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[Gap-dev-discuss] Cynthiune : MP3 bundle and long delays

From: Philippe Roussel
Subject: [Gap-dev-discuss] Cynthiune : MP3 bundle and long delays
Date: Sun, 20 May 2012 20:45:23 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:12.0) Gecko/20120430 Thunderbird/12.0.1


I got the MP3 bundle to seek very fast but I would like you guys to try
to understand the code because I'm missing something.

To suppress the long delay while seeking through a mp3, the following is
enough :

Index: Bundles/MP3/MP3.h
RCS file: /sources/gap/gap/user-apps/Cynthiune/Bundles/MP3/MP3.h,v
retrieving revision 1.2
diff -u -r1.2 MP3.h
--- Bundles/MP3/MP3.h   30 Apr 2012 14:40:29 -0000      1.2
+++ Bundles/MP3/MP3.h   20 May 2012 18:35:06 -0000
@@ -25,7 +25,7 @@

 @protocol Format;

-#define IBUFFER_SIZE 5 * 8192
+#define IBUFFER_SIZE 8192

 typedef mad_fixed_t MadFixed;

If this doesn't add too much overhead on small cpus I think this should
be applied.

BUT, I'd would to love to understand the code in MP3.m
-readNextChunk:withSize so please explain it to me if you can.

In short, from what I understand, when the decoding buffer is empty, we
fread data from the file, IBUFFER_SIZE - iRemain bytes at a time. It
appears that most of the time IBUFFER_SIZE - iRemain = 1 so we're
reading char by char.... For each char read we call decodeInputBuffer()
which won't return BufferHasNoError before the buffer is big enough so
we loop a lot of time before returning a decoded buffer, thus the delay.

I'm quite sure this whole code is wrong and way too complicated but I
could be wrong. Someone should probably read the libmad documentation
and start from scratch.

Anyway, if you have an explanation for this code, I'm all ears :o)


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