feat: add Proficiency.keyFor, Proficiency.hits and Progression.xpToNext

src/progression.lua

@@ -0,0 +1,48 @@
+-- Player experience and leveling. Killing a monster grants xp scaled by the
+-- species catalog; once xp clears the threshold for the current level the
+-- player levels up, gaining permanent hp/attack/defense and a full heal-up on
+-- the hp gained. A single large xp grant can trigger several level-ups in a
+-- row, so the gain loops rather than checking once.
+
+local Progression = {}
+
+-- Threshold to advance from `level` to `level + 1`. Linear on purpose: pure
+-- and easy to reason about in tests, unlike an exponential curve.
+function Progression.xpToNext(level)
+  return 20 + (level - 1) * 15
+end
+
+local function hpGain(level)
+  return 4 + level
+end
+
+local function levelUp(player)
+  player.level = player.level + 1
+  local gain = hpGain(player.level)
+  player.maxHp = player.maxHp + gain
+  player.hp = player.hp + gain
+  player.attack = player.attack + 1
+  player.baseAttack = player.baseAttack + 1
+  if player.level % 2 == 0 then
+    player.defense = player.defense + 1
+    player.baseDefense = player.baseDefense + 1
+  end
+  player.skillPoints = (player.skillPoints or 0) + 1
+  player.xp = player.xp - player.xpNext
+  player.xpNext = Progression.xpToNext(player.level)
+  return string.format("you reach level %d", player.level)
+end
+
+-- Applies an xp gain to the player, running as many level-ups as the gain
+-- covers. Returns the log lines produced, in order.
+function Progression.grantXp(player, amount)
+  if amount <= 0 then return {} end
+  player.xp = player.xp + amount
+  local lines = {}
+  while player.xp >= player.xpNext do
+    lines[#lines + 1] = levelUp(player)
+  end
+  return lines
+end
+
+return Progression