dweiss commented on a change in pull request #460:
URL: https://github.com/apache/lucene/pull/460#discussion_r754613632
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File path: lucene/core/src/java/org/apache/lucene/util/fst/FST.java
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@@ -1000,6 +1027,98 @@ private void writePresenceBits(
assert bytePos - dest == numPresenceBytes;
}
+ private long estimateNodeAddress(
Review comment:
Thanks. I got the intuition right but some parts of this code were
written while I was... away (including byte reversals during serialization),
hence the uncertainty. I keep wondering if there is any other way to get those
deltas... or make the deltas refer to a different placeholder. I recall tricks
like this done back in assembly days on Amigas; basically you had a two-level
data structure - a stream of bytes + known-size "placeholders" for compacting.
data stream: byte1 byte2 byte3 ... byteN byteN+1 ...
address placeholder: (data stream@N), (data stream@M), ...
Each placeholder is a fixed-size offset - the compacting routine receives
the full data stream + placeholders so it has the ability to compute deltas
(from left to right or from right to left) and shift-compact the data stream
without knowing anything about the other data bytes. This way you sort of
postpone the delta-offset computation until you know all of the data and the
upper bound for its size, then reduce.
I'm sure you gave it some thought too - it's just what popped in my head
immediately when I saw your code. The requirement for those two extra methods
on outputs + the need to measure each node (I'd call it explicitly -
computeNodeAddress) is a bit worrying... but then - if there is no visible
slowdown then perhaps I'm just overreacting...
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