mirror of
https://github.com/tsolomko/SWCompression.git
synced 2026-06-23 14:56:41 +00:00
Timofey Solomko
39eea7c42a
Overtime size of source files became unmanageble, so it is better to split them. Also internal zip structure have been renamed to include word zip.
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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var pointerData = DataWithPointer(data: data, bitOrder: .reversed)
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try self.init(&pointerData)
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}
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init(_ pointerData: inout DataWithPointer) 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 = pointerData.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 = pointerData.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 = pointerData.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 = pointerData.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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pointerData.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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pointerData.index += 4
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}
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}
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}
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