Laxsnor Laxsnor - 2 years ago 86
Swift Question

Compression of 2-D array on the fly with iOS

I am currently using Swift to store some data on iOS. The values come as a 2-D integer array, defined as an

. I need to save these integer arrays to disk. Currently, I am using the following function to do so:

func writeDataToFile(data: [[Int]], filename: String){

let fullfile = NSString(string: self.folderpath).stringByAppendingPathComponent(filename+".txt")
var fh = NSFileHandle(forWritingAtPath: fullfile)
if fh == nil{
NSFileManager.defaultManager().createFileAtPath(fullfile, contents: nil, attributes: nil)
fh = NSFileHandle(forWritingAtPath: fullfile)

fh?.writeData("Time: \(filename)\n".dataUsingEncoding(NSUTF16StringEncoding)!)

Currently this function works just fine, but it produces files that are relatively large (1.1mb each - which when you are writing them at 1 Hz, gets huge fast). The arrays written have a fixed size and the values will be from 20000 < x < 35000. Is there a way to compress this data on the fly such that I can later read the data into say Python or some other language? Would it just be easier to use some library like Zip to compress the files into zips after writing? Is there some way to transform the data (without loss of data/fidelity) into an image (for compression purposes, not viewing purposes). There is some metadata that I would like to store along with the 2-D array, such as a timestamp.

Answer Source

Since you are currently saving those as string values, the simplest and fastest size reduction would be to save them as binary values (or base64 encoded strings). Then you could convert all of your int values into 2 byte sets (since unsigned 2 bytes can store up to 65536) and save the values that way. That would go from 5 bytes per int value down to 2 bytes per int value. Immediate savings of 60%.

For the Base64 encoding I use something I found on the internet called NSData+Base64. But in looking that up I just read:

In the iOS 7 and Mac OS 10.9 SDKs, Apple introduced new base64 methods on NSData that make it unnecessary to use a 3rd party base 64 decoding library. What's more, they exposed access to private base64 methods that are retrospectively available back as far as IOS 4 and Mac OS 6.


You could go much further into the compression by realizing that data from one element to the next will likely not change by the entire range, since heat maps will always be gradients. Then you could save the arrays as difference since the last element and likely get that down to a single byte (255 value) change set. But that may lose precision if you are viewing something with a very fast heat gradient (or using a low resolution camera).

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