System.FileArchive.GZip

GZip file decompression

Copyright (c) 2004 John Goerzen, [e-mail]

The GZip format is described in RFC1952.

GZip Files

GZip files contain one or more Sections. Each Section, on disk, begins with a GZip Header, then stores the compressed data itself, and finally stores a GZip Footer.

The Header identifies the file as a GZip file, records the original modification date and time, and, in some cases, also records the original filename and comments.

The Footer contains a GZip CRC32 checksum over the decompressed data as well as a 32-bit length of the decompressed data. The module GZip is used to validate stored CRC32 values.

The vast majority of GZip files contain only one Section. Standard tools that work with GZip files create single-section files by default.

Multi-section files can be created by simply concatenating two existing GZip files together. The standard gunzip and zcat tools will simply concatenate the decompressed data when reading these files back. The decompress function in this module will do the same.

When reading data from this module, please use caution regarding how you access it. For instance, if you are wanting to write the decompressed stream to disk and validate its CRC32 value, you could use the decompress function. However, you should process the entire stream before you check the value of the Bool it returns. Otherwise, you will force Haskell to buffer the entire file in memory just so it can check the CRC32.

Types

data Header

The data structure representing the GZip header. This occurs at the beginning of each Section on disk.

Constructors

Header

Fields

method :: Int

Compression method. Only 8 is defined at present.

flags :: Int

extra :: Maybe String

filename :: Maybe String

comment :: Maybe String

mtime :: Word32

Modification time of the original file

xfl :: Int

Extra flags

os :: Int

Creating operating system

Instances

Eq Header

Show Header

type Section = (Header, String, Footer)

A section represents a compressed component in a GZip file. Every GZip file has at least one.

data GZipError

Constructors

CRCError

CRC-32 check failed

NotGZIPFile

Couldn't find a GZip header

UnknownMethod

Compressed with something other than method 8 (deflate)

UnknownError String

Other problem arose

Instances

Eq GZipError

Show GZipError

Error GZipError

data Footer

Stored on-disk at the end of each section.

Constructors

Footer

Fields

size :: Word32

The size of the original, decompressed data

crc32 :: Word32

The stored GZip CRC-32 of the original, decompressed data

crc32valid :: Bool

Whether or not the stored CRC-32 matches the calculated CRC-32 of the data

Whole-File Processing

decompress :: String -> (String, Maybe GZipError)

Read a GZip file, decompressing all sections that are found.

Returns a decompresed data stream and Nothing, or an unreliable string and Just (error). If you get anything other than Nothing, the String returned should be discarded.

hDecompress

Arguments

:: Handle

Input handle

-> Handle

Output handle

-> IO (Maybe GZipError)

Read a GZip file, decompressing all sections found.

Writes the decompressed data stream to the given output handle.

Returns Nothing if the action was successful, or Just GZipError if there was a problem. If there was a problem, the data written to the output handle should be discarded.

read_sections :: String -> Either GZipError [Section]

Read all sections.

Section Processing

read_header :: String -> Either GZipError (Header, String)

Read the GZip header. Return (Header, Remainder).

read_section :: String -> Either GZipError (Section, String)

Read one section, returning (ThisSection, Remainder)