v0.28.0 (#46): per-region stats — area×population-weighted-median Nation.stats (PR 1 of 2)

shared/protocol.ts

@@ -8,10 +8,12 @@ import type {
   GameState,
   MarkerType,
   Nation,
+  NationStats,
   RelationStatus,
+  RegionStats,
   StatKey,
 } from './types';
-import { MARKER_TYPES } from './types';
+import { MARKER_TYPES, STAT_KEYS } from './types';
 
 /** A numeric change to one of a nation's 0–100 stats. */
 export type StateChange =
@@ -515,3 +517,137 @@ export function nationForPrompt(n: Nation) {
     relations: n.relations,
   };
 }
+
+// --- Per-region stats (v0.28, #46) ------------------------------------------
+//
+// A nation's 0–100 `NationStats` are no longer stored as independent per-nation
+// values: they are DERIVED as the area×population-weighted median of the stats
+// of the regions the nation owns (`GameState.regionStats`, keyed by region key).
+// These pure helpers implement that derivation, the fresh-game / legacy seeding,
+// and the one-time migration of a pre-v0.28 row. They have no runtime deps.
+
+/**
+ * The weighted median of `values` with per-value `weights` (paired by index).
+ * The pairs are sorted by value ascending and the result is the value at the
+ * first point where the cumulative weight reaches at least half the total
+ * weight. An empty input, or a total weight of zero, yields 0. Non-finite or
+ * non-positive weights are treated as 0 (excluded from the median).
+ * Translation-invariant: adding a constant to every value shifts the result by
+ * that constant — so the median of a set of **identical** values is that value
+ * (the invariant the equal-value seeding and legacy migration rely on).
+ * Pure + shared.
+ */
+export function weightedMedian(values: number[], weights: number[]): number {
+  if (values.length === 0) return 0;
+  const pairs = values
+    .map((v, i) => {
+      const w =
+        typeof weights[i] === 'number' && Number.isFinite(weights[i]) && weights[i] > 0
+          ? weights[i]
+          : 0;
+      return { v: Number.isFinite(v) ? v : 0, w };
+    })
+    .filter((p) => p.w > 0);
+  if (pairs.length === 0) return 0;
+  const total = pairs.reduce((s, p) => s + p.w, 0);
+  if (total <= 0) return 0;
+  pairs.sort((a, b) => a.v - b.v);
+  let cum = 0;
+  for (const p of pairs) {
+    cum += p.w;
+    if (cum >= total / 2) return p.v;
+  }
+  return pairs[pairs.length - 1]!.v;
+}
+
+/**
+ * Derive a nation's aggregate {@link NationStats} from the stats of the regions
+ * it owns: each of the four stats is the **area×population-weighted median**
+ * (see {@link weightedMedian}) over the nation's owned region keys. A region's
+ * weight is `(area > 0 ? area : 1) × (population > 0 ? population : 1)` — both
+ * factors fall back to 1 when the region has no geo entry (`geoOf` returns
+ * `undefined` for it), so every region carries weight ≥ 1 and a missing geo
+ * degrades gracefully to a uniform (unweighted) median. A region key with no
+ * `regionStats` entry is skipped. Returns the zero stats when the nation owns
+ * no region with a stats entry (the caller's 0-region guard keeps the nation's
+ * directly-maintained `n.stats` in that case). Pure + shared.
+ */
+export function deriveNationStats(
+  regionKeys: string[],
+  regionStats: Record<string, RegionStats>,
+  geoOf?: (key: string) => { area: number; population: number } | undefined,
+): NationStats {
+  const zero: NationStats = { military: 0, economy: 0, stability: 0, population: 0 };
+  const byStat: Record<StatKey, number[]> = {
+    military: [],
+    economy: [],
+    stability: [],
+    population: [],
+  };
+  const weights: number[] = [];
+  for (const key of regionKeys) {
+    const rs = regionStats[key];
+    if (!rs) continue; // a key with no per-region stats is skipped
+    const g = geoOf?.(key);
+    const a = g && g.area > 0 ? g.area : 1;
+    const p = g && g.population > 0 ? g.population : 1;
+    weights.push(a * p);
+    for (const k of STAT_KEYS) byStat[k].push(rs[k]);
+  }
+  if (weights.length === 0) return zero;
+  const out: NationStats = { military: 0, economy: 0, stability: 0, population: 0 };
+  for (const k of STAT_KEYS) out[k] = clampStat(weightedMedian(byStat[k], weights));
+  return out;
+}
+
+/**
+ * Seed a per-region stats table from a set of nations (used both at game
+ * creation and by the legacy migration): every owned region key gets a **copy**
+ * of its owner's current `NationStats`, so a fresh nation's regions all start
+ * identical (the weighted median of identical values is that value, so the
+ * derived `n.stats` equals the seeded one). A region with two or more owners
+ * (corrupt) is seeded by the first owner encountered; a key owned by nobody gets
+ * no entry. Pure + shared.
+ */
+export function seedRegionStats(nations: Record<string, Nation>): Record<string, RegionStats> {
+  const out: Record<string, RegionStats> = {};
+  for (const n of Object.values(nations)) {
+    for (const key of n.territory) {
+      if (!(key in out)) out[key] = { ...n.stats }; // first owner wins (sole ownership holds normally)
+    }
+  }
+  return out;
+}
+
+/**
+ * (v0.28, #46) One-time migration of a **legacy** (pre-v0.28) `GameState` row
+ * into the per-region-stats model, returning a new state (the input is never
+ * mutated):
+ *  - a row that already has `regionStats` (a v0.28+ save) is returned
+ *    **UNCHANGED** — its `Nation.stats` are already the correct derived medians
+ *    and must not be recomputed on load (that would drift them);
+ *  - a legacy row (`regionStats` absent at runtime) seeds every owned region key
+ *    with a copy of its owner's current stats (so each nation's regions start
+ *    identical), then recomputes each nation's stats as the weighted median of
+ *    its regions (a 0-region nation keeps its stats). Without `geoOf` the
+ *    weighting is uniform, which is weight-invariant for the identical-value
+ *    case, so the recomputed stats equal the originals — the migration is a
+ *    no-op on the visible numbers and only materializes the (previously
+ *    implicit) per-region values. Never throws.
+ */
+export function migrateRegionStats(
+  state: GameState,
+  geoOf?: (key: string) => { area: number; population: number } | undefined,
+): GameState {
+  // A v0.28+ row already carries per-region stats: leave it exactly as saved.
+  if (state.regionStats !== undefined) return state;
+  // Legacy row: materialize per-region stats from each nation's (uniform) stats.
+  const next: GameState = { ...state, regionStats: seedRegionStats(state.nations) };
+  for (const n of Object.values(next.nations)) {
+    if (n.territory.length === 0) continue; // 0-region guard: keep its stats
+    if (n.territory.some((k) => next.regionStats[k])) {
+      n.stats = deriveNationStats(n.territory, next.regionStats, geoOf);
+    }
+  }
+  return next;
+}