-- A Gen 2 party member: stats, moves, level-up and experience.
--
-- Separate from src/pokemon/Pokemon.lua because the struct itself changed:
-- Gen 2 splits `special` into Special Attack and Special Defense, adds a held
-- item, happiness and pokerus, and its level-up moves come from EvosAttacks
-- rather than a Gen 1 learnset table.
--
-- Stat formula is unchanged from Gen 1 (data/pokemon/base_stats + DVs):
--   stat = floor((base * 2 + DV * 2 + floor(sqrt(statExp) / 4)) * level / 100) + 5
--   HP is the same but + level + 10
-- with the Gen 2 twist that a mon's SpA and SpD share one Special DV, which is
-- why a high-Special DV raises both.
--
-- Experience curves come from data/growth_rates.asm, whose `growth_rate` macro
-- documents its own polynomial:
--   [1]/[2] * n^3 + [3] * n^2 + [4] * n - [5]
-- with a sign bit on the n^2 term.  pokemon.lua carries those five numbers per
-- GROWTH_* so this needs no hardcoded table.

local Unown = require("src.core.gen2.Unown")
-- The mod event bus.  pokemon.level_up and pokemon.move_learned are the SAME
-- names src/battle/Experience.lua and src/battle/BattleState.lua raise on
-- Gen 1, with the same payload keys: a mod that watches a Red party watches a
-- Gold one unchanged (docs/mod-api-gen2-compat.md).
local Runtime = require("src.mods.Runtime")

local Mon = {}

Mon.MAX_LEVEL = 100
Mon.PARTY_SIZE = 6

-- DVs are 0..15 each; Attack's low bit pair also decides gender and shininess.
Mon.MAX_DV = 15

local function rand(a, b)
  if love and love.math and love.math.random then
    return love.math.random(a, b)
  end
  return math.random(a, b)
end

function Mon.randomDVs()
  return {
    hp = nil, -- derived below
    attack = rand(0, Mon.MAX_DV),
    defense = rand(0, Mon.MAX_DV),
    speed = rand(0, Mon.MAX_DV),
    special = rand(0, Mon.MAX_DV),
  }
end

-- The HP DV is not stored: it is the low bit of each of the other four
-- (Gen 1 and 2 both build it this way), which is why a perfect-HP mon needs
-- all four others odd.
function Mon.hpDV(dvs)
  local function bit(value) return (value or 0) % 2 end
  return bit(dvs.attack) * 8 + bit(dvs.defense) * 4
    + bit(dvs.speed) * 2 + bit(dvs.special)
end

local function statValue(base, dv, level, statExp)
  local exp = math.floor(math.sqrt(statExp or 0) / 4)
  return math.floor((((base or 1) * 2 + (dv or 0) * 2 + exp) * level) / 100) + 5
end

-- All six stats at a level.  `statExp` is optional per-stat effort.
function Mon.stats(baseStats, dvs, level, statExp)
  baseStats = baseStats or {}
  dvs = dvs or {}
  statExp = statExp or {}
  local hpDv = dvs.hp or Mon.hpDV(dvs)
  local hp = math.floor((((baseStats.hp or 1) * 2 + hpDv * 2
    + math.floor(math.sqrt(statExp.hp or 0) / 4)) * level) / 100)
    + level + 10
  return {
    hp = hp,
    attack = statValue(baseStats.attack, dvs.attack, level, statExp.attack),
    defense = statValue(baseStats.defense, dvs.defense, level, statExp.defense),
    speed = statValue(baseStats.speed, dvs.speed, level, statExp.speed),
    -- One Special DV feeds both special stats, and so does one Special stat
    -- exp: the Gen 2 party struct kept Gen 1's five exp words (macros/ram.asm
    -- box_struct ends them at SpcExp), so SpA and SpD grow together.  The
    -- per-stat keys are still read as a fallback for a record written before
    -- the shared word existed.
    specialAttack = statValue(baseStats.specialAttack, dvs.special, level,
      statExp.special or statExp.specialAttack),
    specialDefense = statValue(baseStats.specialDefense, dvs.special, level,
      statExp.special or statExp.specialDefense),
  }
end

-- The five stat exp words, in struct order.  There is no sixth: see Mon.stats.
Mon.STAT_EXP_ORDER = { "hp", "attack", "defense", "speed", "special" }

-- Each word is 16 bit and GiveExperiencePoints stops it at $ffff rather than
-- letting it wrap (.stat_exp_maxed_out).
Mon.MAX_STAT_EXP = 65535

function Mon.newStatExp()
  return { hp = 0, attack = 0, defense = 0, speed = 0, special = 0 }
end

-- GiveExperiencePoints' .stat_exp_loop (engine/battle/core.asm): the defeated
-- mon's base stats are added to every participant's stat exp, and the loop runs
-- NUM_EXP_STATS = 5 times over a six-entry base stat block, so the Special word
-- takes the loser's Special ATTACK and the Special Defense base stat is never
-- read at all.
--
-- `.EvenlyDivideExpAmongParticipants` divides the base stats in place before
-- any of this, and only when two or more mons took part, which is why the
-- divisor is shared with Mon.experienceGain rather than computed here.
--
-- Pokerus adds the same value a SECOND time (`jr z, .stat_exp_awarded` skips
-- the second add when the byte is zero) -- doubled, not multiplied by a rate,
-- so it stacks with nothing.
-- `halved` is the EXP.SHARE tax: with any holder in the party the whole
-- wEnemyMon base stat block is `srl`'d in place before EITHER pass runs
-- (engine/battle/core.asm, the IsAnyMonHoldingExpShare block ahead of the
-- first GiveExperiencePoints call), so participants and holders both draw
-- stat exp from the halved values.
function Mon.gainStatExp(mon, loserDef, participants, doubled, halved)
  if type(mon) ~= "table" then return nil end
  local base = (loserDef and loserDef.baseStats) or {}
  local share = math.max(1, math.floor(participants or 1))
  mon.statExp = mon.statExp or Mon.newStatExp()
  local gains = {}
  for _, key in ipairs(Mon.STAT_EXP_ORDER) do
    local from = (key == "special") and base.specialAttack or base[key]
    from = from or 0
    if halved then from = math.floor(from / 2) end
    local gain = math.floor(from / share)
    if doubled then gain = gain * 2 end
    local value = (mon.statExp[key] or 0) + gain
    if value > Mon.MAX_STAT_EXP then value = Mon.MAX_STAT_EXP end
    mon.statExp[key] = value
    gains[key] = gain
  end
  return gains
end

-- Total experience needed to *be* `level`, from a GROWTH_* record.
-- The single point every experience calculation resolves its curve through.
-- The merged growth_rates registry wins, then the extractor's own coefficient
-- rows on data.pokemon.growthRates, so a mod-free boot reads exactly the table
-- it always did and a boot with a registered curve reads that instead.
--
-- Both live at the same key space (GROWTH_MEDIUM_FAST and friends), and the
-- registry's Data path is the shared `growth_rates` one Gen 1 uses -- the
-- point of routing it there rather than to a gen2 table is that a mod writes
-- ONE record for both games (src/mods/Builtins.lua's Gen 2 registrant seeds
-- this registry from the coefficient rows, so the vanilla curves are Gold's
-- own).
function Mon.growthFor(data, curve)
  if not curve then return nil end
  local registered = data and data.growth_rates and data.growth_rates[curve]
  if registered then return registered end
  return data and data.pokemon and data.pokemon.growthRates
    and data.pokemon.growthRates[curve]
end

-- Seeds the growth_rates registry with Gold's own curves, as records carrying
-- expForLevel so the id space and the record shape both match Gen 1's.  Called
-- by src/mods/Builtins.lua under Gen 2.  A dataset with no coefficient rows
-- (the ROM-free fixtures) seeds nothing rather than registering broken curves.
function Mon.registerInto(registry, data, owner)
  local rows = data and data.pokemon and data.pokemon.growthRates
  if type(rows) ~= "table" then return end
  for curve, row in pairs(rows) do
    -- the closure holds the coefficient row, so the registered record computes
    -- exactly what the arm below would have
    registry:register(curve, {
      expForLevel = function(level) return Mon.experienceForLevel(row, level) end,
    }, owner)
  end
end

-- `growth` is either the extractor's coefficient row (numerator / denominator /
-- squared / linear / constant, straight off GrowthRates in the ROM) or a
-- growth_rates REGISTRY record, which carries expForLevel(level) instead --
-- the same record shape Gen 1's registry uses (src/pokemon/Growth.lua), so a
-- mod that registers a custom curve writes one record and it works in both
-- games.  A registered curve wins outright; the coefficient arm is what a
-- mod-free boot and every driver still run.
function Mon.experienceForLevel(growth, level)
  if growth and growth.expForLevel then
    return math.max(0, math.floor(growth.expForLevel(level) or 0))
  end
  if not growth then return level * level * level end
  local n = level
  local numerator = growth.numerator or 1
  local denominator = growth.denominator or 1
  local value = math.floor(numerator * n * n * n / denominator)
  value = value + (growth.squared or 0) * n * n
  value = value + (growth.linear or 0) * n
  value = value - (growth.constant or 0)
  return math.max(0, value)
end

-- The level a total experience buys.  Walks up rather than inverting the
-- polynomial, which the cart also does (it only ever compares against the next
-- level's threshold).
function Mon.levelForExperience(growth, experience)
  local level = 1
  while level < Mon.MAX_LEVEL do
    if experience < Mon.experienceForLevel(growth, level + 1) then break end
    level = level + 1
  end
  return level
end

-- The moves a species knows on arrival at `level`: its last four level-up
-- moves at or below it (EvosAttacks order, later moves pushing earlier ones
-- out, which is what makes a caught mon's moveset deterministic).
function Mon.movesAtLevel(def, level, moves)
  local known = {}
  for _, entry in ipairs((def and def.levelMoves) or {}) do
    if entry.level <= level then
      -- A move already known is not learned twice.
      local duplicate = false
      for _, existing in ipairs(known) do
        if existing == entry.move then duplicate = true break end
      end
      if not duplicate then
        known[#known + 1] = entry.move
        if #known > 4 then table.remove(known, 1) end
      end
    end
  end
  local out = {}
  for _, id in ipairs(known) do
    local moveDef = moves and moves[id]
    out[#out + 1] = {
      id = id,
      pp = moveDef and moveDef.pp or 0,
      maxPp = moveDef and moveDef.pp or 0,
    }
  end
  return out
end

-- Build a party member.  `data` needs `pokemon` and `moves`; growth records
-- live on data.pokemon.growthRates (written by the extractor).
function Mon.new(data, species, level, opts)
  opts = opts or {}
  local def = data and data.pokemon and data.pokemon[species]
  if not def then return nil end
  level = math.max(1, math.min(Mon.MAX_LEVEL, level or 5))
  local dvs = opts.dvs or Mon.randomDVs()
  dvs.hp = Mon.hpDV(dvs)
  local statExp = opts.statExp or Mon.newStatExp()
  local stats = Mon.stats(def.baseStats, dvs, level, statExp)
  local growth = Mon.growthFor(data, def.growthRate)
  return {
    species = species,
    name = def.name or species,
    nickname = opts.nickname,
    level = level,
    experience = Mon.experienceForLevel(growth, level),
    dvs = dvs,
    -- The five stat exp words.  A wild or gift mon starts at zero: nothing in
    -- the cart seeds them, MON_STAT_EXP is zeroed by _MoveMon.
    statExp = statExp,
    -- MON_PKRS.  Zero is "never infected"; src/core/gen2/Pokerus.lua owns every
    -- read and write of it after this.
    pokerus = opts.pokerus or 0,
    stats = stats,
    hp = opts.hp or stats.hp,
    maxHp = stats.hp,
    types = def.types,
    moves = opts.moves or Mon.movesAtLevel(def, level, data.moves),
    -- Held item; wild mons roll one from BaseData's two item slots on the cart,
    -- which is not modeled yet, so only scripted gifts carry one.
    item = opts.item,
    status = nil,
    -- 70 for a caught mon, 120 for a gift/hatched one.
    happiness = opts.happiness or 70,
    caughtLevel = level,
    -- shiny.roll / gender.roll get the species and level as context; opts.shiny
    -- still wins, because a FORCED shiny battle (Red Gyarados) is the cart
    -- overriding the roll rather than a roll to be hooked.
    shiny = opts.shiny or Mon.isShiny(dvs,
      { species = species, def = def, level = level }),
    gender = Mon.gender(def, dvs, { species = species, level = level }),
    -- Unown has no gender and no shininess worth looking at, but it does have
    -- a FORM, and the form is the same DVs read a different way
    -- (GetUnownLetter, engine/gfx/load_pics.asm).  Stamped at build time so
    -- every screen that shows an Unown -- the battle pic, the box, the #DEX --
    -- reads one field instead of each redoing the bit shuffle.
    unownLetter = (species == Unown.SPECIES)
      and Unown.letterFromDVs(dvs) or nil,
  }
end

-- AddPartyMon copies wPlayerName into wPartyMonOTs and wPlayerID into MON_ID
-- (move_mon.asm:44-56, :143-149); SendMonIntoBox does the same (:970-994).
function Mon.stampOT(save, mon)
  local player = save and save.player
  if not (mon and player) then return mon end
  if player.id == nil then player.id = rand(0, 65535) end
  mon.ot = mon.ot or player.name
  -- NpcTrade.lua:150: `ot` is what Breeding reads, `otName` what the summary prints.
  mon.otName = mon.otName or mon.ot
  mon.otId = mon.otId or player.id
  return mon
end

-- shiny.roll and gender.roll, two of the names Gen 2 invents: Gen 1 has
-- neither shininess nor gender in the ROM at all (Red's shiny indicator mods
-- read src/pokemon/Stats.lua's virtual pattern, which is a mod-side
-- convention, not an engine seam), so there is no Gen 1 name to share.
--
-- Both wrap the DV-derived roll rather than the mon that comes out of it,
-- because on the cart these ARE the roll: CheckShininess and GetGender read
-- the same two DV bytes LoadEnemyMon just generated, and nothing later can
-- change the answer without changing the DVs.  Wrapping here means a shiny-odds
-- mod and a gender-ratio mod work on every route a mon arrives by -- a wild
-- encounter, a gift, a hatch, a trade -- because Mon.new is the one builder.
--
-- Shared ctx keys:
--   dvs      the DV set being read, exactly as stored on the mon
--   species  the species id, nil when the caller had none to give
--   def      that species' record, nil likewise
--   level    the level the mon is being built at, nil for a bare query
--
-- gender.roll's ctx carries `ratio` as well, BaseData's genderRatio byte, so a
-- mod can shift the threshold rather than restate the whole rule.  A chain that
-- returns something that is not one of "male" / "female" / "unknown" is
-- ignored, because every screen that prints a gender indexes by those three.

-- Gen 2 shininess: the classic DV pattern (Speed/Defense/Special all 10, and
-- Attack in {2,3,6,7,10,11,14,15}).
function Mon.vanillaShiny(dvs)
  if not dvs then return false end
  if dvs.speed ~= 10 or dvs.defense ~= 10 or dvs.special ~= 10 then
    return false
  end
  local attack = dvs.attack or 0
  return attack % 4 == 2 or attack % 4 == 3
end

function Mon.isShiny(dvs, ctx)
  if not Runtime.wantsHook("shiny.roll") then return Mon.vanillaShiny(dvs) end
  local shiny = Runtime.call("shiny.roll", function(c)
    return Mon.vanillaShiny(c.dvs)
  end, { dvs = dvs, species = ctx and ctx.species, def = ctx and ctx.def,
         level = ctx and ctx.level })
  return shiny and true or false
end

-- Gender comes from the Attack DV against the species' ratio threshold: an
-- Attack DV *below* the threshold is female (BaseData's `db GENDER_F12_5` is
-- already scaled out of 256).
function Mon.vanillaGender(def, dvs)
  local ratio = def and def.genderRatio
  if not ratio then return "unknown" end
  if ratio == 0xff then return "unknown" end
  -- The DV is 0..15; the threshold is out of 256 in steps of 16.
  local threshold = math.floor(ratio / 16)
  return ((dvs and dvs.attack or 0) < threshold) and "female" or "male"
end

local GENDERS = { male = true, female = true, unknown = true }

function Mon.gender(def, dvs, ctx)
  if not Runtime.wantsHook("gender.roll") then
    return Mon.vanillaGender(def, dvs)
  end
  local gender = Runtime.call("gender.roll", function(c)
    return Mon.vanillaGender(c.def, c.dvs)
  end, { def = def, dvs = dvs, ratio = def and def.genderRatio,
         species = (ctx and ctx.species) or (def and def.id),
         level = ctx and ctx.level })
  if not GENDERS[gender] then return Mon.vanillaGender(def, dvs) end
  return gender
end

-- Experience for defeating `loser`, per recipient.  Gen 2:
--   exp = baseExp * loserLevel / 7, split among the recipients of the pass,
-- then GiveExperiencePoints' three BoostExp arms in the cart's own order,
-- each a floored x1.5 on the running amount:
--   traded    the mon's OT id differs from the player's (BoostedExpPointsText)
--   trainer   a trainer battle (wBattleMode)
--   luckyEgg  the mon HOLDS a LUCKY_EGG -- checked by item id, not held
--             effect, exactly as the cart's `cp LUCKY_EGG` does
-- opts.halved is the EXP.SHARE tax: any holder in the party halves the base
-- exp byte before either pass (the same `srl` block that halves stat exp).
function Mon.experienceGain(loserDef, loserLevel, participants, trainer, opts)
  opts = opts or {}
  local baseExp = (loserDef and loserDef.baseExp) or 0
  if opts.halved then baseExp = math.floor(baseExp / 2) end
  local value = math.floor(baseExp * (loserLevel or 1) / 7)
  value = math.floor(value / math.max(1, participants or 1))
  if opts.traded then value = math.floor(value * 3 / 2) end
  if trainer then value = math.floor(value * 3 / 2) end
  if opts.luckyEgg then value = math.floor(value * 3 / 2) end
  return math.max(1, value)
end

-- Award experience, level up as far as it reaches, and report what happened so
-- the battle can print "grew to level N!" and offer new moves.
function Mon.gainExperience(mon, amount, data)
  local def = data and data.pokemon and data.pokemon[mon.species]
  local growth = Mon.growthFor(data, def and def.growthRate)
  mon.experience = (mon.experience or 0) + math.max(0, amount or 0)
  local before = mon.level
  local capped = Mon.experienceForLevel(growth, Mon.MAX_LEVEL)
  if mon.experience > capped then mon.experience = capped end
  local after = Mon.levelForExperience(growth, mon.experience)
  if after <= before then
    return { levels = 0, learned = {} }
  end
  mon.level = after
  -- Recompute stats and carry the HP gain, the way the cart adds the delta
  -- rather than refilling.
  local previousMax = mon.maxHp or (mon.stats and mon.stats.hp) or 1
  mon.stats = Mon.stats(def and def.baseStats, mon.dvs, after, mon.statExp)
  mon.maxHp = mon.stats.hp
  mon.hp = math.min(mon.maxHp, (mon.hp or previousMax)
    + (mon.maxHp - previousMax))

  -- pokemon.level_up, once per level crossed and after the stats were
  -- recalculated, exactly as src/battle/Experience.lua raises it on Gen 1 --
  -- a jump of three levels is three events, not one.  `learnable` is the moves
  -- this species learns at exactly that level, the same list Gen 1 carries.
  if Runtime.wants("pokemon.level_up") then
    for level = before + 1, after do
      local learnable = {}
      for _, entry in ipairs((def and def.levelMoves) or {}) do
        if entry.level == level then learnable[#learnable + 1] = entry.move end
      end
      Runtime.emit("pokemon.level_up", {
        mon = mon, level = level, prevLevel = level - 1, learnable = learnable,
      })
    end
  end

  -- Every level-up move between the old and new level is offered.
  local learned = {}
  for _, entry in ipairs((def and def.levelMoves) or {}) do
    if entry.level > before and entry.level <= after then
      learned[#learned + 1] = entry.move
    end
  end
  return { levels = after - before, learned = learned, from = before, to = after }
end

-- Teach a move, or report that all four slots are full so the caller can ask
-- which to forget.
function Mon.learnMove(mon, moveId, data)
  mon.moves = mon.moves or {}
  for _, move in ipairs(mon.moves) do
    if move.id == moveId then return false, "known" end
  end
  local def = data and data.moves and data.moves[moveId]
  local entry = {
    id = moveId,
    pp = def and def.pp or 0,
    maxPp = def and def.pp or 0,
  }
  if #mon.moves >= 4 then return false, "full", entry end
  mon.moves[#mon.moves + 1] = entry
  -- pokemon.move_learned, the payload BattleState:learnMove emits on Gen 1.
  -- This is Gen 2's single choke point for teaching a move -- the level-up
  -- award, an evolution's new move and the TM path all arrive here -- so the
  -- event covers all three rather than only the battle's.
  Runtime.emit("pokemon.move_learned", { mon = mon, moveId = moveId })
  return true
end

-- Which evolution (if any) fires at this level.
function Mon.evolutionAtLevel(def, level)
  for _, entry in ipairs((def and def.evolutions) or {}) do
    if entry.method == "EVOLVE_LEVEL" and (entry.level or 0) <= level then
      return entry
    end
  end
  return nil
end

return Mon