refactor: system init + add new search modifiers

[rmrt1n]

TL;DR

Better system init abstraction (yet again..) and new component search modifiers.

Component search modifiers

I see a lot of helper functions in rampage to filter over the results of Iter, which IMO isn't that great as it makes understanding what a system does harder as you have to jump between different functions. These modifiers hope to fix this by allowing you to filter on the spot so you don't lose context:

Filter results

Filter returns only the entities that matches your predicate:

func IsAlivePlayer(_ cardinal.EntityID, player PlayerSearch) bool {
    return player.HP.Get() > 0
}

for eid, player := range state.Players.Iter().Filter(IsAlivePlayer) {
    // Process only results that match your predicate...
}

Limit results

Limit returns only the first N results.

for eid, player := range state.Players.Iter().Limit(100) {
    // Process results...
}

Return a single element

Single returns a single result. It returns an error if the result is empty or has more than one element. This is a shorthand for finding singleton components. If you want to find the first result of a component that exists in multiple entities, use Limit(1).

eid, leaderboard, err := state.Leaderboard.Iter().Single()
if err != nil {
    // Handle the error
}
// Use the result...

Chaining modifiers

You can chain these functions together for complex searches, e.g.

// Find a single entity that matches the search:
_, account, err := state.Accounts.Iter().
    Filter(func (_ cardinal.EntityID, a AccountSearch) bool {
        return a.AccountData.Get().Persona == persona
    }).
    Single()
assert.That(err == nil)

// Chain multiple filters:
results := state.Players.Iter().Filter(IsAlivePlayer).Filter(HasBuff).Filter(HasPet)
for eid, player := range results {
    // Process only results that match your filters...
}

Internal changes

This PR includes several improvements:

• Moved all system field init to pkg/cardinal/system.go.
  This consolidates everything in one place. I don't know why it took me so long to realize this. The way to go is to just expose ECS APIs and call them from the system fields. It's much cleaner now and less confusing.
• Performance improvements to the system event manager.
  Nothing too special here. The existing version does a lot of boxing because the containers hold interfaces. Replaced it with the same approach as the archetype columns.

[rmrt1n]

• refactor: system init + add new search modifiers #875 👈 (View in Graphite)
• main

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