[OSM-dev] Delta-encoding in PBF
Hello people,
I'm writing a parser in C# for PBF dumps. However, I'm having issues with the
Delta-{en|de}coding.
I thought I succesfully implemented the decoding when I saw correct
latitudes/longitudes, with the following (pseudo-)code:
---8---
function decode(delta) {
ret = []
for (i=0; i delta.Count; i++) {
if (i == 0) {
ret.append(delta[0])
continue
}
ret.append(delta[0] + delta[i])
}
return ret
}
---8---
This worked great with latitudes/longitudes, which I converted with the
OUT = IN * granularity / 10**9
formula I found in the .proto.
However, it seems that I can't understand the way user_sid is encoded. I tried
to apply the same function, but it returned negative numbers at a certain
point, and I obviously had an OutOfRange exception.
I must also say that I originally thought that the delta-encoding worked like
this:
1) write the first number, at list[0]
2) at position n, write the result of list[n-1] - list[n]
Did I just miss the correct documentation?
Thank you,
David
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Re: [OSM-dev] Delta-encoding in PBF
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Op 18-12-10 11:53, David Paleino schreef:
Hello people,
I'm writing a parser in C# for PBF dumps. However, I'm having issues with the
Delta-{en|de}coding.
http://git.openstreetmap.nl/index.cgi/pbf2osm.git/tree/src/main.c#n588
Maybe that helps?
Stefan
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Re: [OSM-dev] Delta-encoding in PBF
On Sat, 18 Dec 2010 13:28:09 +0100, Stefan de Konink wrote:
Op 18-12-10 11:53, David Paleino schreef:
Hello people,
I'm writing a parser in C# for PBF dumps. However, I'm having issues with
the Delta-{en|de}coding.
http://git.openstreetmap.nl/index.cgi/pbf2osm.git/tree/src/main.c#n588
Maybe that helps?
Aha, yes! :)
From that code, it seems like you're using what I thought before, i.e. given x,
x1 and x2, the delta-coded list would contain x, x1-x, x2-x1. And then you're
reconstructing them just by summing up.
So I found my bug. I was summing up deltas between them, instead of
previous result + current delta.
Thank you!,
David
--
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Re: [OSM-dev] Delta-encoding in PBF
On Sat, Dec 18, 2010 at 6:28 AM, Stefan de Konink ste...@konink.de wrote:
-BEGIN PGP SIGNED MESSAGE-
Hash: SHA512
Op 18-12-10 11:53, David Paleino schreef:
Hello people,
I'm writing a parser in C# for PBF dumps. However, I'm having issues with the
Delta-{en|de}coding.
http://git.openstreetmap.nl/index.cgi/pbf2osm.git/tree/src/main.c#n588
Thanks.
Also to add to this, the reference implementation of PBF is the
osmosis implementation, split between Osmosis (at
http://svn.openstreetmap.org/applications/utils/osmosis/trunk/pbf/)
and the application generic part at
https://github.com/scrosby/OSM-binary which contains the master PBF
definitions and is used to build osmbin.jar.
Scott
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