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libakgl/include/akgl/physics.h
Andrew Kesterson 5e213061af Call collide from the simulation, after the move rather than before it 
The collide slot has existed unused since the backend was written. It is wired
in now, and its signature changes from (self, a1, a2) to (self, actor, dt): the
old shape had nowhere to put a normal, a depth, or a piece of map geometry, and
no answer to which of the two actors it was supposed to be resolving.

**Resolution runs after `move`.** That is the decision everything else follows
from. A resolver called from movementlogicfunc runs before gravity, drag and the
move, so it has to predict where the actor will end up -- which means
re-implementing the integrator's arithmetic in the game, and being wrong
whenever the integrator changes. examples/sidescroller/collision.c carries forty
lines of exactly that, and says so.

Only `move` is subdivided. Gravity, drag, the thrust integration and the speed
ellipse all still run once against the whole dt, and sum(subdt) is dt, so an
actor with nothing to collide with takes one sub-step of exactly dt -- `dt/1.0f`
is dt exactly in IEEE-754 -- and follows the arithmetic path it always did. That
is what lets the integrator stay untouched and every recorded physics number
stay valid, and tests/physics.c and tests/physics_sim.c pass unchanged with no
world attached.

Collision is opt-in. A backend with no collision world costs one comparison per
actor per step and does nothing else.

Also here:

- `self->gravity` was never null-checked and is dereferenced unconditionally, so
  a hand-built backend with only `move` filled in crashed rather than reporting.
- Children are re-snapped in a post-pass, gated on collision being on. The
  in-loop snap uses whatever position the parent had when the child's slot came
  up in pool order, which is already sometimes a frame stale; that was harmless
  while a parent only moved by v*dt and is not once a parent can be pushed out
  of a wall mid-step.
- tests/physics.c's deliberate tripwire has been visited. It asserted that
  arcade collide always raised AKERR_API, with a comment saying it existed so
  that implementing collision would have to come back here. What replaces it
  asserts the opt-in contract instead.
- physics_sim.c's hard-coded "%d of 7 simulations" is derived now. A literal
  goes stale the first time somebody adds a simulation, which is this commit.

Three new whole-motion simulations, and the third took two attempts to make
honest:

- Landing and resting: 0.0000 px of drift over 120 steps after settling.
- Walking after landing: 107 px in a second, which is the symptom a player sees
  when a resting actor is caught on the ground it is standing on.
- A fast fall not tunnelling. The first version used gravity, so the step size
  grew every frame and the actor happened to land *inside* the floor rather than
  stepping over it -- it passed with sub-stepping disabled and proved nothing.
  It uses a constant 1200 px/s now, so every step is exactly 60 pixels against a
  32-pixel window in which an overlap exists, and the samples miss it cleanly.
  With sub-stepping the actor stops at y=288; without it, y=1300.

Co-Authored-By: Claude Code <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
2026-08-02 07:05:09 -04:00

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/**
* @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/collision.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 *actor, float32_t dt); /**< Resolve one actor against this backend's collision world for one sub-step. Called by akgl_physics_simulate after every sub-move. A `NULL` slot, or a `NULL` #collision, means nothing collides. */
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. */
struct akgl_CollisionWorld *collision; /**< The collision world this backend resolves against, or `NULL` for none. A backend with no world behaves exactly as one did before collision existed, which is what makes collision opt-in and what keeps the recorded physics baseline honest. */
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 actor The actor that would have been resolved. Required, and checked,
* because a caller passing rubbish should find out regardless of
* which backend happens to be installed.
* @param dt Length of the sub-step, in seconds. Ignored.
* @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_null_collide(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
* @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: resolve one actor against the world.
*
* Called by akgl_physics_simulate after each sub-move, so it resolves against
* where the actor is rather than where it was predicted to be.
*
* **A backend with no #akgl_PhysicsBackend::collision returns success having
* changed nothing.** That is what makes collision opt-in: a program written
* before any of this existed installs a backend, never attaches a world, and
* runs exactly as it did.
*
* @param self The backend. Required.
* @param actor The actor to resolve. Required.
* @param dt Length of the sub-step, in seconds. Carried on each contact so a
* response can scale with it.
* @return `NULL` on success, otherwise an error context owned by the caller.
* @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
* @throws AKGL_ERR_COLLISION If a narrowphase query could not be answered.
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_collide(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
* @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_
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