mirror of
https://github.com/tsolomko/SWCompression.git
synced 2026-06-23 14:56:41 +00:00
Timofey Solomko
369da20c16
I've also removed a most occurrences of inout keyword because DWP (and BitReader) are classes and they are already passed by reference in arguments.
95 lines
3.4 KiB
Swift
95 lines
3.4 KiB
Swift
// Copyright (c) 2017 Timofey Solomko
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// Licensed under MIT License
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//
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// See LICENSE for license information
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import Foundation
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/// Represents a Zlib archive's header.
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public struct ZlibHeader {
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/// Supported compression methods in zlib archive.
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public enum CompressionMethod: Int {
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/// The only one supported compression method (Deflate).
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case deflate = 8
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}
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/// Levels of compression which can be used to create Zlib archive.
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public enum CompressionLevel: Int {
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/// Fastest algorithm.
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case fastestAlgorithm = 0
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/// Fast algorithm.
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case fastAlgorithm = 1
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/// Default algorithm.
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case defaultAlgorithm = 2
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/// Slowest algorithm but with maximum compression.
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case slowAlgorithm = 3
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}
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/// Compression method of archive. Currently, always equals to `.deflate`.
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public let compressionMethod: CompressionMethod
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/// Level of compression used in archive.
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public let compressionLevel: CompressionLevel
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/// Size of 'window': moving interval of data which was used to make archive.
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public let windowSize: Int
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/**
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Initializes the structure with the values from Zlib `archive`.
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If data passed is not actually a Zlib archive, `ZlibError` will be thrown.
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- Parameter archive: Data archived with zlib.
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- Throws: `ZlibError`. It may indicate that either archive is damaged or
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it might not be archived with Zlib at all.
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*/
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public init(archive data: Data) throws {
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let bitReader = BitReader(data: data, bitOrder: .reversed)
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try self.init(bitReader)
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}
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init(_ bitReader: BitReader) throws {
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// First four bits are compression method.
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// Only compression method = 8 (DEFLATE) is supported.
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let compressionMethod = bitReader.intFromBits(count: 4)
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guard compressionMethod == 8 else { throw ZlibError.wrongCompressionMethod }
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self.compressionMethod = .deflate
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// Remaining four bits indicate window size.
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// For Deflate it must not be more than 7.
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let compressionInfo = bitReader.intFromBits(count: 4)
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guard compressionInfo <= 7 else { throw ZlibError.wrongCompressionInfo }
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let windowSize = 1 << (compressionInfo + 8)
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self.windowSize = windowSize
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// compressionMethod and compressionInfo combined are needed later for integrity check.
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let cmf = compressionInfo << 4 + compressionMethod
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// Next five bits are fcheck bits which are supposed to be integrity check.
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let fcheck = bitReader.intFromBits(count: 5)
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// Sixth bit indicate if archive contain Adler-32 checksum of preset dictionary.
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let fdict = bitReader.intFromBits(count: 1)
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// Remaining bits indicate compression level.
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guard let compressionLevel = ZlibHeader.CompressionLevel(rawValue:
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bitReader.intFromBits(count: 2)) else { throw ZlibError.wrongCompressionLevel }
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self.compressionLevel = compressionLevel
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// fcheck, fdict and compresionLevel together make flags byte which is used in integrity check.
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let flags = compressionLevel.rawValue << 6 + fdict << 5 + fcheck
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guard (UInt(cmf) * 256 + UInt(flags)) % 31 == 0 else { throw ZlibError.wrongFcheck }
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// If preset dictionary is present 4 bytes will be skipped.
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if fdict == 1 {
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bitReader.index += 4
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}
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}
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}
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