feat: add tierFrom, Resistances.tierOf and wordKey

src/resistances.lua

@@ -0,0 +1,67 @@
+-- Elemental damage typing. Every hit that lands - melee, a bolt, a thrown
+-- weapon, a hazard tick, a status effect's damage-over-time - is tagged with
+-- one of the types below, and this module decides how much of it actually
+-- gets through. A defender's tier for a type comes from up to two places: its
+-- own base profile (bestiary data, e.g. a dragon's own hide) and whatever it
+-- has equipped (weapon/armor/ring/bow, e.g. a ring of cold resistance). The
+-- single best (most protective) tier across those sources wins - immune beats
+-- resist beats weak beats the normal default - so a found ring can cancel out
+-- an innate vulnerability, but nothing ever stacks two resists into a second
+-- discount or two weaknesses into a bigger penalty.
+
+local Resistances = {}
+
+Resistances.PHYSICAL = "physical"
+Resistances.FIRE = "fire"
+Resistances.COLD = "cold"
+Resistances.POISON = "poison"
+Resistances.LIGHTNING = "lightning"
+Resistances.ARCANE = "arcane"
+
+Resistances.TYPES = {
+  Resistances.PHYSICAL, Resistances.FIRE, Resistances.COLD,
+  Resistances.POISON, Resistances.LIGHTNING, Resistances.ARCANE,
+}
+
+Resistances.IMMUNE = "immune"
+Resistances.RESIST = "resist"
+Resistances.WEAK = "weak"
+Resistances.NORMAL = "normal"
+
+local RANK = { immune = 3, resist = 2, weak = 1, normal = 0 }
+
+-- Slots checked alongside an entity's own `resist` table, in a fixed order so
+-- the result never depends on Lua's unspecified pairs() iteration order.
+local EQUIP_SLOTS = { "weapon", "armor", "ring", "bow" }
+
+local function tierFrom(source, damageType)
+  return source and source.resist and source.resist[damageType]
+end
+
+-- The single tier that governs `damageType` incoming against `entity`, folding
+-- in its own base profile and every equipped slot that carries one.
+function Resistances.tierOf(entity, damageType)
+  local best = tierFrom(entity, damageType) or Resistances.NORMAL
+  for _, slot in ipairs(EQUIP_SLOTS) do
+    local tier = tierFrom(entity[slot], damageType)
+    if tier and RANK[tier] > RANK[best] then
+      best = tier
+    end
+  end
+  return best
+end
+
+-- Scales a already-computed `amount` of `damageType` damage by `entity`'s
+-- tier for that type. Immune drops it to nothing outright rather than merely
+-- halving it, so it can floor damage below the "always at least 1" guarantee
+-- combat.lua's own minimum otherwise enforces.
+function Resistances.apply(entity, amount, damageType)
+  if amount <= 0 then return amount end
+  local tier = Resistances.tierOf(entity, damageType or Resistances.PHYSICAL)
+  if tier == Resistances.IMMUNE then return 0 end
+  if tier == Resistances.RESIST then return math.floor(amount / 2) end
+  if tier == Resistances.WEAK then return math.floor(amount * 1.5) end
+  return amount
+end
+
+return Resistances