How to boost::serialize S2PointIndex

[GoroYeh-HRI]

Hi,

If I want to use boost::serialize() to store class S2PointIndex as binary stream, how do I do it? Do I need to implement my own serialization function for S2PointIndex ? Thanks.

[smcallis]

It looks like we don't have an Encode() method on that one, so I think the answer is yes. Fortunately it's pretty simple under the hood.

On Tue, Oct 31, 2023 at 3:47 PM GoroYeh-HRI @.***> wrote:

Hi,

If I want to use boost::serialize() to store class S2PointIndex as binary stream, how do I do it? Do I need to implement my own serialization function for S2PointIndex ? Thanks.

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[GoroYeh-HRI]

Hi @smcallis , thanks for your reply. Could you elaborate more on the way to do it?

I'm currently tracing class S2PointIndex and think I need to serialize S2Point, S2CellId, and s2internal::BTreeMultimap. The last one BTreeMultimap is a little bit complicated, so I'm stuck at this point.

Could you provide more details on how to implement serialize() function for this class? Thank you very much!

[smcallis]

S2Point and S2CellId already have Encode/Init methods so they can serialize themselves for you. s2internal::BTreeMultimap is just absl::btree_multimap. I'd probably just iterate that container and store pairs of (S2CellId, [list of data]), then read that back into a new absl::btree_multimap when decoding. That might not be the absolute most efficient encoding space-wise but it is simple. I'd worry more about the fact that PointIndex takes a template parameter for the data, so you'll have to serialize the data type too.

[GoroYeh-HRI]

S2Point and S2CellId already have Encode/Init methods so they can serialize themselves for you. s2internal::BTreeMultimap is just absl::btree_multimap. I'd probably just iterate that container and store pairs of (S2CellId, [list of data]), then read that back into a new absl::btree_multimap when decoding. That might not be the absolute most efficient encoding space-wise but it is simple. I'd worry more about the fact that PointIndex takes a template parameter for the data, so you'll have to serialize the data type too.

Thank you so much for your reply! So your point is, for S2Point and S2CellId I don't need to write any codes to serialize them? And for s2internal::BTreeMultimap, in addition to the container I need to handle the parameter.

I noticed the BTreeMultimap is instantiated like this: container_internal::btree<container_internal:: map_params<Key, Value, Compare, Alloc,/*TargetNodeSize=*/256,/*IsMulti=*/true>>

Did you mean I'll need to serialize map_params<> ? I noticed map_params inherits common_params which inherits common_policy_traits. I wonder how deep I should trace in order to successfully store all the necessary data. Thank you!

[smcallis]

You'll have to write code to hook S2Point and S2CellId into boost::serialize, but you can probably do that using their existing Encode/Init methods. I don't think you'll need to worry about storing the map params, that's a runtime detail different from storing the data in a serialized form.