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function wp_unique_filename( $dir, $filename, $unique_filename_callback = null ) {
	// Sanitize the file name before we begin processing.
	$filename = sanitize_file_name( $filename );
	$ext2     = null;

	// Initialize vars used in the wp_unique_filename filter.
	$number        = '';
	$alt_filenames = array();

	// Separate the filename into a name and extension.
	$ext  = pathinfo( $filename, PATHINFO_EXTENSION );
	$name = pathinfo( $filename, PATHINFO_BASENAME );

	if ( $ext ) {
		$ext = '.' . $ext;
	}

	// Edge case: if file is named '.ext', treat as an empty name.
	if ( $name === $ext ) {
		$name = '';
	}

	/*
	 * Increment the file number until we have a unique file to save in $dir.
	 * Use callback if supplied.
	 */
	if ( $unique_filename_callback && is_callable( $unique_filename_callback ) ) {
		$filename = call_user_func( $unique_filename_callback, $dir, $name, $ext );
	} else {
		$fname = pathinfo( $filename, PATHINFO_FILENAME );

		// Always append a number to file names that can potentially match image sub-size file names.
		if ( $fname && preg_match( '/-(?:d+xd+|scaled|rotated)$/', $fname ) ) {
			$number = 1;

			// At this point the file name may not be unique. This is tested below and the $number is incremented.
			$filename = str_replace( "{$fname}{$ext}", "{$fname}-{$number}{$ext}", $filename );
		}

		/*
		 * Get the mime type. Uploaded files were already checked with wp_check_filetype_and_ext()
		 * in _wp_handle_upload(). Using wp_check_filetype() would be sufficient here.
		 */
		$file_type = wp_check_filetype( $filename );
		$mime_type = $file_type['type'];

		$is_image    = ( ! empty( $mime_type ) && str_starts_with( $mime_type, 'image/' ) );
		$upload_dir  = wp_get_upload_dir();
		$lc_filename = null;

		$lc_ext = strtolower( $ext );
		$_dir   = trailingslashit( $dir );

		/*
		 * If the extension is uppercase add an alternate file name with lowercase extension.
		 * Both need to be tested for uniqueness as the extension will be changed to lowercase
		 * for better compatibility with different filesystems. Fixes an inconsistency in WP < 2.9
		 * where uppercase extensions were allowed but image sub-sizes were created with
		 * lowercase extensions.
		 */
		if ( $ext && $lc_ext !== $ext ) {
			$lc_filename = preg_replace( '|' . preg_quote( $ext ) . '$|', $lc_ext, $filename );
		}

		/*
		 * Increment the number added to the file name if there are any files in $dir
		 * whose names match one of the possible name variations.
		 */
		while ( file_exists( $_dir . $filename ) || ( $lc_filename && file_exists( $_dir . $lc_filename ) ) ) {
			$new_number = (int) $number + 1;

			if ( $lc_filename ) {
				$lc_filename = str_replace(
					array( "-{$number}{$lc_ext}", "{$number}{$lc_ext}" ),
					"-{$new_number}{$lc_ext}",
					$lc_filename
				);
			}

			if ( '' === "{$number}{$ext}" ) {
				$filename = "{$filename}-{$new_number}";
			} else {
				$filename = str_replace(
					array( "-{$number}{$ext}", "{$number}{$ext}" ),
					"-{$new_number}{$ext}",
					$filename
				);
			}

			$number = $new_number;
		}

		// Change the extension to lowercase if needed.
		if ( $lc_filename ) {
			$filename = $lc_filename;
		}

		/*
		 * Prevent collisions with existing file names that contain dimension-like strings
		 * (whether they are subsizes or originals uploaded prior to #42437).
		 */

		$files = array();
		$count = 10000;

		// The (resized) image files would have name and extension, and will be in the uploads dir.
		if ( $name && $ext && @is_dir( $dir ) && str_contains( $dir, $upload_dir['basedir'] ) ) {
			/**
			 * Filters the file list used for calculating a unique filename for a newly added file.
			 *
			 * Returning an array from the filter will effectively short-circuit retrieval
			 * from the filesystem and return the passed value instead.
			 *
			 * @since 5.5.0
			 *
			 * @param array|null $files    The list of files to use for filename comparisons.
			 *                             Default null (to retrieve the list from the filesystem).
			 * @param string     $dir      The directory for the new file.
			 * @param string     $filename The proposed filename for the new file.
			 */
			$files = apply_filters( 'pre_wp_unique_filename_file_list', null, $dir, $filename );

			if ( null === $files ) {
				// List of all files and directories contained in $dir.
				$files = @scandir( $dir );
			}

			if ( ! empty( $files ) ) {
				// Remove "dot" dirs.
				$files = array_diff( $files, array( '.', '..' ) );
			}

			if ( ! empty( $files ) ) {
				$count = count( $files );

				/*
				 * Ensure this never goes into infinite loop as it uses pathinfo() and regex in the check,
				 * but string replacement for the changes.
				 */
				$i = 0;

				while ( $i <= $count && _wp_check_existing_file_names( $filename, $files ) ) {
					$new_number = (int) $number + 1;

					// If $ext is uppercase it was replaced with the lowercase version after the previous loop.
					$filename = str_replace(
						array( "-{$number}{$lc_ext}", "{$number}{$lc_ext}" ),
						"-{$new_number}{$lc_ext}",
						$filename
					);

					$number = $new_number;
					++$i;
				}
			}
		}

		/*
		 * Check if an image will be converted after uploading or some existing image sub-size file names may conflict
		 * when regenerated. If yes, ensure the new file name will be unique and will produce unique sub-sizes.
		 */
		if ( $is_image ) {
			$output_formats = wp_get_image_editor_output_format( $_dir . $filename, $mime_type );
			$alt_types      = array();

			if ( ! empty( $output_formats[ $mime_type ] ) ) {
				// The image will be converted to this format/mime type.
				$alt_mime_type = $output_formats[ $mime_type ];

				// Other types of images whose names may conflict if their sub-sizes are regenerated.
				$alt_types   = array_keys( array_intersect( $output_formats, array( $mime_type, $alt_mime_type ) ) );
				$alt_types[] = $alt_mime_type;
			} elseif ( ! empty( $output_formats ) ) {
				$alt_types = array_keys( array_intersect( $output_formats, array( $mime_type ) ) );
			}

			// Remove duplicates and the original mime type. It will be added later if needed.
			$alt_types = array_unique( array_diff( $alt_types, array( $mime_type ) ) );

			foreach ( $alt_types as $alt_type ) {
				$alt_ext = wp_get_default_extension_for_mime_type( $alt_type );

				if ( ! $alt_ext ) {
					continue;
				}

				$alt_ext      = ".{$alt_ext}";
				$alt_filename = preg_replace( '|' . preg_quote( $lc_ext ) . '$|', $alt_ext, $filename );

				$alt_filenames[ $alt_ext ] = $alt_filename;
			}

			if ( ! empty( $alt_filenames ) ) {
				/*
				 * Add the original filename. It needs to be checked again
				 * together with the alternate filenames when $number is incremented.
				 */
				$alt_filenames[ $lc_ext ] = $filename;

				// Ensure no infinite loop.
				$i = 0;

				while ( $i <= $count && _wp_check_alternate_file_names( $alt_filenames, $_dir, $files ) ) {
					$new_number = (int) $number + 1;

					foreach ( $alt_filenames as $alt_ext => $alt_filename ) {
						$alt_filenames[ $alt_ext ] = str_replace(
							array( "-{$number}{$alt_ext}", "{$number}{$alt_ext}" ),
							"-{$new_number}{$alt_ext}",
							$alt_filename
						);
					}

					/*
					 * Also update the $number in (the output) $filename.
					 * If the extension was uppercase it was already replaced with the lowercase version.
					 */
					$filename = str_replace(
						array( "-{$number}{$lc_ext}", "{$number}{$lc_ext}" ),
						"-{$new_number}{$lc_ext}",
						$filename
					);

					$number = $new_number;
					++$i;
				}
			}
		}
	}

	/**
	 * Filters the result when generating a unique file name.
	 *
	 * @since 4.5.0
	 * @since 5.8.1 The `$alt_filenames` and `$number` parameters were added.
	 *
	 * @param string        $filename                 Unique file name.
	 * @param string        $ext                      File extension. Example: ".png".
	 * @param string        $dir                      Directory path.
	 * @param callable|null $unique_filename_callback Callback function that generates the unique file name.
	 * @param string[]      $alt_filenames            Array of alternate file names that were checked for collisions.
	 * @param int|string    $number                   The highest number that was used to make the file name unique
	 *                                                or an empty string if unused.
	 */
	return apply_filters( 'wp_unique_filename', $filename, $ext, $dir, $unique_filename_callback, $alt_filenames, $number );
}

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