OpenSubtitles Hash

OSHash.swift

import Foundation

public class OSHash {
    enum HashError: Error {
        case invalidFile
        case fileToSmall
    }
    
    private static let chunkSize: Int = 65536
    private static let byteCount = 8
    
    func hashValue(_ fileURL: URL) throws -> UInt64  {
        try self.hashValue(fileURL.path)
    }
    
    func hashValue(_ filePath: String) throws -> UInt64  {
        guard let fileHandler = FileHandle(forReadingAtPath: filePath) else {
            throw HashError.invalidFile
        }
        let fileDataBegin: Data = fileHandler.readData(ofLength: OSHash.chunkSize) as Data
        fileHandler.seekToEndOfFile()
        
        let fileSize: UInt64 = fileHandler.offsetInFile
        guard (UInt64(OSHash.chunkSize) <= fileSize) else {
            fileHandler.closeFile()
            throw HashError.fileToSmall
        }
        
        fileHandler.seek(toFileOffset: max(0, fileSize - UInt64(OSHash.chunkSize)))
        var hash: UInt64 = fileSize
        
        let fileDataEnd: Data = fileHandler.readData(ofLength: OSHash.chunkSize)
        
        fileDataBegin.withUnsafeBytes { buffer in
            let binded = buffer.bindMemory(to: UInt64.self)
            let data_bytes = UnsafeBufferPointer<UInt64>(
                start: binded.baseAddress,
                count: fileDataBegin.count/OSHash.byteCount)
            hash = data_bytes.reduce(into: hash,  { hash = $0 &+ $1 })
        }
        
        fileDataEnd.withUnsafeBytes { buffer in
            let binded = buffer.bindMemory(to: UInt64.self)
            let data_bytes = UnsafeBufferPointer<UInt64>(
                start: binded.baseAddress,
                count: fileDataEnd.count/OSHash.byteCount)
            hash = data_bytes.reduce(into: hash,  { hash = $0 &+ $1 })
        }
        
        fileHandler.closeFile()
        return hash
    }
}