libakgl/include/akgl/physics.h

/**
 * @file physics.h
 * @brief The pluggable physics backend, and the two implementations that ship.
 *
 * Same shape as the renderer: a record of function pointers plus an initializer
 * that fills it in. `null` accepts every call and changes nothing, which is what
 * a menu screen or a test harness wants; `arcade` applies gravity, drag, thrust
 * and a speed cap. Selecting between them is akgl_physics_factory, driven by the
 * `physics.engine` configuration property -- not a branch in the simulation.
 *
 * The model an actor is simulated under, and the fields on akgl_Actor that carry
 * it:
 *
 * | Term                  | Fields       | Comes from                                    |
 * |-----------------------|--------------|-----------------------------------------------|
 * | thrust                | `tx, ty, tz` | the actor's own acceleration while it is moving |
 * | environmental         | `ex, ey, ez` | gravity, less drag                            |
 * | velocity              | `vx, vy, vz` | thrust + environmental                        |
 * | max speed             | `sx, sy, sz` | the character; caps the thrust vector, not velocity |
 * | acceleration          | `ax, ay, az` | the character                                 |
 *
 * Each step: movement logic, then gravity, then drag, then velocity, then move.
 * Because the cap is applied to thrust rather than to velocity, gravity can
 * carry an actor faster than its stated top speed -- which is the point, for a
 * falling one.
 *
 * The cap is on the thrust **vector**, not on each axis separately. Capping the
 * axes independently lets the corner of the box through: an actor holding two
 * directions at once got both caps at once and travelled their diagonal, 41%
 * faster than either alone. Scaling to the ellipse instead keeps a character
 * whose horizontal and vertical speeds differ moving at the ratio it asked for.
 */

/**
 * @brief Default bound on one simulation step, in seconds.
 *
 * A twentieth of a second: three frames at 60 Hz, so an ordinary dropped frame
 * or two passes through untouched, and a load or a breakpoint does not.
 */
#define AKGL_PHYSICS_DEFAULT_MAX_TIMESTEP  0.05

#ifndef _AKGL_PHYSICS_H_
#define _AKGL_PHYSICS_H_

#include <SDL3/SDL.h>
#include <akerror.h>
#include <akgl/actor.h>
#include <akgl/iterator.h>
#include <akgl/staticstring.h>

/** @brief Defines a pluggable physics backend and its environmental parameters. */
typedef struct akgl_PhysicsBackend {
    akerr_ErrorContext AKERR_NOIGNORE *(*simulate)(struct akgl_PhysicsBackend *self, akgl_Iterator *opflags);            /**< Step the whole world. Both backends point this at akgl_physics_simulate. */
    akerr_ErrorContext AKERR_NOIGNORE *(*gravity)(struct akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);    /**< Apply environmental acceleration to one actor. */
    akerr_ErrorContext AKERR_NOIGNORE *(*collide)(struct akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2);     /**< Resolve a collision between two actors. Not called by the simulation loop yet. */
    akerr_ErrorContext AKERR_NOIGNORE *(*move)(struct akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);       /**< Commit one actor's velocity to its position. */

    float64_t drag_x;              /**< Fraction of environmental velocity shed per second along x. 0 disables. From `physics.drag.x`. */
    float64_t drag_y;              /**< Drag along y. From `physics.drag.y`. */
    float64_t drag_z;              /**< Drag along z. From `physics.drag.z`. */
    float64_t gravity_x;           /**< Acceleration along x, world units per second squared. Applied toward screen left. From `physics.gravity.x`. */
    float64_t gravity_y;           /**< Acceleration along y. Applied down the screen, since y grows downward. From `physics.gravity.y`. */
    float64_t gravity_z;           /**< Acceleration along z. Applied away from the camera. From `physics.gravity.z`. */
    SDL_Time gravity_time;      /**< Timestamp of the previous step, in nanoseconds. Stamped by akgl_physics_simulate and seeded by the initializers; the simulation's `dt` is measured from this. */
    float64_t max_timestep;     /**< Longest step the simulation will take, in seconds. A hitch longer than this advances the world by this much instead. 0 disables the bound. From `physics.max_timestep`, default #AKGL_PHYSICS_DEFAULT_MAX_TIMESTEP. */
} akgl_PhysicsBackend;

/**
 * @brief Gravity for the null backend: accept the call and change nothing.
 * @param self  The backend. Required -- the one thing this does check.
 * @param actor The actor that would have been accelerated. Ignored; `NULL` is
 *              accepted.
 * @param dt    Seconds since the previous step. Ignored.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self is `NULL`.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_gravity(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
 * @brief Collision for the null backend: accept the call and change nothing.
 *
 * Note that this *succeeds* where the arcade backend's collide refuses -- the
 * null backend's contract is "nothing collides", which is an answer, not a gap.
 *
 * @param self The backend. Required.
 * @param a1   First actor. Ignored; `NULL` is accepted.
 * @param a2   Second actor. Ignored; `NULL` is accepted.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self is `NULL`.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2);
/**
 * @brief Movement for the null backend: accept the call and change nothing.
 *
 * Actors under the null backend do not move on their own. Anything that sets an
 * actor's `x`/`y` directly still works -- this only declines to integrate
 * velocity.
 *
 * @param self  The backend. Required.
 * @param actor The actor that would have moved. Ignored; `NULL` is accepted.
 * @param dt    Seconds since the previous step. Ignored.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self is `NULL`.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_move(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
 * @brief Install the null backend's methods.
 *
 * Reads no configuration and touches no gravity or drag fields, so whatever was
 * in them stays -- harmlessly, since none of the null methods look.
 *
 * @param self The backend to initialize. Required.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self is `NULL`.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_init_null(akgl_PhysicsBackend *self);


/**
 * @brief Accelerate one actor's environmental velocity by gravity.
 *
 * Adds `gravity * dt` to the actor's `ex`/`ey`/`ez` on each axis whose gravity
 * is non-zero, with the sign chosen for screen space: x pulls left, y pulls
 * down, z pulls away from the camera. It touches only the environmental term, so
 * an actor's own thrust is unaffected and the speed cap does not apply -- which
 * is why a falling actor keeps accelerating past its character's top speed.
 *
 * @param self  The backend supplying the gravity constants. Required.
 * @param actor The actor to accelerate. Required.
 * @param dt    Seconds since the previous step. A `dt` of 0 is a no-op; a
 *              negative one accelerates backwards.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_gravity(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
 * @brief Collision for the arcade backend. Not implemented.
 *
 * Deliberately loud rather than silently permissive: unlike
 * akgl_physics_null_collide, which means "nothing collides", this means "nobody
 * has written this yet", and a caller that reaches it should find out.
 *
 * @param self The backend. Required, and checked before the refusal.
 * @param a1   First actor. Never examined.
 * @param a2   Second actor. Never examined.
 * @return Never `NULL`.
 * @throws AKERR_NULLPOINTER If @p self is `NULL`.
 * @throws AKERR_API Otherwise, always, with the message "Not implemented".
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2);
/**
 * @brief Commit an actor's velocity to its position.
 *
 * The last step of a simulation tick: `position += velocity * dt` on all three
 * axes. It does not clamp to the map, consult the tilemap, or test for
 * collisions -- an actor will walk straight through a wall and off the edge of
 * the world.
 *
 * @param self  The backend. Required, though nothing on it is read.
 * @param actor The actor to move. Required. Its `vx`/`vy`/`vz` must already have
 *              been computed; akgl_physics_simulate does that immediately
 *              before calling this.
 * @param dt    Seconds since the previous step.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_move(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
 * @brief Install the arcade backend's methods and read its constants from configuration.
 *
 * Reads `physics.gravity.x`, `.y`, `.z` and `physics.drag.x`, `.y`, `.z` from
 * the property registry, all defaulting to `"0.0"` -- so an arcade backend with
 * no configuration behaves like the null one until something is set.
 *
 * @param self The backend to initialize. Required. Its method pointers are
 *             installed before the properties are read, so a failure part-way
 *             leaves a usable backend with some constants still at their
 *             previous values.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self is `NULL`.
 * @throws AKERR_VALUE If one of the six properties is set to something that is
 *         not a number.
 * @throws ERANGE If one of them does not fit a `double`.
 * @throws AKGL_ERR_HEAP If the string pool is exhausted.
 *
 * @note With #AKGL_REGISTRY_PROPERTIES uninitialized every property reads back
 *       as its default, so this silently configures zero gravity and zero drag
 *       rather than reporting anything. See the warning in registry.h.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_init_arcade(akgl_PhysicsBackend *self);

/**
 * @brief Select and initialize a physics backend by name.
 *
 * The dispatch point for the whole subsystem: `"null"` and `"arcade"` are the
 * two names, and akgl_game_init passes whatever the `physics.engine` property
 * holds. Adding a backend means adding a name here, not a branch in the
 * simulation.
 *
 * @param self The backend to initialize. Required.
 * @param type The backend name. Required. Matched on its leading characters, so
 *             `"nullify"` selects `null` and `"arcadia"` selects `arcade`.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self or @p type is `NULL`.
 * @throws AKERR_KEY If @p type matches neither name. The message quotes what was
 *         asked for.
 * @throws AKERR_* Whatever the selected initializer raises.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_factory(akgl_PhysicsBackend *self, akgl_String *type);

/**
 * @brief Step every live actor forward by the time elapsed since the previous call.
 *
 * Shared by both backends -- what differs is the `gravity` and `move` they point
 * at. For each live actor, in pool order:
 *
 * - a child actor is snapped to its parent's position plus its own `v` as an
 *   offset, and skipped entirely -- children do not simulate;
 * - an actor with no character is skipped, since the speed and acceleration
 *   constants live there;
 * - thrust accumulates along an axis the actor is moving on, then the thrust
 *   *vector* is scaled to the ellipse the character's per-axis maximum speeds
 *   describe;
 * - the backend's `gravity` runs, drag is applied to the environmental term,
 *   velocity becomes environmental plus thrust, and the backend's `move`
 *   commits it.
 *
 * `dt` is measured from `self->gravity_time`, which is stamped at the end and
 * seeded by akgl_physics_init_arcade and akgl_physics_init_null. It is then
 * bounded by `self->max_timestep`: a step longer than that advances the world
 * by `max_timestep` instead of by the whole elapsed time.
 *
 * That bound is not a nicety. Anything that stalls a frame -- a level load
 * between initialization and the first step, a breakpoint, a dragged window, an
 * alt-tab -- otherwise arrives as one enormous `dt`, and one enormous `dt`
 * moves every actor by however far its velocity carries it in that whole
 * interval. A quarter-second load under ordinary platformer gravity is a
 * hundred pixels of fall between the first frame and the second, straight
 * through whatever was underneath. Advancing the world in slow motion during a
 * hitch is the trade every game engine makes here.
 *
 * @param self    The backend. Required, along with its `move` pointer.
 * @param opflags Iterator flags. Optional -- `NULL` means "simulate everything".
 *                Only #AKGL_ITERATOR_OP_LAYERMASK is honoured, restricting the
 *                step to actors on `layerid`; the update, render, and release
 *                bits are ignored here.
 * @return `NULL` on success, otherwise an error context owned by the caller.
 * @throws AKERR_NULLPOINTER If @p self or `self->move` is `NULL`.
 * @throws AKERR_* Whatever an actor's `movementlogicfunc`, or the backend's
 *         `gravity` or `move`, raises. The first failure aborts the whole step,
 *         leaving the actors already processed advanced and the rest not.
 *
 * @note An actor's `movementlogicfunc` can raise AKGL_ERR_LOGICINTERRUPT to say
 *       "skip the rest of the simulation for me this tick". It is handled here
 *       and does not propagate -- it is a control signal wearing an error's
 *       clothes, not a failure. Only `movementlogicfunc` gets that treatment:
 *       the `gravity` and `move` calls return straight out of the loop on any
 *       status, LOGICINTERRUPT included, so a backend must not use it to opt an
 *       actor out from there.
 */
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_simulate(akgl_PhysicsBackend *self, akgl_Iterator *opflags);

#endif // _AKGL_PHYSICS_H_