[OSM-dev] Delta-encoding in PBF

[David Paleino]

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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