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Bug 766331 - Don't rescan old data for Ogg timestamps - r=cpearce

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nsOggReader::RangeEndTime() relied on CRC values from valid ogg
pages to keep track of already-searched data when looking for a
end timestamp. This caused quadratic behaviour searching in a
section of a file with no valid pages.

Instead, remember where we last looked, and backoff immediately
if we go more than the maximum length of a page into previously
scanned data. This avoids searching data we've eliminated as
containing valid Ogg pages and lets us search backward with O(N)
instead.
This commit is contained in:
Ralph Giles 2012-06-25 09:38:56 -07:00
parent 89736bf22a
commit 585c620ea4
1 changed files with 11 additions and 0 deletions
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11
content/media/ogg/nsOggReader.cpp
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@ -780,7 +780,9 @@ PRInt64 nsOggReader::RangeEndTime(PRInt64 aStartOffset,
// we've previously encountered before, we'll either backoff again if we
// haven't found an end time yet, or return the last end time found.
const int step = 5000;
const int maxOggPageSize = 65306;
PRInt64 readStartOffset = aEndOffset;
PRInt64 readLimitOffset = aEndOffset;
PRInt64 readHead = aEndOffset;
PRInt64 endTime = -1;
PRUint32 checksumAfterSeek = 0;
@ -802,6 +804,12 @@ PRInt64 nsOggReader::RangeEndTime(PRInt64 aStartOffset,
checksumAfterSeek = 0;
ogg_sync_reset(&sync.mState);
readStartOffset = NS_MAX(static_cast<PRInt64>(0), readStartOffset - step);
// There's no point reading more than the maximum size of
// an Ogg page into data we've previously scanned. Any data
// between readLimitOffset and aEndOffset must be garbage
// and we can ignore it thereafter.
readLimitOffset = NS_MIN(readLimitOffset,
readStartOffset + maxOggPageSize);
readHead = NS_MAX(aStartOffset, readStartOffset);
}
@ -827,6 +835,9 @@ PRInt64 nsOggReader::RangeEndTime(PRInt64 aStartOffset,
NS_ENSURE_SUCCESS(res, -1);
}
readHead += bytesRead;
if (readHead > readLimitOffset) {
mustBackOff = true;
}
// Update the synchronisation layer with the number
// of bytes written to the buffer