Up to the top of the hill and back again!
Jun. 14th, 2003 12:29 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
lovingbothThere was this video file I downloaded where the first 256 bytes were some rubbish, with the 'real file' starting immediately after.
This causes all the media players to reject it.
Hmm, if I had the spare disc space, it'd be easy - write a short Turbo Pascal program to copy file a from offset n to file b.
I don't, so I have to do it in place.
Ok, write a short TP program to do that - set i := 0 then repeat 'seek to position i+n, read n bytes, seek to position i, write them, add n to i' until you've done it all.
Quick test - yep, I managed the end conditions correctly - it worked whether or not the file was a multiple of n.
Add some code to print i each time through the loop, so I can see just how slowly it works through this nearly a gigabyte long file. I can leave it going overnight if need be...
OK, deep breath... go!
Coo, it does a million bytes a second! :)
It's quickly finished and...
... the file is still not valid :(
It looks ok in a hex viewer - you can see it was produced by nandub, for example - but obviously something's wrong.
Oh well, delete it.
This causes all the media players to reject it.
Hmm, if I had the spare disc space, it'd be easy - write a short Turbo Pascal program to copy file a from offset n to file b.
I don't, so I have to do it in place.
Ok, write a short TP program to do that - set i := 0 then repeat 'seek to position i+n, read n bytes, seek to position i, write them, add n to i' until you've done it all.
Quick test - yep, I managed the end conditions correctly - it worked whether or not the file was a multiple of n.
Add some code to print i each time through the loop, so I can see just how slowly it works through this nearly a gigabyte long file. I can leave it going overnight if need be...
OK, deep breath... go!
Coo, it does a million bytes a second! :)
It's quickly finished and...
... the file is still not valid :(
It looks ok in a hex viewer - you can see it was produced by nandub, for example - but obviously something's wrong.
Oh well, delete it.