how to reverse order of NSMutableData

Question

I want to reverse bytes of NSMutableData.

I am generating nsmutabledata object from a audio file. and playing it using data. my purpose is play audio reverse.

If I can reverse the NSMutableData I will be success.

Here is my code

NSString *inputVideoPath = [[NSBundle mainBundle] pathForResource:@"chalne" ofType:@"mp3"];

NSMutableData *wave=[NSMutableData dataWithContentsOfURL:[NSURL fileURLWithPath:inputVideoPath]];

avPlayer = [[AVAudioPlayer alloc] initWithData:wave error:nil ];
[avPlayer prepareToPlay];
[avPlayer play];

It will not work. To play it in reverse it also needs to be an audio file (or segments) not just a reversed byte-array. — Templar, Aug 01 '12 at 14:01
Then what is the way to play it in reverse order... Can you give me some code example please... — Shivomkara Chaturvedi, Aug 03 '12 at 05:04
Check out this post, there is an example about how to do it: http://stackoverflow.com/questions/7373977/objective-c-ios-whats-the-simplest-way-to-play-an-audio-file-backwards — , Aug 21 '12 at 21:44

score 3 · Accepted Answer · edited May 04 '13 at 03:52

3

Simply reversing the order of the bits will not work, because a bytestream of sound data has a specific format. You'll want to reverse the samples, not the bytestream as a whole. However, as to answer 'how to reverse the bytes of an NSData', this ought to work (typed it out, may have typos):

NSData *myData;
const char *bytes = [myData bytes];
char *reverseBytes = malloc(sizeof(char) * [myData length]);
int index = [myData length] - 1;
for (int i = 0; i < [myData length]; i++)
    reverseBytes[index--] = bytes[i];
NSData *reversedData = [NSData dataWithBytes:reverseBytes length:[myData length]]
free(reverseBytes);

edited May 04 '13 at 03:52

denis2342

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answered Aug 01 '12 at 14:07

CrimsonDiego

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Very good anser it reverse the data but as you suggested it does not play. May you please let me know the way how to play a audio file in reverse order. – Shivomkara Chaturvedi Aug 03 '12 at 05:03

denis2342 · Answer 2 · 2013-05-04T03:53:22.033

I fixed the above example and tested it. now it can be used in a category:

- (NSData*) reversedData
{
    NSData *myData = self;

    const char *bytes = [myData bytes];

    NSUInteger datalength = [myData length];

    char *reverseBytes = malloc(sizeof(char) * datalength);
    NSUInteger index = datalength - 1;

    for (int i = 0; i < datalength; i++)
        reverseBytes[index--] = bytes[i];

    NSData *reversedData = [NSData dataWithBytesNoCopy:reverseBytes length: datalength freeWhenDone:YES];

    return reversedData;
}

Dávid Kaszás · Answer 3 · 2014-11-27T11:50:49.857

0

-(NSData *)reverseData
{
    NSMutableData *data = [[NSMutableData alloc] init];
    for(int i = (int)self.length - 1; i >=0; i--){
        [data appendBytes: &self.bytes[i] length:1];
    }
    return [data copy];
}

edited Nov 27 '14 at 11:50

answered Nov 26 '14 at 15:00

Dávid Kaszás

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how to reverse order of NSMutableData

3 Answers3