# ============================================================================
# tween.ludic — fluent, stateful tween handles the engine advances (#48).
#
# The pure Tween.* namespace (emit_anim.ludic) interpolates a single value from a
# game-managed timer. This adds the *sequenced* layer the follow-up asked for: a
# disposable handle that chains several segments — tween, delay, tween — and that
# the engine advances one tick per frame, so a game fires a multi-step motion once
# and just reads the current value:
#
#     var h = Tween.to(0, 100, 30, 2)      # ease-out 0 -> 100 over 30 ticks
#     h = Tween.delay(h, 15)               # hold 15 ticks
#     h = Tween.chain(h, 0, 30, 1)         # then ease-in 100 -> 0 over 30 ticks
#     # ...each frame the engine advances it...
#     sprite.x = Tween.value(h)
#     if Tween.done(h) { ... }
#
# Everything is integer + Q16.16-free (the ease curves run in 0..1024 fixed-point,
# like Motion), so a sequence plays identically on every run and headless — free
# replays and lockstep. The handle pool is fixed-size; Tween.to reuses a finished
# slot, so a game that starts and finishes tweens forever never runs out.
# ============================================================================

const TW_MAX:  int = 64            # concurrent handles
const TW_SEGS: int = 8             # segments per handle (chain depth)

# per-handle state
var tw_used:  words = null         # slot in use (1) or free (0)
var tw_seg:   words = null         # index of the segment currently playing
var tw_nseg:  words = null         # number of segments queued
var tw_tick:  words = null         # ticks elapsed inside the current segment
var tw_value: words = null         # OUTPUT: the value this frame
var tw_done:  words = null         # OUTPUT: 1 once every segment has finished

# per-segment state (flat: handle * TW_SEGS + seg)
var tw_kind:  words = null         # 0 = tween (from->to), 1 = delay (hold)
var tw_from:  words = null
var tw_to:    words = null
var tw_dur:   words = null         # segment length in ticks
var tw_ease:  words = null         # 0 linear, 1 in, 2 out, 3 in-out (Motion's curves)

function tw_init() -> void {
  if tw_used == null {
    tw_used  = words(TW_MAX)
    tw_seg   = words(TW_MAX)
    tw_nseg  = words(TW_MAX)
    tw_tick  = words(TW_MAX)
    tw_value = words(TW_MAX)
    tw_done  = words(TW_MAX)
    tw_kind  = words(TW_MAX * TW_SEGS)
    tw_from  = words(TW_MAX * TW_SEGS)
    tw_to    = words(TW_MAX * TW_SEGS)
    tw_dur   = words(TW_MAX * TW_SEGS)
    tw_ease  = words(TW_MAX * TW_SEGS)
  }
}

# claim a free handle (a finished or never-used slot). -1 if the pool is full.
function tw_alloc() -> int {
  tw_init()
  var i = 0
  while i < TW_MAX {
    if tw_used[i] == 0 { return i }
    i += 1
  }
  # none free: reuse the first finished handle so long-lived games don't leak.
  i = 0
  while i < TW_MAX {
    if tw_done[i] != 0 { return i }
    i += 1
  }
  return -1
}

# append one segment to a handle (internal). Silently ignored past TW_SEGS.
function tw_push(h: int, kind: int, from: int, to: int, dur: int, ease: int) -> void {
  if (h < 0) or (h >= TW_MAX) { return }
  let n = tw_nseg[h]
  if n >= TW_SEGS { return }
  let s = h * TW_SEGS + n
  tw_kind[s] = kind
  tw_from[s] = from
  tw_to[s]   = to
  tw_dur[s]  = dur
  tw_ease[s] = ease
  tw_nseg[h] = n + 1
}

# start a new tween handle: from -> to over `dur` ticks with easing `ease`.
function tween_to(from: int, to: int, dur: int, ease: int) -> int {
  let h = tw_alloc()
  if h < 0 { return -1 }
  tw_used[h]  = 1
  tw_seg[h]   = 0
  tw_nseg[h]  = 0
  tw_tick[h]  = 0
  tw_value[h] = from
  tw_done[h]  = 0
  tw_push(h, 0, from, to, dur, ease)
  return h
}

# chain a tween segment after the handle's current queue: continues from where the
# previous segment ends, so the game only names the new target. Returns the handle
# for fluent chaining.
function tween_chain(h: int, to: int, dur: int, ease: int) -> int {
  if (h < 0) or (h >= TW_MAX) { return h }
  var from = tw_value[h]
  let n = tw_nseg[h]
  if n > 0 {
    let last = h * TW_SEGS + (n - 1)
    if tw_kind[last] == 0 { from = tw_to[last] }     # continue from the previous tween's end
  }
  tw_push(h, 0, from, from + (to - from), dur, ease)  # `to` is the absolute target
  return h
}

# chain a pause of `ticks` ticks (the value holds). Returns the handle.
function tween_delay(h: int, ticks: int) -> int {
  if (h < 0) or (h >= TW_MAX) { return h }
  tw_push(h, 1, 0, 0, ticks, 0)
  return h
}

# the value of a handle this frame.
function tween_value(h: int) -> int {
  if (h < 0) or (h >= TW_MAX) { return 0 }
  return tw_value[h]
}

# has every segment of the handle finished?
function tween_done(h: int) -> bool {
  if (h < 0) or (h >= TW_MAX) { return true }
  return tw_done[h] != 0
}

# are two handles both finished? — the completion of a parallel pair. Independent
# handles advance together each frame, so running several at once *is* parallel;
# this is the "all done" query over a pair.
function tween_parallel(a: int, b: int) -> bool {
  return tween_done(a) and tween_done(b)
}

# free a handle immediately (stop and dispose). Its value is frozen where it was.
function tween_stop(h: int) -> void {
  if (h < 0) or (h >= TW_MAX) { return }
  tw_used[h] = 0
  tw_done[h] = 1
}

# floor of a/b for non-negative a (the tick counter only ever rises).
function tw_div(a: int, b: int) -> int {
  if b <= 0 { return 0 }
  return a / b
}

# the same integer easing curves Motion uses (progress carried in 0..1024).
function tween_ease(t: int, ease: int) -> int {
  if ease == 1 { return t * t / 1024 }
  if ease == 2 { let u = 1024 - t;  return 1024 - (u * u / 1024) }
  if ease == 3 {
    if t < 512 { return (t * t / 1024) * 2 }
    let u = 1024 - t
    return 1024 - (u * u / 1024) * 2
  }
  return t
}

# advance one active handle by a tick: interpolate within the current segment,
# roll over to the next when it ends, latch done past the last.
function tw_advance_one(h: int) -> void {
  if tw_done[h] != 0 { return }
  let n = tw_nseg[h]
  if n == 0 { tw_done[h] = 1;  return }
  var seg = tw_seg[h]
  if seg >= n { tw_done[h] = 1;  return }
  var tick = tw_tick[h] + 1
  let s   = h * TW_SEGS + seg
  let dur = tw_dur[s]
  let kind = tw_kind[s]
  if kind == 0 {                                     # a tween segment
    var t = 1024
    if dur > 0 { t = tw_div(tick * 1024, dur) }
    if t > 1024 { t = 1024 }
    let te = tween_ease(t, tw_ease[s])
    tw_value[h] = tw_from[s] + (tw_to[s] - tw_from[s]) * te / 1024
  }
  # (a delay segment holds tw_value unchanged.)
  if tick >= dur {                                   # this segment finished: advance
    if kind == 0 { tw_value[h] = tw_to[s] }          # rest exactly on the target
    seg += 1
    tw_seg[h]  = seg
    tw_tick[h] = 0
    if seg >= n { tw_done[h] = 1 }
  } else {
    tw_tick[h] = tick
  }
}

# the engine-owned system: advance every active tween handle one tick. Inserted
# into the Update phase when a game uses Tween.to (emit_game.ludic).
function esys_tween() -> void {
  if tw_used == null { return }
  var i = 0
  while i < TW_MAX {
    if (tw_used[i] != 0) and (tw_done[i] == 0) { tw_advance_one(i) }
    i += 1
  }
}