Comment by durkie | Hacker News Reader

Comment by durkie | Hacker News Reader

i was just starting a project and considering the use of s2. i'm going with geohash instead because I would get different values for the s2 cell based on implementation.

ruby:

S2Cells::S2LatLon.new(34.1234, -84.1234).to_s2_id(30)

=> -8577780315821624425

S2Cells::S2LatLon.new(34.1234, -84.1234).to_s2_id(1)

=> -8358680908399640576

https://gojekfarm.github.io/s2-calc/ agrees with these values

python's s2sphere (from sidewalk labs, seems like it'd be more of a reference implementation) gives very different numbers:

s2sphere.Cell.from_lat_lng(s2sphere.LatLng.from_degrees(34.1234, -84.1234))

=> Cell: face 4, level 30, orientation 0, id 9868963757887927191

This discrepancy feels like it'd be some kind of signed/unsigned integer issue, but I didn't think that would be a thing in Ruby. It also doesn't seem like the level=1 cell value in Ruby would be such a large, specific number. I'm missing something.

For what it’s worth, at least in the case you described, it is indeed a case of signed vs unsigned representations. That is, the numbers

-8577780315821624425

and

9868963757887927191

have the exact same underlying 64-bit representation, which is all that matters for S2 (you can verify this by checking their difference is exactly 2^64). The former is just interpreting the 64-bit string as a signed int, while the latter is unsigned.

Separately, the level 1 number you mention

-8358680908399640576

is fairly simple if you look at its binary representation; almost all of its trailing digits are 0.

Basically, you really shouldn’t think of S2 values as integers, but more as their underlying length 64 bit string.

thank you!

S2 cell IDs are usually uint64, so the Ruby version looks wrong. If you cast that int64 to uint64 you get the same value as the python version.

Level 1 IDs look like large random numbers in decimal, but when viewed in binary they have many trailing zeros, that are used to truncate to make the tokens.

thanks!

"The reference implementation of the S2 library is written in C++. Versions in other languages are ported from the C++ code, and may not have the same robustness, performance, or features as the C++ version."

From https://s2geometry.io/about/overview

yes, that's the true "reference" implementation, i just figured a repo from sidewalk labs (a google company) would be maybe more authoritative.

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