<?php
/**
 * Decides whether decoding an image fits in PHP's memory limit.
 *
 * @package Fitzaro
 */

namespace Fitzaro\Image;

defined( 'ABSPATH' ) || exit;

/**
 * A 40 MP photo decodes to about 160 MB in GD, and many shared hosts allow 128 MB.
 * Checking before decoding turns a fatal "memory exhausted" into a clear rejection.
 */
final class MemoryBudget {

	/**
	 * GD truecolor stores 4 bytes per pixel; the extra byte covers row overhead.
	 */
	private const BYTES_PER_PIXEL = 5;

	/**
	 * Free bytes, or null when the memory limit is unlimited.
	 *
	 * @var int|null
	 */
	private $available;

	/**
	 * Creates a budget.
	 *
	 * @param int|null $available Free bytes, or null for unlimited.
	 */
	public function __construct( ?int $available ) {
		$this->available = $available;
	}

	/**
	 * Budget from the current PHP settings and usage.
	 */
	public static function from_runtime(): self {
		$limit = self::to_bytes( (string) ini_get( 'memory_limit' ) );

		return new self( null === $limit ? null : $limit - memory_get_usage( true ) );
	}

	/**
	 * Whether decoding a $width × $height image and producing a $max_side output fits.
	 *
	 * @param int $width    Source width.
	 * @param int $height   Source height.
	 * @param int $max_side Longest side of the output.
	 */
	public function fits( int $width, int $height, int $max_side ): bool {
		if ( null === $this->available ) {
			return true;
		}

		// Source bitmap + resized copy + flattened canvas of the output size.
		$needed = $width * $height * self::BYTES_PER_PIXEL + 2 * $max_side * $max_side * self::BYTES_PER_PIXEL;

		return $needed <= $this->available;
	}

	/**
	 * Converts a php.ini size ("128M", "1G", "-1") to bytes; null means unlimited.
	 *
	 * @param string $value Ini value.
	 */
	public static function to_bytes( string $value ): ?int {
		$value = trim( $value );
		if ( '' === $value || '-1' === $value ) {
			return null;
		}

		$number = (int) $value;
		$unit   = strtolower( substr( $value, -1 ) );
		$factor = array(
			'k' => 1024,
			'm' => 1024 ** 2,
			'g' => 1024 ** 3,
		);

		return $number * ( $factor[ $unit ] ?? 1 );
	}
}