HoneyBounce

PHP port of charmbracelet/harmonica — damped-spring physics + Newtonian projectile simulation for animation. Pure math; no terminal dependency.

composer require sugarcraft/honey-bounce

Spring

Damped harmonic oscillator (Ryan-Juckett's algorithm). Choose dampingRatio: < 1 oscillates, = 1 is critical (no overshoot, fastest convergence), > 1 is over-damped.

use SugarCraft\Bounce\Spring;

$spring = new Spring(
    deltaTime:        Spring::fps(60),  // 1/60 of a second
    angularFrequency: 6.0,              // rad/sec
    dampingRatio:     1.0,              // critical
);
$pos = 0.0;
$vel = 0.0;
$target = 100.0;

for ($frame = 0; $frame < 60; $frame++) {
    [$pos, $vel] = $spring->update($pos, $vel, $target);
    echo sprintf("frame %2d  pos=%.2f  vel=%.2f\n", $frame, $pos, $vel);
}

Spring::fps(int $n) returns 1.0 / $n for the deltaTime — pair with the same $n per-second simulation cadence.

Projectile

Newtonian-physics simulator for arcs / bouncing balls / particle effects.

use SugarCraft\Bounce\{Point, Projectile, Vector};

$p = Projectile::new(
    deltaTime:    Spring::fps(60),
    position:     Point::zero(),
    velocity:     new Vector(5.0, -10.0),
    acceleration: Projectile::gravity(),  // (0, 9.81) — Y-down
);
for ($i = 0; $i < 60; $i++) {
    $p = $p->update();
    echo sprintf("t=%2d  pos=(%.1f, %.1f)\n", $i, $p->position->x, $p->position->y);
}

Gravity constants: Projectile::GRAVITY (9.81) and Projectile::TERMINAL_GRAVITY (53.0). Helper factories Projectile::gravity() and Projectile::terminalGravity() return Y-axis Vector instances ready to drop into the constructor.

SugarCraft\Bounce\Gravity exposes the same vectors as static accessors at the package level — Gravity::standard(), Gravity::terminal(), Gravity::standardYDown(), Gravity::terminalYDown() — so call sites translating from harmonica's package-level Gravity / TerminalGravity constants read uniformly.

Damping-ratio regimes

The dampingRatio argument to Spring picks one of three classical behaviours:

• Under-damped (ζ < 1) — oscillates around the target, amplitudes decaying each cycle. Picks for "bouncy" feel.
• Critically-damped (ζ = 1) — fastest convergence with no overshoot. The default for "snap to value" animations.
• Over-damped (ζ > 1) — converges without overshoot but slower than critical. Picks for slow, weighty motion.

Negative damping ratios are clamped to 0 (a pure oscillator with no decay would never settle).

Coordinate systems

Both Vector and Point are 3D (x, y, z) — the constructor's $z defaults to 0.0 so existing 2D call sites still compile unchanged. Use the third dimension when porting demos that need a Z axis (parallax / depth-shaded particle systems).

The Y-axis convention is Y-up by default to match upstream harmonica: Gravity::standard() returns (0, -9.81, 0) so increasing Y means "up the screen". Terminal renderers usually grow downward — flip to Gravity::standardYDown() (or its Projectile::gravityYDown() alias) when you want gravity to pull toward the bottom of the grid without manually negating every coordinate.

Projectile::update() returns a new Projectile instance each call (immutable-with-pattern); upstream Projectile.Update() returns the new Point and mutates the receiver in place. Read the new position from result->position rather than $p->position().

Public API

• Spring — __construct($dt, $ω, $ζ) / update($pos, $vel, $target) / Spring::fps(int).
• Projectile — Projectile::new(...) / update() / position() / velocity() / acceleration() / gravity() / terminalGravity() / gravityYDown() / terminalGravityYDown() / GRAVITY / TERMINAL_GRAVITY.
• Gravity — package-level static accessors mirroring harmonica's Gravity / TerminalGravity constants: standard(), terminal(), standardYDown(), terminalYDown().
• Vector — immutable 3D vector with add / sub / scale / length / dot / cross / Vector::zero().
• Point — immutable 3D point with add(Vector) / distance / Point::zero().

Test

cd honey-bounce && composer install && vendor/bin/phpunit

Demos

Projectile motion

Spring physics