# Design

## Context

Input, placement, HiDPI, and grid are implemented. Simulation state (`sim_paused`, `ACTION_PAUSE`, `ACTION_STEP`, speed actions) exists but is unused. See proposal.md.

## Goals / Non-Goals

**Goals:**
- Conway tick loop with adjustable interval.
- Pause/resume, stepping, speed control.
- Paused-only cell editing via mouse, including drag painting.

**Non-Goals:**
- Pattern detection or statistics.
- Undo/redo.
- Multi-threading.

## Decisions

### D1: Tick timing
- Use monotonic clock in the frame loop; accumulate elapsed time; when >= interval, compute next generation and reset accumulator.

### D2: Next generation buffer
- Allocate a second grid buffer; compute into it, then swap pointers (or copy). Prefer swap of `cells` pointers with size unchanged.

### D3: Editing while paused
- Pointer button handler sets/clears cells if `sim_paused`; drag paints using motion events while button held.

### D4: Speed bounds
- Interval range 0.05s..1.0s (20 tps..1 tps), step factor 2.

### D5: Default bindings
- Add `p`, `n`, `+`/`-` to default config (merge).

## Risks / Trade-offs

- [Tick cost on large grids] -> grid is small (50x50), fine.
- [Editing during tick race] -> editing only while paused avoids races.

## Open Questions

None.