add support for consistent unique id generation base on index tuple

[sanderr]

Consider below model:

entity A:
    number n
end
index A(n)

entity AContainer:
end
AContainer.a [0:] -- A

and assume you want to generate a unique id for each A in an AContainer instance (for example the rule number from the bics bootcamp day 3). It would be nice to support generation of such an id.

A proof of concept that still needs some work:

`main.cf:

entity TestEntity:
	string unique
end

index TestEntity(unique)

implement TestEntity using std::none

collector = IdCollector(instance_index_field = "unique")

Identifiable(collector = collector, instance = TestEntity(unique = "0"))
Identifiable(collector = collector, instance = TestEntity(unique = "1"))
Identifiable(collector = collector, instance = TestEntity(unique = "2"))
Identifiable(collector = collector, instance = TestEntity(unique = "3"))
Identifiable(collector = collector, instance = TestEntity(unique = "4"))

libs/auto_id/model/_init.cf

# entity responsible for id assignment to it's instances based on the instances' field with name instance_index_field. In practice, this should be a list of strings to support multi-field indices.
entity IdCollector:
	string instance_index_field
end

implement IdCollector using collect_ids

# will receive an id from its IdCollector
entity Identifiable:
	number id
end

index Identifiable(instance, collector)

IdCollector.instances [0:] -- Identifiable.collector [1]
# TODO: use inheritance over composition? Instances could inherit from Identifiable instead of binding them together.
Identifiable.instance [1] -- std::Entity

implement Identifiable using std::none


implementation collect_ids for IdCollector:
        # plugin returns a dict from index value to assigned id. TODO: this method only works for single-field string indices
	id_map = map_unique_ids(self.instances, self.instance_index_field, 11)
	for i in self.instances:
		instance_index_value = i.instance.unique
		i.id = id_map[instance_index_value]
		std::print("unique: {{i.instance.unique}}, id: {{i.id}}")
	end
end

libs/auto_id/plugins/__init__.py

from operator import attrgetter

from inmanta.plugins import plugin


@plugin
def map_unique_ids(collection: "list", index_field: "string", offset: "number") -> "dict":
    """
    """
    sorted_collection = sorted(
        collection,
        key=lambda i: attrgetter(index_field)(i.instance)
    )
    return {attrgetter(index_field)(instance.instance): offset + i for i, instance in enumerate(sorted_collection)}

# vim: set tabstop=4 shiftwidth=4 expandtab:

[sanderr]

An important consideration here is whether std is really the best place to achieve this. Above example is not very intuitive to use. If that can not be improved, built-in compiler support might be more relevant.