PopulationProcessor.hpp

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#pragma once
 
#include "SpatialHashProcessor.hpp"
 
namespace MayaFlux::Nodes::Network {
class GpuFieldOperator;
}
 
namespace MayaFlux::Buffers {
 
/**
 * @struct PopulationConfig
 * @brief Parameters for GPU-local population dynamics: destruction and
 *        spawn-as-copy over a seed's fixed, over-allocated capacity.
 *
 * hash.particle_count here is expected to already be live_count plus
 * whatever reserve capacity SpatialFieldConfig::reserve_fraction asked for:
 * the caller overrides SpatialHashConfig::particle_count to that total
 * before constructing this, so every hash/claim stage already dispatches
 * over the full reserve range. live_count is the boundary this struct
 * itself adds: everything at or past it starts dead, and is the only region
 * PopulationSpawnProcessor is allowed to write a new particle into.
 *
 * This mechanism is not reflected to the CPU. Whatever PhysicsOperator
 * simulates is the authoritative population; what exists here is GPU-local
 * bookkeeping about which of the seed's records currently render as alive,
 * rebuilt from scratch on the next reseed. ClaimAccumulateProcessor is the
 * one enforcement point: it never lets anything past live_count reach
 * PhysicsOperator's own bond/mass tracking, however far spawn has grown the
 * live-looking population beyond it.
 */
struct PopulationConfig {
    SpatialHashConfig hash;
    uint32_t live_count;
 
    /**
     * @brief Declare the two state fields population dynamics needs.
     * @param buffer Buffer to declare on. Already-present names are left
     *        untouched, matching every other declare_fields in this family.
     *
     * Declares mutation_alive at hash.particle_count elements (the total
     * seed + reserve capacity) and mutation_spawn_cursor as a single
     * element. Neither is reset by any per-cycle stage: PopulationInitProcessor
     * writes both exactly once, and every cycle after that only
     * ClaimAccumulateProcessor (destroy) and PopulationSpawnProcessor (spawn)
     * ever touch them again, forward-only, until the next reseed.
     */
    void declare_fields(const std::shared_ptr<NetworkGeometryBuffer>& buffer) const;
};
 
/**
 * @class PopulationInitProcessor
 * @brief One-shot: splits mutation_alive into live/reserve, zeros the
 *        reserve region's vertex records, and seeds mutation_spawn_cursor.
 *
 * Dispatches exactly once, on the first cycle it attaches to a ready
 * buffer, then refuses every subsequent dispatch via on_before_execute: a
 * second pass would reset every destroy/spawn decision made since the
 * first. Must run before HashCountProcessor/HashScatterProcessor's first
 * dispatch reads mutation_alive, so it belongs at the front of the chain,
 * ahead of HashClearProcessor.
 *
 * Zeroing the reserve region's vertex records (not just marking them dead)
 * matters because nothing else guarantees what those bytes held before this
 * ran: a stray large or NaN position/size would be a visible artifact if
 * alive-gating ever has a bug, rather than invisible.
 */
class MAYAFLUX_API PopulationInitProcessor : public NetworkStateFieldProcessor {
public:
    explicit PopulationInitProcessor(const PopulationConfig& config);
 
protected:
    void on_buffer_ready() override;
 
    /** @brief True only on the first call; every call after returns false. */
    bool on_before_execute(
        Portal::Graphics::CommandBufferID cmd_id,
        const std::shared_ptr<VKBuffer>& buffer) override;
 
private:
    struct Params {
        uint32_t live_count;
        uint32_t total_count;
        uint32_t stride_words;
    };
 
    Params m_params;
    bool m_done {};
};
 
/**
 * @class PopulationSpawnProcessor
 * @brief Copies a crowded live particle's full vertex record into a fresh
 *        reserve slot, claimed via a monotonic bump allocator.
 *
 * One thread per particle (dead particles skip immediately). Walks the same
 * 27-cell neighbourhood HashDensityColorProcessor does, over the completed
 * hash, and when a live particle's real neighbour count clears
 * spawn_density_threshold, does atomicAdd(spawn_cursor[0], 1u) to claim the
 * next slot. A claim landing at or past total_count is over capacity and is
 * silently dropped: the reserve never grows past what
 * SpatialFieldConfig::reserve_fraction allocated at wiring time, matching
 * "always over-allocate at seed time" rather than any live growth.
 *
 * The claimed slot is guaranteed to belong to no other thread this
 * dispatch (atomicAdd returns a distinct value per caller) and to be
 * read by nothing else this cycle (every stage that consumes hash_*
 * mutation_* state already ran earlier in the chain), so the copy has no
 * write hazard with any other thread. The one thing it does not
 * synchronise against is its own claimed slot's thread running the same
 * dispatch: that thread may observe mutation_alive for its own index
 * before or after this write lands, but either way it only ever reads,
 * never writes, so the race is benign: worst case, a freshly spawned
 * particle waits one extra cycle before it can itself become a source.
 *
 * Density is global, not cluster-scoped: a crowded region spawns regardless
 * of which PhysicsOperator collection its particles came from.
 *
 * Reads spawn_density_threshold from the owning GpuFieldOperator
 * fresh whenever its revision() changes, the same on_before_execute check
 * every other tunable in this family uses.
 */
class MAYAFLUX_API PopulationSpawnProcessor : public NetworkStateFieldProcessor {
public:
    PopulationSpawnProcessor(
        const PopulationConfig& config,
        std::shared_ptr<Nodes::Network::GpuFieldOperator> particle_op);
 
protected:
    void on_buffer_ready() override;
 
    bool on_before_execute(
        Portal::Graphics::CommandBufferID cmd_id,
        const std::shared_ptr<VKBuffer>& buffer) override;
 
private:
    struct Params {
        GridPushConstants grid;
        float spawn_density_threshold;
    };
 
    Params m_params;
    std::shared_ptr<Nodes::Network::GpuFieldOperator> m_particle_op;
    uint64_t m_built_revision;
};
 
} // namespace MayaFlux::Buffers