Grapple Swing

A grapple-hook mechanic: fire a rope at an anchor point and free-fall becomes a pendulum swing. Reel the rope in or out mid-swing to trade arc for height, and release at the top of the arc to sling off with full momentum. The demo is a small gap-crossing arena — three girders strung across a pit, cleared by chaining swings rather than jumping.

Reuse

The mechanic lives in src/grapple-swing.js as a framework-free ES module:

• createGrappler(opts) — returns a state object; call update(dt, input) each frame.
• opts: gravity, airControl, maxAirSpeed, minRope, maxRope, reelSpeed, releaseBoost.
• attach(anchor) / release() — the module doesn’t pick anchors or know about a level; the caller decides what’s grappleable (nearest in range, line-of-sight, whatever fits) and calls attach({x, y}). release() keeps whatever velocity the swing built up, times releaseBoost.
• input: { moveX: -1..1, reel: -1..1 } — air control while free or swinging, reel in/out while attached.
• State exposes pos, vel, attached, anchor, ropeLength — draw the rope as a line from anchor to pos, however fits the project.

The core swing step: once attached, if the distance to the anchor exceeds ropeLength, the position is projected back onto that circle and the outward-radial component of velocity is stripped, leaving only the tangential component. That’s the whole pendulum — no trig, no rope segments.

Port notes for DragonRuby: update() is ~30 lines of pure state math with no DOM dependencies — translate directly into a tick method; the rope draws as one line primitive from anchor to pos.

Gotchas

• The constraint only fires when the player is outside the rope radius — swinging in tighter than ropeLength (e.g. after reeling in) is unconstrained until it stretches taut again, which is correct pendulum behavior but can surprise you if you expected a rigid rod.
• release() is momentum-conserving by design; tune releaseBoost down to 1.0 if a project wants a pure sling with no extra pop.
• The demo in demo/ bundles its own copy of the module (self-contained by contract). If you touch src/, re-copy it into demo/.