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49 lines
2.1 KiB
Text
49 lines
2.1 KiB
Text
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local jecs = require("@jecs")
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local pair = jecs.pair
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local world = jecs.world()
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local Likes = world:component()
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--[[
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Fragmentation is a property of archetype-based ECS implementations where entities
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are spread out over more archetypes as the number of different component combinations
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increases. The overhead of fragmentation is visible in two areas:
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- Archetype creation
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- Queries (queries have to match & iterate more archetypes)
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Games that make extensive use of relationships might observe high levels of
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fragmentation, as relationships can introduce many different combinations of
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components. While the Jecs storage is optimized for supporting large amounts
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(hundreds of thousands) of archetypes, fragmentation is a factor to consider
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when using relationships.
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To improve the speed of evaluating queries, Jecs has indices that store all
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archetypes for a given component ID. Whenever a new archetype is created, it is
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registered with the indices for the IDs the archetype has, including IDs for
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relationship pairs.
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While registering an archetype for a relationship index is not more expensive
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than registering an archetype for a regular index, an archetype with relationships
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has to also register itself with the appropriate wildcard indices for its
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relationships. For example, an archetype with relationship pair(Likes, Apples)
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registers itself with the pair(Likes, Apples), pair(Likes, jecs.Wildcard) and
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pair(jecs.Wildcard, Apples) indices. For this reason, creating new archetypes
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with relationships has a higher overhead than an archetype without relationships.
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]]
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local alice = world:entity()
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local bob = world:entity()
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local charlie = world:entity()
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local e1 = world:entity()
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world:add(e1, pair(Likes, alice)) -- Creates archetype [pair(Likes, alice)]
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local e2 = world:entity()
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world:add(e2, pair(Likes, bob)) -- Creates archetype [pair(Likes, bob)]
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local e3 = world:entity()
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world:add(e3, pair(Likes, charlie)) -- Creates archetype [pair(Likes, charlie)]
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-- Each different target creates a new archetype, leading to fragmentation
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-- This is why relationships can increase archetype count significantly
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