Battle royale vertical slice

Battle Royale vertical slice — implementation spec (v0.1)

Authoritative design: DESIGN.md. This file is the slice scope: the smallest set of engine changes that makes a battle-royale match playable, watchable, and decisive, so we can answer "is this fun?" from a replay.

Non-goals for the slice: renaming src/ctf/**, manifest/league upload, golden-hash corpus, item roster expansion, minimap/zoom broadcast work.

Mode gating (hard requirement)

Everything below hangs off a new config field mode ("ctf" default, "ffa" = battle royale). With mode == "ctf" the sim must stay byte-identical to the fork point: no new RNG draws, no gameHash shape change, existing replay fixtures and tests untouched, no GameVersion bump.

config.json stays the CTF default. The slice adds config.br.json for the battle-royale match, used by the demo script.

A. FFA core

1. mode: "ffa", numPlayers (2..16, validated; everything derives from it — no array[16, ...] state). Unknown-config-key warnings must still fire.
2. Single life: lives = 1, no respawn (respawnTimer never rearms), spawn hitPoints = 20. Fists deal 2 damage at contact range; low/mid/heavy gun tiers deal 2/3/5 damage with 150%/100%/60% cooldowns. Spray and grenade hits remain 4 damage (cross-trench grenade splash remains 1).
3. No hearts/flags in ffa: none spawned, no carry/capture/steal path reachable, no capture scoring. Endzone/pedestal geometry may remain as terrain.
4. Spawn pads: procedurally spaced ring for any N — equal distance to center, with every FFA player starting unarmed. A seed-derived integer offset rotates seat ownership of the unchanged pad set per episode; the mapping is bijective and CTF spawn positions are untouched. Fists are the fallback weapon, while rectangular-arena-anchored low/mid/heavy bands offer a broad weak pickup across the board, an intermediate current-strength pickup, and a scarce heavy weapon in the center risk/reward contest. The bands do not collapse when the safe zone reaches its final floor. Pads use maximum pairwise spacing, snapped to reachable floor, never a fixed array. Teams are irrelevant in ffa: per-player identity comes from color/skin (distinct hue per slot), and no code path may branch on team for damage, vision, scoring, or win.
5. End conditions: <= 1 alive, or maxTicks cap. gameover must always name a winner — never -1 while at least one player joined.
6. Placement total order (no draws): alive-at-cap > later death tick > more kills > more damage dealt > lower slot. Same-tick deaths: damage dealt, then slot. Needs a per-player damageDealt counter (in-sim, deterministic).

B. Scoring

reward per player = survivalSeconds * 1 (accrued live, one point per 24 ticks alive) + podium (1st +100, 2nd +40, 3rd +15) + 10 per kill (last damager) + 4 per assist, split among the other damagers of the victim within the last 240 ticks (integer split, remainder dropped — deterministic). Config knobs with these defaults: survivalPointsPerSec, killPoints, assistPoints, assistWindowTicks, podiumPoints. Environmental/ring deaths credit nobody.

C. Ring — a fence, not a clock

Circular safe zone, centered on the map center, radius shrinking linearly from "covers the whole arena" to a floor of 3% of arena area, reached at ringShrinkSec (default 150 s) and then constant. Outside: 1 HP per ringDamageTicks (default 48 = one HP per 2 s), no scaling. Integer math only (compare squared distances; no floats anywhere in the sim). Schedule is fully described in player_config so policies can plan. Config: ringEnabled, ringShrinkSec, ringFloorAreaPct, ringDamageTicks.

The shipped baseline doctrine closes on the nearest living enemy when three or fewer players remain, rather than disengaging while hurt; ring safety and normal aim/fire gating still apply.

D. Proximity chat

Reuse the existing shout system; in ffa, delivery is gated by the recipient being inside the sender's vision radius and LOS (same test as visible.agents), for both broadcast and dm. Out-of-range sends are legal and return/record as blocked rather than erroring. Keep the 1 msg/s cooldown. Raising ShoutMaxChars is optional for the slice — only do it if the chat chip rendering stays legible; otherwise leave it and note it.

E. Arena

config.br.json uses generated terrain (mapPath: "gen" or a pool entry) at ~5x the CTF arena area (CTF arena is 1235x659 ≈ 814k px², so target ≈ 4.0M px², e.g. ~2760x1470) with structures and trenches for cover. If the generator's size bands can't reach that with symmetry validation passing, report the largest that does and say so — do not silently ship a small map.

F. Baseline bot + demo

Baseline doctrine (slice-level, in priority order): stay inside the ring → disengage and heal when hurt (break LOS first) → take fights only with an advantage (target is critical, or we outnumber) → otherwise roam/loot → hail agents that enter vision. With three or fewer players alive, it instead closes on the nearest enemy while retaining ring safety and normal aim/fire gates. It only has to make matches end decisively and read well on the broadcast; it does not have to be strong.

Demo: one command that runs a full headless N-bot match (server + N baselines), prints the final ranking + scores, and writes a .bitreplay that plays in the existing replay viewer.

G. Verification

• nim check src/ctf.nim clean.
• Full existing test suite green (nim c -r -d:release tests/tests.nim from repo root) — proof that mode: "ctf" is untouched.
• New tests: N-derived spawn ring for N in {2, 5, 16}; single-life elimination and HP pool; ring damage only outside and only on schedule; placement total order incl. the same-tick-death tiebreak; scoring arithmetic (survival + podium + kill + assist split); chat delivered in range and blocked out of range; ffa match always ends with a named winner (both by wipe and by cap).
• Determinism: same seed twice → identical tick hashes, at N=4 and N=16.
• docs/ENV_VARIATION.md row for every new config field (repo rule).