Resize only one specifc size of image

Function code:

function wp_generate_attachment_metadata_for_single_size( $attachment_id, $file, $sizes_for_resizing = '' ) {
		$attachment = get_post( $attachment_id );

		$metadata = array();
		$support = false;
		if ( preg_match('!^image/!', get_post_mime_type( $attachment )) && file_is_displayable_image($file) ) {
			$imagesize = getimagesize( $file );
			$metadata['width'] = $imagesize[0];
			$metadata['height'] = $imagesize[1];

			// Make the file path relative to the upload dir.
			$metadata['file'] = _wp_relative_upload_path($file);

			// Make thumbnails and other intermediate sizes.
			global $_wp_additional_image_sizes;

			$sizes = array();
			foreach ( get_intermediate_image_sizes() as $s ) {
				$sizes[$s] = array( 'width' => '', 'height' => '', 'crop' => false );
				if ( isset( $_wp_additional_image_sizes[$s]['width'] ) )
					$sizes[$s]['width'] = intval( $_wp_additional_image_sizes[$s]['width'] ); // For theme-added sizes
				else
					$sizes[$s]['width'] = get_option( "{$s}_size_w" ); // For default sizes set in options
				if ( isset( $_wp_additional_image_sizes[$s]['height'] ) )
					$sizes[$s]['height'] = intval( $_wp_additional_image_sizes[$s]['height'] ); // For theme-added sizes
				else
					$sizes[$s]['height'] = get_option( "{$s}_size_h" ); // For default sizes set in options
				if ( isset( $_wp_additional_image_sizes[$s]['crop'] ) )
					$sizes[$s]['crop'] = $_wp_additional_image_sizes[$s]['crop']; // For theme-added sizes
				else
					$sizes[$s]['crop'] = get_option( "{$s}_crop" ); // For default sizes set in options
			}

			/**
			 * Filter the image sizes automatically generated when uploading an image.
			 *
			 * @since 2.9.0
			 *
			 * @param array $sizes An associative array of image sizes.
			 */
			$sizes = apply_filters( 'intermediate_image_sizes_advanced', $sizes );

			$editor = wp_get_image_editor( $file );

			if ( $sizes_for_resizing ) {

				$metadata_sizes = array();
				foreach ( $sizes as $key => $value ) {
					if ( ! isset( $sizes_for_resizing[$key] ) ) {
						$filename_parts = explode('.', basename( $file ) );
						$metadata_sizes[$key] = array(
							'file' => $filename_parts[0] . '-' . $value['width'] . 'x' . $value['height'] . '.' . $filename_parts[1],
							'width' => intval( $value['width'] ),
							'height' => intval( $value['height'] ),
							'mime-type' => get_post_mime_type( $attachment ),
						);
					}
				}

				if ( ! is_wp_error( $editor ) ) {
					$metadata['sizes'] = $editor->multi_resize( $sizes_for_resizing );
					$metadata['sizes'] = array_merge( $metadata_sizes, $metadata['sizes'] );
				}

			} else {
				if ( $sizes ) {
					if ( ! is_wp_error( $editor ) ) {
						$metadata['sizes'] = $editor->multi_resize( $sizes );
					}
				} else {
					$metadata['sizes'] = array();
				}
			}

			// Fetch additional metadata from EXIF/IPTC.
			$image_meta = wp_read_image_metadata( $file );
			if ( $image_meta )
				$metadata['image_meta'] = $image_meta;

		}

		if ( $support && ! empty( $metadata['image']['data'] ) ) {
			// Check for existing cover.
			$hash = md5( $metadata['image']['data'] );
			$posts = get_posts( array(
				'fields' => 'ids',
				'post_type' => 'attachment',
				'post_mime_type' => $metadata['image']['mime'],
				'post_status' => 'inherit',
				'posts_per_page' => 1,
				'meta_key' => '_cover_hash',
				'meta_value' => $hash
			) );
			$exists = reset( $posts );

			if ( ! empty( $exists ) ) {
				update_post_meta( $attachment_id, '_thumbnail_id', $exists );
			} else {
				$ext = '.jpg';
				switch ( $metadata['image']['mime'] ) {
				case 'image/gif':
					$ext = '.gif';
					break;
				case 'image/png':
					$ext = '.png';
					break;
				}
				$basename = str_replace( '.', '-', basename( $file ) ) . '-image' . $ext;
				$uploaded = wp_upload_bits( $basename, '', $metadata['image']['data'] );
				if ( false === $uploaded['error'] ) {
					$image_attachment = array(
						'post_mime_type' => $metadata['image']['mime'],
						'post_type' => 'attachment',
						'post_content' => '',
					);
					/**
					 * Filter the parameters for the attachment thumbnail creation.
					 *
					 * @since 3.9.0
					 *
					 * @param array $image_attachment An array of parameters to create the thumbnail.
					 * @param array $metadata         Current attachment metadata.
					 * @param array $uploaded         An array containing the thumbnail path and url.
					 */
					$image_attachment = apply_filters( 'attachment_thumbnail_args', $image_attachment, $metadata, $uploaded );

					$sub_attachment_id = wp_insert_attachment( $image_attachment, $uploaded['file'] );
					add_post_meta( $sub_attachment_id, '_cover_hash', $hash );
					$attach_data = wp_generate_attachment_metadata( $sub_attachment_id, $uploaded['file'] );
					wp_update_attachment_metadata( $sub_attachment_id, $attach_data );
					update_post_meta( $attachment_id, '_thumbnail_id', $sub_attachment_id );
				}
			}
		}

		// Remove the blob of binary data from the array.
		unset( $metadata['image']['data'] );

		/**
		 * Filter the generated attachment meta data.
		 *
		 * @since 2.1.0
		 *
		 * @param array $metadata      An array of attachment meta data.
		 * @param int   $attachment_id Current attachment ID.
		 */
		return apply_filters( 'wp_generate_attachment_metadata', $metadata, $attachment_id );
	}