ludic/selfhost/frontend/actions.ludic

# actions.ludic — 0.R: actions and reducers. What happened is an action, a typed record; what it
# means for a state is that state's reducer, in the module that owns it:
#
#   action PickUp { item: int }
#   reducer Pack on PickUp(p: mut Pack, a: PickUp) { push(p.items, a.item) }
#   ...
#   dispatch PickUp { item: 3 }
#
# `dispatch` queues the action; the queue is drained - every reducer of each action run in turn,
# each supplied its own state - at the end of every phase of the frame loop, after each ludic.base
# phase, or where the program calls drain_actions(). A reducer writes exactly its own state and may
# READ others, declared between it and the action. Actions a reducer dispatches go behind the queue,
# never re-entrant, and a queue still growing after ACTION_PASSES rounds stops the program.
var g_act_names: []pointer = new []pointer     # the actions, in the order read
var g_act_nodes: []Node = new []Node
var g_red_nodes: []Node = new []Node            # the reducers
var g_red_state: []pointer = new []pointer
var g_red_action: []pointer = new []pointer
var g_dsp_ids: []Node = new []Node              # each dispatch's action number, filled at the end
var g_dsp_names: []pointer = new []pointer
var g_dsp_at: []Node = new []Node
var g_act_main: pointer = null                  # the program's own file, where the drain is written
const ACTION_PASSES: int = 64

# action NAME { fields } - a record
function parse_action() -> void {
  let c = parse_component()
  c.uns = 3
  push(prog, c)
  push(g_act_names, c.s)
  push(g_act_nodes, c)
}
# reducer STATE on ACTION(s: mut STATE, a: ACTION) { ... }
function parse_reducer() -> void {
  let st = toks[pi + 1].text
  if toks[pi + 1].kind != TK_ID or not (toks[pi + 2].text == "on") or toks[pi + 3].kind != TK_ID {
    perr("a reducer is `reducer State on Action(s: mut State, a: Action) { ... }`")
  }
  let act = toks[pi + 3].text
  pi += 2
  toks[pi].text = "function"                     # read the rest as a function named for the action
  let f = parse_fn()
  f.s = `ludic_reduce__{act}__{st}`
  f.vis = 1
  push(g_red_nodes, f)
  push(g_red_state, st)
  push(g_red_action, act)
  push(prog, f)
}
# dispatch ACTION { fields } - a statement: the action made and queued
function parse_dispatch() -> Node {
  let at = toks[pi]
  pi += 1
  let name = eat_id()
  let nw = node(E_NEW)
  nw.s = name
  if is_op("{") { nw.a = record() }
  let call = node(E_CALL)
  let callee = node(E_ID)
  callee.s = "ludic_act_push"
  callee.pos = -1
  call.a = callee
  call.pos = -1                                  # the compiler's call: the queue is supplied
  let id = node(E_INT)
  id.pos = -1
  push(call.kids, id)
  push(call.kids, nw)
  push(g_dsp_ids, id)
  push(g_dsp_names, name)
  push(g_dsp_at, nw)
  let s = node(S_EXPR)
  s.a = call
  s.line = at.line
  return s
}
function act_index(name: pointer) -> int {
  var i = 0
  while i < len(g_act_names) {
    if (g_act_names[i] == name) { return i }
    i += 1
  }
  return -1
}
function act_err(n: Node, msg: pointer) -> void {
  g_err_file = n.file
  g_err_line = n.line
  g_parsing = false
  perr(msg)
}
# after the program is read: the reducers checked, each dispatch numbered, and the queue, its push
# and its drain written
function actions_finish() -> void {
  var i = 0
  while i < len(g_red_nodes) {
    red_check(i)
    i += 1
  }
  i = 0
  while i < len(g_dsp_ids) {
    let k = act_index(g_dsp_names[i])
    if k < 0 { act_err(g_dsp_at[i], `dispatch {g_dsp_names[i]}: {g_dsp_names[i]} is not an action - declare it with action {g_dsp_names[i]} {{ ... }}`) }
    g_dsp_ids[i].ival = k
    i += 1
  }
  # the queue and the drain are the program's, written in its own file: in the file of the first
  # action they would belong to its module, and it would call every module's reducers
  var file = g_parse_file
  if g_act_main != null { file = g_act_main }
  let line = 1
  # no actions: drain_actions() is still there (a ludic.base runner calls it), and does nothing
  if len(g_act_names) == 0 {
    vw_parse("export function drain_actions() -> void {\n}\n", file, line, 1)
    return
  }
  vw_parse(act_src(), file, line, 1)
}
function red_check(i: int) -> void {
  let f = g_red_nodes[i]
  let st = g_red_state[i]
  let act = g_red_action[i]
  if act_index(act) < 0 { act_err(f, `reducer {st} on {act}: {act} is not an action - declare it with action {act} {{ ... }}`) }
  if not is_state_ty(st) { act_err(f, `reducer {st} on {act}: {st} is not a state`) }
  var ps = new []Node
  var k = 0
  while k < len(f.kids) {
    if f.kids[k].kind == N_PARAM { push(ps, f.kids[k]) }
    k += 1
  }
  # between its state and the action: states it only READS, never another to write
  k = 1
  while k < len(ps) - 1 {
    let p = ps[k]
    if not is_state_ty(p.ty) { act_err(f, `reducer {st} on {act}: {p.s} is a {p.ty} - between its state and the action a reducer takes only states it reads; what it needs to know rides in the action`) }
    if p.uns == 1 { act_err(f, `reducer {st} on {act}: a reducer writes one state, and {p.s} is a mut {p.ty} - read it ({p.s}: {p.ty}), or dispatch an action {p.ty}'s own reducer takes`) }
    k += 1
  }
  if len(ps) < 2 or not (ps[0].ty == st) or not (ps[len(ps) - 1].ty == act) {
    act_err(f, `reducer {st} on {act}: its parameters are its state, any states it reads, and the action - ({reg_lower(st)}: mut {st}, a: {act})`)
  }
  var j = 0
  while j < i {
    if (g_red_state[j] == st) and (g_red_action[j] == act) { act_err(f, `reducer {st} on {act} is declared twice (first at {g_red_nodes[j].file}:{itoa(g_red_nodes[j].line)})`) }
    j += 1
  }
}
# the queue, its push and its drain: each action's reducers in the order of their states' names
function act_src() -> pointer {
  var src = "export state LudicActions {\n  kinds: []int = new []int\n  vals: []pointer = new []pointer\n  head: int = 0\n  depth: int = 0\n}\n"
  src = src + "export function ludic_act_push(q: mut LudicActions, k: int, v: pointer) -> void {\n  push(q.kinds, k)\n  push(q.vals, v)\n}\n"
  src = src + "export function drain_actions(q: mut LudicActions) -> void {\n  if q.depth > 0 { return }\n  q.depth = 1\n  var pass = 0\n  var round_end = len(q.kinds)\n"
  src = src + "  while q.head < len(q.kinds) {\n    if q.head == round_end {\n      pass += 1\n      round_end = len(q.kinds)\n"
  src = src + `      if pass >= {itoa(ACTION_PASSES)} {{ ludic_act_runaway(q.kinds[q.head]) }}\n    }}\n`
  src = src + "    let k = q.kinds[q.head]\n    let v = q.vals[q.head]\n    q.head += 1\n"
  var a = 0
  while a < len(g_act_names) {
    let rs = red_sorted(g_act_names[a])
    if len(rs) > 0 {
      src = src + `    if k == {itoa(a)} {{\n`
      var r = 0
      while r < len(rs) {
        src = src + `      {g_red_nodes[rs[r]].s}(v)\n`
        r += 1
      }
      src = src + "    }\n"
    }
    a += 1
  }
  src = src + "  }\n  q.kinds = new []int\n  q.vals = new []pointer\n  q.head = 0\n  q.depth = 0\n}\n"
  src = src + "export function ludic_act_runaway(k: int) -> void {\n  var name = \"?\"\n"
  a = 0
  while a < len(g_act_names) {
    src = src + `  if k == {itoa(a)} {{ name = "{g_act_names[a]}" }}\n`
    a += 1
  }
  src = src + "  print(`actions: {name} is still being dispatched after " + itoa(ACTION_PASSES) + " rounds of reducers - a reducer dispatches what dispatches it`)\n  exit(1)\n}\n"
  return src
}
function red_sorted(act: pointer) -> []int {
  let out = new []int
  var i = 0
  while i < len(g_red_nodes) {
    if (g_red_action[i] == act) { push(out, i) }
    i += 1
  }
  var a = 1
  while a < len(out) {
    let x = out[a]
    var b = a - 1
    while b >= 0 and reg_str_less(g_red_state[x], g_red_state[out[b]]) {
      out[b + 1] = out[b]
      b -= 1
    }
    out[b + 1] = x
    a += 1
  }
  return out
}
# a call to the queue from anywhere: the queue is the runtime's to supply, like a state an entry point
# is given (drain_actions() from a ludic.base runner or a program's own loop)
function is_action_builtin(name: pointer) -> bool {
  return (name == "drain_actions") or (name == "ludic_act_push")
}