::getKubernetesLimit();
        } else {
            $correctedCountLimit = $countLimit > 0
                ? $countLimit
                : max(1, $totalCoreCount + $countLimit);
        }

        if (null !== $correctedCountLimit && $availableCores > $correctedCountLimit) {
            $availableCores = $correctedCountLimit;
        }

        return new ParallelisationResult(
            $reservedCpus,
            $countLimit,
            $loadLimit,
            $systemLoadAverage,
            $correctedCountLimit,
            $correctedSystemLoadAverage ?? $systemLoadAverage,
            $totalCoreCount,
            (int) $availableCores
        );
    }

    /**
     * @throws NumberOfCpuCoreNotFound
     *
     * @return positive-int
     */
    public function getCount(): int
    {
        // Memoize result
        if (null === $this->count) {
            $this->count = $this->findCount();
        }

        return $this->count;
    }

    /**
     * @param positive-int $fallback
     *
     * @return positive-int
     */
    public function getCountWithFallback(int $fallback): int
    {
        try {
            return $this->getCount();
        } catch (NumberOfCpuCoreNotFound $exception) {
            return $fallback;
        }
    }

    /**
     * This method is mostly for debugging purposes.
     */
    public function trace(): string
    {
        $output = [];

        foreach ($this->finders as $finder) {
            $output[] = sprintf(
                'Executing the finder "%s":',
                $finder->toString()
            );
            $output[] = $finder->diagnose();

            $cores = $finder->find();

            if (null !== $cores) {
                $output[] = 'Result found: '.$cores;

                break;
            }

            $output[] = '–––';
        }

        return implode(PHP_EOL, $output);
    }

    /**
     * @throws NumberOfCpuCoreNotFound
     *
     * @return positive-int
     */
    private function findCount(): int
    {
        foreach ($this->finders as $finder) {
            $cores = $finder->find();

            if (null !== $cores) {
                return $cores;
            }
        }

        throw NumberOfCpuCoreNotFound::create();
    }

    /**
     * @throws NumberOfCpuCoreNotFound
     *
     * @return array{CpuCoreFinder, positive-int}
     */
    public function getFinderAndCores(): array
    {
        foreach ($this->finders as $finder) {
            $cores = $finder->find();

            if (null !== $cores) {
                return [$finder, $cores];
            }
        }

        throw NumberOfCpuCoreNotFound::create();
    }

    /**
     * @return positive-int|null
     */
    public static function getKubernetesLimit(): ?int
    {
        $finder = new EnvVariableFinder('KUBERNETES_CPU_LIMIT');

        return $finder->find();
    }

    private static function checkCountLimit(?int $countLimit): void
    {
        if (0 === $countLimit) {
            throw new InvalidArgumentException(
                'The count limit must be a non zero integer. Got "0".'
            );
        }
    }

    private static function checkLoadLimit(?float $loadLimit): void
    {
        if (null === $loadLimit) {
            return;
        }

        if ($loadLimit < 0. || $loadLimit > 1.) {
            throw new InvalidArgumentException(
                sprintf(
                    'The load limit must be in the range [0., 1.], got "%s".',
                    $loadLimit
                )
            );
        }
    }

    private static function checkSystemLoadAverage(?float $systemLoadAverage): void
    {
        if (null !== $systemLoadAverage && $systemLoadAverage < 0.) {
            throw new InvalidArgumentException(
                sprintf(
                    'The system load average must be a positive float, got "%s".',
                    $systemLoadAverage
                )
            );
        }
    }
}
