49 lines
1.8 KiB
Swift
49 lines
1.8 KiB
Swift
import Foundation
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func resolvePayloadSize(dataType: DataType, bytes: Data, sourceEndianness: Endianness) -> UInt32? {
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func endianBased<T>(_ a: T, _ b: T) -> T { return sourceEndianness == .big ? a : b }
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guard let leadingByte = endianBased(bytes.first, bytes.last) else { return nil }
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let initialSizeDefinition = leadingByte & 0b0001_1111
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if dataType == .pointer {
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return UInt32((initialSizeDefinition &>> 3) + 1)
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}
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let getDataSlice = { (numberOfBytesToFetch: Int) -> Data? in
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let startOffset = dataType.isExtended ? 2 : 1
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if bytes.count < startOffset + numberOfBytesToFetch { return nil }
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guard let sliceStart = bytes.index(
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endianBased(bytes.startIndex, bytes.endIndex),
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offsetBy: endianBased(startOffset, -startOffset),
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limitedBy: endianBased(bytes.endIndex, bytes.startIndex)
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) else { return nil }
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guard let sliceEnd = bytes.index(
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sliceStart,
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offsetBy: endianBased(numberOfBytesToFetch, -numberOfBytesToFetch),
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limitedBy: endianBased(bytes.endIndex, bytes.startIndex)
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) else { return nil }
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return bytes.subdata(
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in: Range(
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uncheckedBounds: (
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lower: endianBased(sliceStart, sliceEnd),
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upper: endianBased(sliceEnd, sliceStart)
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)
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)
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)
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}
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let specialRepresentation: (bytesToSlice: Int, increment: UInt32)
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switch initialSizeDefinition {
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case 29: specialRepresentation = (1, 29)
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case 30: specialRepresentation = (2, 285)
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case 31: specialRepresentation = (3, 65_821)
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default: return UInt32(initialSizeDefinition)
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}
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guard let dataSlice = getDataSlice(specialRepresentation.bytesToSlice) else { return nil }
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return UInt32(
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dataSlice.padded(for: UInt32.self, as: sourceEndianness),
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sourceEndianness: sourceEndianness
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) + specialRepresentation.increment
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} |