-- The shiny sparkle: the flash a Pokemon makes when it first appears.
--
-- The games announce a shiny with a burst of stars over the sprite the
-- instant it lands, before the first text box. This is that moment, in the
-- diorama: a ring of additive stars that springs outward from the mon's
-- chest, rises, and fades over about three quarters of a second.
--
-- ------- where the moment IS
--
-- Harder than it sounds, because the two battle paths arrive differently:
--
--   the model rung   a Pokemon grows out of its ball -- Stadium.update
--                    already finds that frame (the POOF_ANIM edge) and
--                    calls StadiumMon:beginGrow.
--   the pic rung     a WILD foe is simply THERE on the first frame, with
--                    no poof and no grow at all. There is no animation to
--                    hang off.
--
-- So the arming edge is neither of those: it is the frame a side's OCCUPANT
-- changes (Stadium's `session.at[side] ~= battler` test, the same identity
-- the mode already uses because a trainer leading with two Rattata changes
-- occupant without changing species). That edge fires for a send-out, a
-- switch and a wild foe alike, which is exactly the set of moments a shiny
-- should announce itself.
--
-- ------- drawn additively, and why it survives the flash
--
-- Stars are light, so they add rather than cover: `Voxel3D.blend("add")`,
-- the same treatment the Poke Ball's glow gets. They are drawn inside the
-- battle's flash window alongside the cards and models, so a sparkle during
-- a hit flash is lit by it like everything else rather than floating over
-- it as a separate layer.

-- the mod namespace (see main.lua): V.require loads a sibling module
local V = ...

local Voxel3D = V.require("Voxel3D")
local Mat4 = V.require("Mat4")
local BattleBillboard = V.require("BattleBillboard")

local ShinyFx = {}

-- ------- shape and timing

ShinyFx.LIFE = 0.75          -- seconds from spring to gone
ShinyFx.STARS = 10           -- around the ring
ShinyFx.RISE = 10            -- world units the ring climbs over its life
ShinyFx.SPREAD = 7           -- how far out the ring opens
ShinyFx.CHEST = 9            -- height above the tile the burst starts at
ShinyFx.SIZE = 3.2           -- a star's world size at full brightness

-- one per side, nil when nothing is playing
local live = { player = nil, enemy = nil }
local star = nil             -- the generated star image, built once

-- ------- the star
--
-- Generated rather than shipped: it is a four-pointed twinkle, which is a
-- cheap closed form (a radial falloff times a cross-shaped spike term) and
-- costs nothing next to an asset that would have to be authored, packed,
-- loaded and kept in step with the rest of the mod's art.
local function starImage()
  if star ~= nil then return star or nil end
  if not (love and love.image and love.graphics) then
    star = false
    return nil
  end
  local ok, img = pcall(function()
    local N = 32
    local data = love.image.newImageData(N, N)
    local c = (N - 1) / 2
    for y = 0, N - 1 do
      for x = 0, N - 1 do
        local dx, dy = (x - c) / c, (y - c) / c
        local r = math.sqrt(dx * dx + dy * dy)
        -- the body: a soft core that is gone by the edge of the square
        local core = math.max(0, 1 - r)
        core = core * core * core
        -- the spikes: bright along the two axes, narrow, and reaching
        -- further out than the core does
        local ax, ay = math.abs(dx), math.abs(dy)
        local spike = math.max(0, 1 - ax * 6) * math.max(0, 1 - ay)
                    + math.max(0, 1 - ay * 6) * math.max(0, 1 - ax)
        local a = math.min(1, core + spike * 0.55)
        -- white with the faintest warm cast, so a sparkle over a cool
        -- model still reads as light rather than as a blue smear
        data:setPixel(x, y, 1, 1, 0.97, a)
      end
    end
    return love.graphics.newImage(data)
  end)
  star = (ok and img) or false
  return star or nil
end

-- ------- arming

-- Start (or restart) the burst on one side. Restarting rather than ignoring
-- a second call is deliberate: a shiny that faints and is sent back out
-- should sparkle again.
function ShinyFx.arm(side)
  if side ~= "player" and side ~= "enemy" then return end
  live[side] = { t = 0 }
end

function ShinyFx.clear(side)
  if side then live[side] = nil else live.player, live.enemy = nil, nil end
end

function ShinyFx.active(side)
  if side then return live[side] ~= nil end
  return live.player ~= nil or live.enemy ~= nil
end

function ShinyFx.update(dt)
  dt = dt or 0
  for _, side in ipairs({ "player", "enemy" }) do
    local s = live[side]
    if s then
      s.t = s.t + dt
      if s.t >= ShinyFx.LIFE then live[side] = nil end
    end
  end
end

-- ------- drawing

-- Eased so the ring leaves fast and settles, which is what a spark does;
-- linear looks like a diagram of a spark.
local function easeOut(u) return 1 - (1 - u) * (1 - u) end

-- Draw whatever is playing. `arena` and `groundY` come from the scene, the
-- same two the mon cards are placed from, so a sparkle lands where its
-- Pokemon is standing rather than where the layout thinks it should be.
function ShinyFx.draw(arena, groundY, pull)
  if not arena then return end
  local img = starImage()
  if not img then return end
  local mesh = BattleBillboard.mesh()
  if not mesh then return end

  local drew = false
  for _, side in ipairs({ "player", "enemy" }) do
    local s = live[side]
    local cell = (side == "player") and arena.player or arena.enemy
    if s and cell then
      local u = math.min(1, s.t / ShinyFx.LIFE)
      local e = easeOut(u)
      -- bright immediately, then out: the announcement is the first frame
      local alpha = 1 - u * u
      local x, z = cell[1], cell[2]
      local yaw = BattleBillboard.yawToward(x, z, Voxel3D.eye)
      local baseY = groundY + ShinyFx.CHEST + ShinyFx.RISE * e

      if not drew then
        Voxel3D.blend("add")
        Voxel3D.seams(false)
        Voxel3D.glass(false)
        drew = true
      end

      for i = 1, ShinyFx.STARS do
        -- the ring is offset half a step per side so the two sides do not
        -- twinkle in lockstep when both are shiny
        local a = (i / ShinyFx.STARS) * math.pi * 2
                  + (side == "player" and 0.31 or 0)
        local r = ShinyFx.SPREAD * e
        -- stars shrink as they fade, and alternate size so the ring reads
        -- as scattered rather than as a cog
        local k = ShinyFx.SIZE * (1 - u * 0.6) * ((i % 2 == 0) and 0.7 or 1)
        local ox = math.cos(a) * r
        local oy = math.sin(a) * r * 0.55   -- flattened: we look from low
        local m = Mat4.mul(
          Mat4.mul(Mat4.translate(x, baseY, z), Mat4.rotateY(yaw)),
          Mat4.mul(Mat4.translate(ox, oy, 0), Mat4.scale(k, k, 1)))
        love.graphics.setColor(1, 1, 1, alpha)
        Voxel3D.draw(mesh, img, m, pull)
      end
    end
  end

  if drew then
    love.graphics.setColor(1, 1, 1, 1)
    Voxel3D.glass(true)
    Voxel3D.seams(true)
    Voxel3D.blend("alpha")
  end
end

-- Drop the generated image (hot reload, or a graphics context that went
-- away) -- the same contract StadiumPack.invalidate honours.
function ShinyFx.invalidate()
  if star and star.release then pcall(star.release, star) end
  star = nil
  ShinyFx.clear()
end

return ShinyFx