In Swift, how do I read an existing binary file in

问题:

As part of my projects, I have a binary data file consisting of a large series of 32 bit integers that one of my classes reads in on initialization. In my C++ library, I read it in with the following initializer:

Evaluator::Evaluator() {
    m_HandNumbers.resize(32487834);
    ifstream inputReader;

    inputReader.open("/path/to/file/7CHands.dat", ios::binary);

    int inputValue;
    for (int x = 0; x < 32487834; ++x) {
        inputReader.read((char *) &inputValue, sizeof (inputValue));
        m_HandNumbers[x] = inputValue;
    }
    inputReader.close();
};

and in porting to Swift, I decided to read the entire file into one buffer (it's only about 130 MB) and then copy the bytes out of the buffer.

So, I've done the following:

public init() {
    var inputStream = NSInputStream(fileAtPath: "/path/to/file/7CHands.dat")!
    var inputBuffer = [UInt8](count: 32478734 * 4, repeatedValue: 0)
    inputStream.open()
    inputStream.read(&inputBuffer, maxLength: inputBuffer.count)
    inputStream.close()
}

and it works fine in that when I debug it, I can see inputBuffer contains the same array of bytes that my hex editor says it should. Now, I'd like to get that data out of there effectively. I know it's stored in.. whatever format you call it where the least significant bytes are first (i.e. the number 0x00011D4A is represented as '4A1D 0100' in the file). I'm tempted to just iterate through it manually and calculate the byte values by hand, but I'm wondering if there's a quick way I can pass an array of [Int32] and have it read those bytes in. I tried using NSData, such as with:

    let data = NSData(bytes: handNumbers, length: handNumbers.count * sizeof(Int32))
    data.getBytes(&inputBuffer, length: inputBuffer.count)

but that didn't seem to load the values (all the values were still zero). Can anyone please help me convert this byte array into some Int32 values? Better yet would be to convert them to Int (i.e. 64 bit integer) just to keep my variable sizes the same across the project.

回答1:

Not sure about your endian-ness, but I use the following function. The difference from your code is using NSRanges of the actual required type, rather than lengths of bytes. This routine reads one value at a time (it's for ESRI files whose contents vary field by field), but should be easily adaptable.

func getBigIntFromData(data: NSData, offset: Int) -> Int {
    var rng = NSRange(location: offset, length: 4)
    var i = [UInt32](count: 1, repeatedValue:0)

    data.getBytes(&i, range: rng)
    return Int(i[0].bigEndian)// return Int(i[0]) for littleEndian
}

回答2:

Grimxn provided the backbone of the solution to my problem, which showed me how to read sections of the buffer into an array; he then showed me a way to read the entire buffer in all at once. Rather than convert all of the items of the array needlessly to Int, I simply read the array into the buffer as UInt32 and did the casting to Int in the function that accesses that array.

For now, since I don't have my utility class defined yet, I integrated Grimxn's code directly into my initializer. The class initializer now looks like this:

public class Evaluator {
    let HandNumberArraySize = 32487834

    var handNumbers: [Int32]

    public init() {
        let data = NSData(contentsOfFile: "/path/to/file/7CHands.dat")!
        var dataRange = NSRange(location: 0, length: HandNumberArraySize * 4)
        handNumbers = [Int32](count: HandNumberArraySize, repeatedValue: 0)
        data.getBytes(&handNumbers, range: dataRange)

        println("Evaluator loaded successfully")
    }

...

}

... and the function that references them is now:

public func cardVectorToHandNumber(#cards: [Int], numberToUse: Int) -> Int {
    var output: Int

    output = Int(handNumbers[53 + cards[0] + 1])

    for i in 1 ..< numberToUse {
        output = Int(handNumbers[output + cards[i] + 1])
    }

    return Int(handNumbers[output])
}

Thanks to Grimxn and thanks once again to StackOverflow for helping me in a very real way!