mirror of
https://github.com/tsolomko/SWCompression.git
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Timofey Solomko
93 lines
3.2 KiB
Swift
93 lines
3.2 KiB
Swift
// Copyright (c) 2022 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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import BitByteData
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/// Represents the header of a Zlib archive.
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public struct ZlibHeader {
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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. Always `.deflate` for Zlib archives.
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public let compressionMethod: CompressionMethod = .deflate
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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 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 reader = LsbBitReader(data: data)
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try self.init(reader)
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}
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init(_ reader: LsbBitReader) throws {
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// Valid Zlib header must contain at least 2 bytes of data.
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guard reader.bytesLeft >= 2
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else { throw ZlibError.wrongCompressionMethod }
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// compressionMethod and compressionInfo combined are needed later for integrity check.
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let cmf = reader.byte()
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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 = cmf & 0xF
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guard compressionMethod == 8
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else { throw ZlibError.wrongCompressionMethod }
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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 = (cmf & 0xF0) >> 4
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guard compressionInfo <= 7
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else { throw ZlibError.wrongCompressionInfo }
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let windowSize = 1 << (compressionInfo.toInt() + 8)
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self.windowSize = windowSize
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// fcheck, fdict and compresionLevel together make flags byte which is used in integrity check.
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let flags = reader.byte()
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// First five bits are fcheck bits which are used for integrity check:
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// let fcheck = flags & 0x1F
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// Sixth bit indicate if archive contain Adler-32 checksum of preset dictionary.
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let fdict = (flags & 0x20) >> 5
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// Remaining bits indicate compression level.
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guard let compressionLevel = ZlibHeader.CompressionLevel(rawValue: (flags.toInt() & 0xC0) >> 6)
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else { throw ZlibError.wrongCompressionLevel }
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self.compressionLevel = compressionLevel
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guard (UInt(cmf) * 256 + UInt(flags)) % 31 == 0
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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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reader.offset += 4
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}
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}
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}
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