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40 lines
1.9 KiB
Text
40 lines
1.9 KiB
Text
Basic parsing method
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====================
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In an MPEG-2 Program Stream, every chunk of data starts with at least 23
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zeros and a one. This is followed by a byte of ID. At any given point I
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can search for the next 3-byte string equal to 1 to find the next chunk.
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I assume these start codes are aligned on byte boundaries. I might be
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wrong, in which case this thing has to be rewritten at some point in the
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future.
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This means there are two basic modes of operation. The first is the
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simple search for the next start code. The second is a continuation mode,
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where data from the previous buffer is available to attempt to complete a
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chunk. Hopefully the majority of time will be spent in the first mode, as
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that is where the most efficiency is, since there's no copying of partial
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chunks.
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The parsing is done as a state machine, as long as there's data left in
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the buffer, something is attempted. What is attempted is based on the
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state of the parser (gee, so that's why they call it a state machine <g>).
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The stages are:
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1) looking for sync (have_sync == FALSE)
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a) have some zeros (zeros > 0)
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2) getting ID (have_sync == TRUE, id == 0)
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3) decoding the chunk contents (have_sync == TRUE, id != 0)
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Mechanism for handling cross-buffer chunks of data
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==================================================
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The problem: if data were to come to the parser in 64-byte chunks, the
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pack head would be split across at least two buffers, possibly three. Up
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front I will make the assumption that no one will be sending buffers of
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less size than the largest chunk (header, PES packet), such that no chunk
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will be split across more than two buffers.
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If we take the pack header as an example, when the stream starts out, we
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can assume that the it starts at the beginning of the buffer and doesn't
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exceed the bounds of it. However, if we're mid-stream and starting
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another pack, it can be split across two buffers.
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