Add PF2e encounter difficulty calculation with 5-tier budget system

Implements PF2e encounter difficulty alongside the existing D&D system.
PF2e uses creature level vs party level to derive XP, compares against
5-tier budgets (Trivial/Low/Moderate/Severe/Extreme), and adjusts
thresholds for party size. The indicator shows 4 bars in PF2e mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/domain/src/encounter-difficulty.ts

@@ -1,7 +1,7 @@
 import type { RulesEdition } from "./rules-edition.js";
 
-/** Abstract difficulty severity: 0 = negligible, 3 = maximum. Maps to filled bar count. */
-export type DifficultyTier = 0 | 1 | 2 | 3;
+/** Abstract difficulty severity: 0 = negligible, up to 4 (PF2e Extreme). Maps to filled bar count. */
+export type DifficultyTier = 0 | 1 | 2 | 3 | 4;
 
 export interface DifficultyThreshold {
 	readonly label: string;
@@ -18,6 +18,8 @@ export interface DifficultyResult {
 	readonly adjustedXp: number | undefined;
 	/** 2014 only: true when the multiplier was shifted due to party size (<3 or 6+). */
 	readonly partySizeAdjusted: boolean | undefined;
+	/** PF2e only: the derived party level used for XP calculation. */
+	readonly partyLevel: number | undefined;
 }
 
 /** Maps challenge rating strings to XP values (standard 5e). */
@@ -160,6 +162,133 @@ function getEncounterMultiplier(
 	};
 }
 
+/**
+ * PF2e: XP granted by a creature based on its level relative to party level.
+ * Key is (creature level − party level), clamped to [−4, +4].
+ */
+const PF2E_LEVEL_DIFF_XP: Readonly<Record<number, number>> = {
+	[-4]: 10,
+	[-3]: 15,
+	[-2]: 20,
+	[-1]: 30,
+	0: 40,
+	1: 60,
+	2: 80,
+	3: 120,
+	4: 160,
+};
+
+/** PF2e base encounter budget thresholds for a party of 4. */
+const PF2E_THRESHOLDS_BASE = {
+	trivial: 40,
+	low: 60,
+	moderate: 80,
+	severe: 120,
+	extreme: 160,
+} as const;
+
+/** PF2e per-PC adjustment to each threshold (added per PC beyond 4, subtracted per PC fewer). */
+const PF2E_THRESHOLD_ADJUSTMENTS = {
+	trivial: 10,
+	low: 15,
+	moderate: 20,
+	severe: 30,
+	extreme: 40,
+} as const;
+
+/**
+ * Derives PF2e party level from PC levels.
+ * Returns the mode (most common level). If no unique mode, returns
+ * the average rounded to the nearest integer.
+ */
+export function derivePartyLevel(levels: readonly number[]): number {
+	if (levels.length === 0) return 0;
+	if (levels.length === 1) return levels[0];
+
+	const counts = new Map<number, number>();
+	for (const l of levels) {
+		counts.set(l, (counts.get(l) ?? 0) + 1);
+	}
+
+	let maxCount = 0;
+	let mode: number | undefined;
+	let isTied = false;
+
+	for (const [level, count] of counts) {
+		if (count > maxCount) {
+			maxCount = count;
+			mode = level;
+			isTied = false;
+		} else if (count === maxCount) {
+			isTied = true;
+		}
+	}
+
+	if (!isTied && mode !== undefined) return mode;
+
+	const sum = levels.reduce((a, b) => a + b, 0);
+	return Math.round(sum / levels.length);
+}
+
+/** Returns PF2e XP for a creature given its level and the party level. */
+export function pf2eCreatureXp(
+	creatureLevel: number,
+	partyLevel: number,
+): number {
+	const diff = Math.max(-4, Math.min(4, creatureLevel - partyLevel));
+	return PF2E_LEVEL_DIFF_XP[diff] ?? 0;
+}
+
+function calculatePf2eBudget(partySize: number) {
+	const adjustment = partySize - 4;
+	return {
+		trivial: Math.max(
+			0,
+			PF2E_THRESHOLDS_BASE.trivial +
+				adjustment * PF2E_THRESHOLD_ADJUSTMENTS.trivial,
+		),
+		low: Math.max(
+			0,
+			PF2E_THRESHOLDS_BASE.low + adjustment * PF2E_THRESHOLD_ADJUSTMENTS.low,
+		),
+		moderate: Math.max(
+			0,
+			PF2E_THRESHOLDS_BASE.moderate +
+				adjustment * PF2E_THRESHOLD_ADJUSTMENTS.moderate,
+		),
+		severe: Math.max(
+			0,
+			PF2E_THRESHOLDS_BASE.severe +
+				adjustment * PF2E_THRESHOLD_ADJUSTMENTS.severe,
+		),
+		extreme: Math.max(
+			0,
+			PF2E_THRESHOLDS_BASE.extreme +
+				adjustment * PF2E_THRESHOLD_ADJUSTMENTS.extreme,
+		),
+	};
+}
+
+function scanCombatantsPf2e(
+	combatants: readonly CombatantDescriptor[],
+	partyLevel: number,
+) {
+	let totalCreatureXp = 0;
+
+	for (const c of combatants) {
+		if (c.creatureLevel !== undefined) {
+			const xp = pf2eCreatureXp(c.creatureLevel, partyLevel);
+			if (c.side === "enemy") {
+				totalCreatureXp += xp;
+			} else {
+				totalCreatureXp -= xp;
+			}
+		}
+	}
+
+	return { totalCreatureXp: Math.max(0, totalCreatureXp) };
+}
+
 /** All standard 5e challenge rating strings, in ascending order. */
 export const VALID_CR_VALUES: readonly string[] = Object.keys(CR_TO_XP);
 
@@ -171,6 +300,7 @@ export function crToXp(cr: string): number {
 export interface CombatantDescriptor {
 	readonly level?: number;
 	readonly cr?: string;
+	readonly creatureLevel?: number;
 	readonly side: "party" | "enemy";
 }
 
@@ -247,6 +377,41 @@ export function calculateEncounterDifficulty(
 	combatants: readonly CombatantDescriptor[],
 	edition: RulesEdition,
 ): DifficultyResult {
+	if (edition === "pf2e") {
+		const partyLevels: number[] = [];
+		for (const c of combatants) {
+			if (c.level !== undefined && c.side === "party") {
+				partyLevels.push(c.level);
+			}
+		}
+
+		const partyLevel = derivePartyLevel(partyLevels);
+		const { totalCreatureXp } = scanCombatantsPf2e(combatants, partyLevel);
+		const budget = calculatePf2eBudget(partyLevels.length);
+		const thresholds: DifficultyThreshold[] = [
+			{ label: "Trivial", value: budget.trivial },
+			{ label: "Low", value: budget.low },
+			{ label: "Moderate", value: budget.moderate },
+			{ label: "Severe", value: budget.severe },
+			{ label: "Extreme", value: budget.extreme },
+		];
+
+		return {
+			tier: determineTier(totalCreatureXp, [
+				budget.low,
+				budget.moderate,
+				budget.severe,
+				budget.extreme,
+			]),
+			totalMonsterXp: totalCreatureXp,
+			thresholds,
+			encounterMultiplier: undefined,
+			adjustedXp: undefined,
+			partySizeAdjusted: undefined,
+			partyLevel,
+		};
+	}
+
 	const { totalMonsterXp, monsterCount, partyLevels } =
 		scanCombatants(combatants);
 
@@ -268,6 +433,7 @@ export function calculateEncounterDifficulty(
 			encounterMultiplier: undefined,
 			adjustedXp: undefined,
 			partySizeAdjusted: undefined,
+			partyLevel: undefined,
 		};
 	}
 
@@ -294,5 +460,6 @@ export function calculateEncounterDifficulty(
 		encounterMultiplier,
 		adjustedXp,
 		partySizeAdjusted,
+		partyLevel: undefined,
 	};
 }