FS.GG.Game.Core 0.4.0

FS.GG.Game.Core

BCL-only deterministic simulation core for FS.GG.Game — the platform's bottom layer. Depends on nothing but FSharp.Core and reaches up to no other FS-GG package.

The sim vocabulary

The shared types — Point, Rect, Circle, ConvexPolygon, and the detection values Contact (a single-axis penetration manifold), Manifold (its multi-point form) and RayHit (a segment-cast result) — live directly in the FS.GG.Game.Core namespace, so open FS.GG.Game.Core is all a caller needs. The tile index Cell is namespace-level too. Everything below is a [<RequireQualifiedAccess>] module, called as Grids.cellAt, Rng.nextInt, and so on.

• Rng — SplitMix64 seeded RNG (ofSeed/nextInt/nextFloat/nextBool/split), byte-identical across runs.
• FixedStep — fixed-timestep accumulator drain (drain/drainWith). No wall-clock read, so a scripted frame time replays exactly.
• Loop — the fixed-step double-buffered loop built on FixedStep.drain (advance/alpha): the world as of the last completed step (Current), the one before it (Previous), and the carried sub-step time (Accumulator). Renderers interpolate Previous → Current by alpha, so motion is smooth at any frame rate while the sim only ever advances by whole fixed steps. The default for any continuously-moving simulation.
• InputCommand — the abstract, device-free vocabulary of input intents (Command.MoveNorth/…/Fire/Pause) as pure data. The policy half of the input split: Game.Core owns which commands exist; the render/input layer owns how a device makes one and never learns a game action.

Collision — detection, then response

• Geometry — the detection surface: overlap and containment for AABB, circle, and convex polygon (intersects/contains/aabbContact/circleContact/polygonContact/obbPolygon), swept tests (sweptIntersects), and segment casts (segmentAabbHit/segmentCircleHit/segmentPolygonHit/…). It returns manifests as values and never resolves them.
• Resolution — the response layer that consumes a detection Contact and produces transforms (pushOut/slide/push; knockback is a deprecated shim over push). Deliberately separate from detection; it never re-detects.
• SpatialGrid — uniform spatial partitioning for the broad phase (build/query/queryRadius).
• Physics — an opt-in mini rigid-body engine: a world of bodies, a broad phase, a narrow phase, and a semi-implicit Euler step with a warm-started sequential-impulse contact solver over bodies that sleep once they settle. Mass is derived from a shape's area, not given. Arcade Resolution stays the first-class default; neither module knows the other.

The grid

• Pathfinding — grid A*/BFS over walkable cells (astar/bfs), plus the many-to-one AI substrate distanceField/flowField/reachableWithin.
• Grids — the parts of a square grid: Edge, Vertex, GridSpec, the six adjacency conversions, and the pixel↔cell map (cellRect/cellCenter/edgeSegment/cellAt). Names the boundary between two tiles, which is where a wall lives.

Sight

• Los — point-to-point grid line of sight over a caller-supplied opacity predicate (line/supercover/trace/lineOfSight).
• Fov — grid field of view by symmetric shadowcasting (fov). Opacity is not walkability: this module consults opacity only.
• Visibility — the continuous-space sibling: exact point-to-point sight against arbitrary occluder segments, and the full angular-sweep visibility polygon (raySegment/isVisible/polygon). It can answer occlusion between two moving bodies, which a tile walk cannot.

Pathfinding, Los, and Fov all take terrain as a pure Cell -> bool predicate — the framework holds no map, so one terrain plugs into all three.

Combat and decision

• Ballistics — projectile weapons over continuous space: a swept advance that cannot tunnel (a fast round is a segment, never a point), a lead / intercept solve, and a splash query with a caller-chosen falloff. Lifetimes are counted in whole ticks, not float seconds, so a replay cannot drift.
• Effects — the mitigation layer: what one incoming hit becomes at the target. It separates the damage Source (a Declared attack — the only thing cover and armor reduce — vs Collision/Environmental/Periodic) from the damage kind, which stays generic in 'K (Game.Core never learns what Frost is).
• Ai — the decision layer's vocabulary: a comparable AgentId that fixes iteration order (never hash order, which would diverge a replay), and the fog-of-war TeamView — an abstract value of what a team knows, whose sightings decay from spotted into ghosts. Its LOS is a caller-supplied oracle, so perception policy stays with the game.

Where it came from

Three provenances:

• Extracted from FS.GG.UI.Canvas / FS.GG.UI.Scene and reimplemented BCL-only, per ADR-0022 — the shared types, Geometry, Rng, FixedStep, Pathfinding, SpatialGrid.
• Promoted from the frozen starter fragments in FS.GG.Rendering (template/fragments/*), which every game that wanted them previously had to copy and diverge from — Los, Visibility, Grids.
• Authored here through the SDD lifecycle, because the org had nothing to promote — Fov, Resolution, Loop, InputCommand, Ballistics, Effects, Ai, and Physics.

Guarantees

Headlessly testable — zero Skia, zero Scene. Purity, totality (degenerate and non-finite input degrades to a documented value rather than throwing), and determinism (integer logic / float presentation) are the contract each module's .fsi states and its tests hold it to — so identical input replays byte-identically across runs, and a module is safe to call from a replayed simulation step. Cross-platform byte-identity is unconditional for the integer / grid layer; the float-heavy modules scope it (Visibility to |coord| <= 1e6) or defer full lockstep to a later fixed-point ADR (Physics.checksum), so trust the per-module .fsi, not one blanket claim.

To project sim state onto drawables, add FS.GG.Game.Render.

House style: .fsi is the sole public surface; net10.0; -preview channel. See FS-GG/FS.GG.Game and ADR-0022.