2.4 KiB
Location filter
Using DHT, geopositional ranges could be used as a prefix to scope indexes and route queries.
Earth circumference is around 40000 km.
Divided into 256 units, each unit would span around 150 km, which would constitute a feasible full-day roundtrip with motorized transport.
Further divided into 16 units, each unit would span about 10km, which would constitute a feasible full-day roundtrip on foot (which is the reality of mobility for most people in the world).
Further divided into 64 units, each unit would span around 150 metres, which is a couple of minutes' walk, or the equivalent of "what exists where I am right now," i.e. the smallest resolution needed in practical terms.
In practice, the units would be defined by degrees, conventionally starting at 0N 0E.
Location filter base representation
The above allows us to express the location scope in a series of three bit fields,
- layer 1: bit 0-255: units of ~150 km
- layer 2: bit 256-269: units of ~10km
- layer 3: bit 270-333: units of ~0.15km
Thus, the filter can be represented within 42 bytes per axis, or 84 bytes total.
Indexer scopes
Indexers will declare the geopositional scope of their relevance.
This would most likely be defined in layer 1 only. If an indexer only defines a single bit in layer 1, the protocol could advise that layer 2 is inspected for better granularity. And again the same with layer 2 and layer 3.
Ask and bid scopes
A store would define its origin in coordinates, from which a location filter bit field would be compiled. The scope should always be defined in terms layer 3.
When the ask or bid is announced on the network, the forwarders and indexers can make a simple bitwise comparison to determine whether it matches its scope, and/or choose where to forward it to. This will localize the gossip and limit network congestion.
Filter optimizations
The filter could optionally be shortened by omitting the layer(s) that dont carry information (e.g. scope is only defined in terms of layer 1).
The network could maintain consensus on e.g. the max scope limit any indexer may provide, and/or whether the scopes must be contiguous. If the max limit was 1 unit in layer 1, then the layer one could be represented by a single byte only, whose numeric value defines the single relevant sector.
Similarily, if scopes must be contiguous, only two numeric values would be needed; one to define the offset and one to define the number of units.