| //! This module contains functionality for generating a [zlib](https://tools.ietf.org/html/rfc1950) |
| //! header. |
| //! |
| //! The Zlib header contains some metadata (a window size and a compression level), and optionally |
| //! a block of data serving as an extra dictionary for the compressor/decompressor. |
| //! The dictionary is not implemented in this library. |
| //! The data in the header aside from the dictionary doesn't actually have any effect on the |
| //! decompressed data, it only offers some hints for the decompressor on how the data was |
| //! compressed. |
| |
| use std::io::{Write, Result}; |
| |
| // CM = 8 means to use the DEFLATE compression method. |
| const DEFAULT_CM: u8 = 8; |
| // CINFO = 7 Indicates a 32k window size. |
| const DEFAULT_CINFO: u8 = 7 << 4; |
| const DEFAULT_CMF: u8 = DEFAULT_CM | DEFAULT_CINFO; |
| |
| // No dict by default. |
| #[cfg(test)] |
| const DEFAULT_FDICT: u8 = 0; |
| // FLEVEL = 0 means fastest compression algorithm. |
| const _DEFAULT_FLEVEL: u8 = 0 << 7; |
| |
| // The 16-bit value consisting of CMF and FLG must be divisible by this to be valid. |
| const FCHECK_DIVISOR: u8 = 31; |
| |
| #[allow(dead_code)] |
| #[repr(u8)] |
| pub enum CompressionLevel { |
| Fastest = 0 << 6, |
| Fast = 1 << 6, |
| Default = 2 << 6, |
| Maximum = 3 << 6, |
| } |
| |
| /// Generate FCHECK from CMF and FLG (without FCKECH )so that they are correct according to the |
| /// specification, i.e (CMF*256 + FCHK) % 31 = 0. |
| /// Returns flg with the FCHKECK bits added (any existing FCHECK bits are ignored). |
| fn add_fcheck(cmf: u8, flg: u8) -> u8 { |
| let rem = ((usize::from(cmf) * 256) + usize::from(flg)) % usize::from(FCHECK_DIVISOR); |
| |
| // Clear existing FCHECK if any |
| let flg = flg & 0b11100000; |
| |
| // Casting is safe as rem can't overflow since it is a value mod 31 |
| // We can simply add the value to flg as (31 - rem) will never be above 2^5 |
| flg + (FCHECK_DIVISOR - rem as u8) |
| } |
| |
| /// Write a zlib header with an empty dictionary to the writer using the specified |
| /// compression level preset. |
| pub fn write_zlib_header<W: Write>(writer: &mut W, level: CompressionLevel) -> Result<()> { |
| writer.write_all(&get_zlib_header(level)) |
| } |
| |
| /// Get the zlib header for the `CompressionLevel` level using the default window size and no |
| /// dictionary. |
| pub fn get_zlib_header(level: CompressionLevel) -> [u8; 2] { |
| let cmf = DEFAULT_CMF; |
| [cmf, add_fcheck(cmf, level as u8)] |
| } |
| |
| #[cfg(test)] |
| mod test { |
| use super::DEFAULT_CMF; |
| use super::*; |
| |
| #[test] |
| fn test_gen_fcheck() { |
| let cmf = DEFAULT_CMF; |
| let flg = super::add_fcheck( |
| DEFAULT_CMF, |
| CompressionLevel::Default as u8 | super::DEFAULT_FDICT, |
| ); |
| assert_eq!(((usize::from(cmf) * 256) + usize::from(flg)) % 31, 0); |
| } |
| |
| #[test] |
| fn test_header() { |
| let header = get_zlib_header(CompressionLevel::Fastest); |
| assert_eq!( |
| ((usize::from(header[0]) * 256) + usize::from(header[1])) % 31, |
| 0 |
| ); |
| } |
| } |