feat(gameplay): add versioned SRD rules migration

Co-authored-by: multica-agent <github@multica.ai>

packages/game-engine/src/index.ts

@@ -1,5 +1,5 @@
 import { randomInt, randomUUID } from 'node:crypto'
-import type { AbilityKey, Character, CheckRequest, DiceRoll, ProposedEvent, SkillKey } from '@dng/shared'
+import type { AbilityKey, Character, CharacterRules, CheckRequest, DamageType, DiceRoll, ProposedEvent, SkillKey } from '@dng/shared'
 
 export interface RandomSource {
   integer(min: number, max: number): number
@@ -16,6 +16,11 @@ export function abilityModifier(score: number): number {
   return Math.floor((score - 10) / 2)
 }
 
+export function proficiencyBonus(level: number): number {
+  if (!Number.isInteger(level) || level < 1 || level > 20) throw new Error('Character level must be between 1 and 20')
+  return 2 + Math.floor((level - 1) / 4)
+}
+
 export function rollDice(formula: string, random: RandomSource = secureRandom) {
   const match = /^(\d+)d(\d+)(?:([+-])(\d+))?$/.exec(formula)
   if (!match) throw new Error(`Unsupported dice formula: ${formula}`)
@@ -86,6 +91,54 @@ function usesProficiency(request: CheckRequest): boolean {
   return request.proficient === true
 }
 
+function proficiencyMultiplier(request: CheckRequest, actor: Character): number {
+  const rules = actor.rules
+  if (!rules) return usesProficiency(request) ? 1 : 0
+  if (request.kind === 'skill' && request.skill) {
+    if (rules.skillExpertise.includes(request.skill)) return 2
+    if (rules.skillProficiencies.includes(request.skill)) return 1
+    return rules.legacyProficiencyFallback && request.proficient === true ? 1 : 0
+  }
+  if (request.kind === 'savingThrow' && request.ability) {
+    if (rules.savingThrowProficiencies.includes(request.ability)) return 1
+    return rules.legacyProficiencyFallback && request.proficient === true ? 1 : 0
+  }
+  if (request.kind === 'attack') return request.proficient === false ? 0 : 1
+  return rules.legacyProficiencyFallback && request.proficient === true ? 1 : 0
+}
+
+function normalizedStatuses(character: Character): Set<string> {
+  return new Set(character.statuses.map(status => status.trim().toLowerCase()))
+}
+
+function effectiveRollMode(request: CheckRequest, actor: Character, target?: Character): CheckRequest['mode'] {
+  let advantage = request.mode === 'advantage' ? 1 : 0
+  let disadvantage = request.mode === 'disadvantage' ? 1 : 0
+  const actorStatuses = normalizedStatuses(actor)
+  const targetStatuses = target ? normalizedStatuses(target) : new Set<string>()
+
+  if (['ability', 'skill', 'attack'].includes(request.kind) && actorStatuses.has('poisoned')) disadvantage += 1
+  if (request.kind === 'attack') {
+    if (actorStatuses.has('blinded') || actorStatuses.has('restrained') || actorStatuses.has('prone')) disadvantage += 1
+    if (actorStatuses.has('invisible')) advantage += 1
+    if (targetStatuses.has('blinded') || targetStatuses.has('restrained') || targetStatuses.has('paralyzed')
+      || targetStatuses.has('petrified') || targetStatuses.has('stunned') || targetStatuses.has('unconscious')) advantage += 1
+    if (targetStatuses.has('invisible')) disadvantage += 1
+  }
+  if (request.kind === 'savingThrow' && request.ability === 'dex' && actorStatuses.has('restrained')) disadvantage += 1
+
+  if (advantage && disadvantage) return 'normal'
+  if (advantage) return 'advantage'
+  if (disadvantage) return 'disadvantage'
+  return 'normal'
+}
+
+function automaticallyFailedSave(request: CheckRequest, actor: Character): boolean {
+  if (request.kind !== 'savingThrow' || !request.ability || !['str', 'dex'].includes(request.ability)) return false
+  const statuses = normalizedStatuses(actor)
+  return ['paralyzed', 'petrified', 'stunned', 'unconscious'].some(status => statuses.has(status))
+}
+
 export function resolveCheck(
   request: CheckRequest,
   actor: Character,
@@ -112,11 +165,27 @@ export function resolveCheck(
     }
   }
 
+  if (request.kind === 'deathSavingThrow') {
+    const statuses = normalizedStatuses(actor)
+    if (actor.hp !== 0 || statuses.has('stable') || statuses.has('dead')) {
+      throw new Error('Death saving throws require an unstable character at 0 HP')
+    }
+    const result = random.integer(1, 20)
+    return {
+      id: randomUUID(), checkKind: request.kind, formula: '1d20', rolls: [result], kept: [result],
+      modifier: 0, total: result, difficulty: 10, success: result >= 10,
+      actorId: actor.id, targetId: null, createdAt: new Date().toISOString(),
+    }
+  }
+
   const ability = abilityForCheck(request)
-  const diceCount = request.mode === 'normal' ? 1 : 2
+  const mode = effectiveRollMode(request, actor, target)
+  const diceCount = mode === 'normal' ? 1 : 2
   const rolls = Array.from({ length: diceCount }, () => random.integer(1, 20))
-  const keptValue = request.mode === 'advantage' ? Math.max(...rolls) : request.mode === 'disadvantage' ? Math.min(...rolls) : rolls[0]!
-  const modifier = abilityModifier(actor.abilities[ability]) + (usesProficiency(request) ? actor.proficiency : 0)
+  const keptValue = mode === 'advantage' ? Math.max(...rolls) : mode === 'disadvantage' ? Math.min(...rolls) : rolls[0]!
+  const proficiency = actor.rules ? proficiencyBonus(actor.rules.level) : actor.proficiency
+  const exhaustionPenalty = actor.rules ? actor.rules.exhaustion * 2 : 0
+  const modifier = abilityModifier(actor.abilities[ability]) + proficiency * proficiencyMultiplier(request, actor) - exhaustionPenalty
   const total = keptValue + modifier
   if (request.kind === 'attack' && (!request.targetId || !target || target.id !== request.targetId)) {
     throw new Error('Attack rolls require the active target character')
@@ -125,18 +194,19 @@ export function resolveCheck(
   const automaticAttackOutcome = request.kind === 'attack'
     ? keptValue === 20 ? true : keptValue === 1 ? false : null
     : null
+  const automaticSaveOutcome = automaticallyFailedSave(request, actor) ? false : null
   return {
     id: randomUUID(),
     checkKind: request.kind,
-    formula: request.mode === 'normal'
+    formula: mode === 'normal'
       ? `1d20${modifier >= 0 ? '+' : ''}${modifier}`
-      : `2d20${request.mode === 'advantage' ? 'kh1' : 'kl1'}${modifier >= 0 ? '+' : ''}${modifier}`,
+      : `2d20${mode === 'advantage' ? 'kh1' : 'kl1'}${modifier >= 0 ? '+' : ''}${modifier}`,
     rolls,
     kept: [keptValue],
     modifier,
     total,
     difficulty,
-    success: difficulty === null ? null : automaticAttackOutcome ?? total >= difficulty,
+    success: difficulty === null ? null : automaticAttackOutcome ?? automaticSaveOutcome ?? total >= difficulty,
     actorId: actor.id,
     targetId: request.targetId ?? null,
     createdAt: new Date().toISOString(),
@@ -193,26 +263,145 @@ export interface AppliedState {
   audit: Array<{ event: ProposedEvent; before: unknown; after: unknown }>
 }
 
-export function applyMechanicalEvents(characters: Character[], events: ProposedEvent[]): AppliedState {
-  const next = characters.map(character => ({ ...character, inventory: [...character.inventory], statuses: [...character.statuses] }))
+function cloneRules(rules: CharacterRules | undefined): CharacterRules | undefined {
+  return rules ? structuredClone(rules) : undefined
+}
+
+function addStatus(character: Character, status: string): void {
+  if (!character.statuses.some(current => current.toLowerCase() === status.toLowerCase())) character.statuses.push(status)
+}
+
+function removeStatus(character: Character, status: string): void {
+  character.statuses = character.statuses.filter(current => current.toLowerCase() !== status.toLowerCase())
+}
+
+function adjustedDamage(value: number, damageType: DamageType | null | undefined, rules: CharacterRules | undefined): number {
+  let damage = Math.max(0, value)
+  if (!damageType || !rules) return damage
+  if (rules.damageImmunities.includes(damageType)) return 0
+  if (rules.damageResistances.includes(damageType)) damage = Math.floor(damage / 2)
+  if (rules.damageVulnerabilities.includes(damageType)) damage *= 2
+  return damage
+}
+
+function applyDeathSavingThrows(characters: Character[], rolls: DiceRoll[]): void {
+  for (const roll of rolls.filter(candidate => candidate.checkKind === 'deathSavingThrow')) {
+    const character = characters.find(candidate => candidate.id === roll.actorId)
+    if (!character?.rules || character.hp !== 0 || normalizedStatuses(character).has('dead')) continue
+    const natural = roll.kept[0]
+    if (natural === 20) {
+      character.hp = 1
+      character.rules.deathSaves = { successes: 0, failures: 0 }
+      removeStatus(character, 'unconscious')
+      removeStatus(character, 'stable')
+      continue
+    }
+    if (roll.success) character.rules.deathSaves.successes = Math.min(3, character.rules.deathSaves.successes + 1)
+    else character.rules.deathSaves.failures = Math.min(3, character.rules.deathSaves.failures + (natural === 1 ? 2 : 1))
+
+    if (character.rules.deathSaves.successes >= 3) {
+      character.rules.deathSaves = { successes: 0, failures: 0 }
+      addStatus(character, 'stable')
+    }
+    if (character.rules.deathSaves.failures >= 3) {
+      removeStatus(character, 'unconscious')
+      removeStatus(character, 'stable')
+      addStatus(character, 'dead')
+    }
+  }
+}
+
+function applyRest(character: Character, rest: string): void {
+  if (!character.rules || normalizedStatuses(character).has('dead')) return
+  const longRest = rest.toLowerCase() === 'long'
+  for (const resource of Object.values(character.rules.resources)) {
+    if (longRest ? resource.recovery !== 'none' : resource.recovery === 'shortRest') resource.current = resource.max
+  }
+  if (!longRest || character.hp < 1) return
+  character.hp = character.maxHp
+  character.rules.temporaryHp = 0
+  character.rules.exhaustion = Math.max(0, character.rules.exhaustion - 1)
+  character.rules.deathSaves = { successes: 0, failures: 0 }
+}
+
+export function applyMechanicalEvents(characters: Character[], events: ProposedEvent[], rolls: DiceRoll[] = []): AppliedState {
+  const next = characters.map(character => ({
+    ...character,
+    inventory: [...character.inventory],
+    statuses: [...character.statuses],
+    rules: cloneRules(character.rules),
+  }))
   const audit: AppliedState['audit'] = []
 
+  applyDeathSavingThrows(next, rolls)
+
   for (const event of events) {
-    if (!['damage', 'healing', 'inventory', 'status'].includes(event.type)) continue
+    if (!['damage', 'healing', 'inventory', 'status', 'temporaryHp', 'resource', 'rest'].includes(event.type)) continue
     const target = next.find(character => character.id === event.targetId)
     if (!target) throw new Error(`Unknown event target: ${event.targetId}`)
     const before = structuredClone(target)
 
-    if (event.type === 'damage') target.hp = Math.max(0, target.hp - Math.max(0, event.value ?? 0))
-    if (event.type === 'healing') target.hp = Math.min(target.maxHp, target.hp + Math.max(0, event.value ?? 0))
+    if (event.type === 'damage') {
+      const damage = adjustedDamage(event.value ?? 0, event.damageType, target.rules)
+      const priorHp = target.hp
+      const absorbed = Math.min(target.rules?.temporaryHp ?? 0, damage)
+      if (target.rules) target.rules.temporaryHp -= absorbed
+      const hpDamage = damage - absorbed
+      const remainingDamageAtZero = Math.max(0, hpDamage - priorHp)
+      const criticalAtZero = priorHp === 0 && rolls.some(roll =>
+        roll.checkKind === 'attack'
+        && roll.actorId === event.actorId
+        && roll.targetId === event.targetId
+        && roll.kept[0] === 20,
+      )
+      target.hp = Math.max(0, priorHp - hpDamage)
+      if (target.hp === 0 && !normalizedStatuses(target).has('dead')) {
+        if (remainingDamageAtZero >= target.maxHp) {
+          removeStatus(target, 'unconscious')
+          removeStatus(target, 'stable')
+          addStatus(target, 'dead')
+        } else {
+          addStatus(target, 'unconscious')
+          if (priorHp === 0 && target.rules) {
+            removeStatus(target, 'stable')
+            target.rules.deathSaves.failures = Math.min(3, target.rules.deathSaves.failures + (criticalAtZero ? 2 : 1))
+            if (target.rules.deathSaves.failures >= 3) {
+              removeStatus(target, 'unconscious')
+              addStatus(target, 'dead')
+            }
+          }
+        }
+      }
+    }
+    if (event.type === 'healing' && !normalizedStatuses(target).has('dead')) {
+      target.hp = Math.min(target.maxHp, target.hp + Math.max(0, event.value ?? 0))
+      if (target.hp > 0) {
+        removeStatus(target, 'unconscious')
+        removeStatus(target, 'stable')
+        if (target.rules) target.rules.deathSaves = { successes: 0, failures: 0 }
+      }
+    }
     if (event.type === 'inventory' && event.item) {
       if ((event.value ?? 1) >= 0 && !target.inventory.includes(event.item)) target.inventory.push(event.item)
       if ((event.value ?? 1) < 0) target.inventory = target.inventory.filter(item => item !== event.item)
     }
     if (event.type === 'status' && event.item) {
-      if ((event.value ?? 1) >= 0 && !target.statuses.includes(event.item)) target.statuses.push(event.item)
-      if ((event.value ?? 1) < 0) target.statuses = target.statuses.filter(status => status !== event.item)
+      if (event.item.toLowerCase() === 'exhaustion' && target.rules) {
+        target.rules.exhaustion = Math.max(0, Math.min(6, target.rules.exhaustion + (event.value ?? 1)))
+        if (target.rules.exhaustion >= 6) addStatus(target, 'dead')
+      } else {
+        if ((event.value ?? 1) >= 0) addStatus(target, event.item)
+        if ((event.value ?? 1) < 0) removeStatus(target, event.item)
+      }
     }
+    if (event.type === 'temporaryHp' && target.rules) {
+      target.rules.temporaryHp = Math.max(target.rules.temporaryHp, Math.max(0, event.value ?? 0))
+    }
+    if (event.type === 'resource' && event.item && target.rules?.resources[event.item]) {
+      const resource = target.rules.resources[event.item]!
+      resource.current = Math.max(0, Math.min(resource.max, resource.current + (event.value ?? -1)))
+    }
+    if (event.type === 'rest' && event.item) applyRest(target, event.item)
     audit.push({ event, before, after: structuredClone(target) })
   }