# ludic.base

The vocabulary a game made of mechanic packages shares. A mechanic (fishing, needs, a shop, the
weather) depends on `ludic.base` and on nothing else - never on another mechanic. The game is the
only code that knows two mechanics, and it wires them together.

(`ludic.core` is a different, older package: the engine's canonical ECS components, used by the
examples under `examples/library/`.)

```ludic
import "ludic.base"
```

## The rules

1. **A mechanic uses only this.** Its module line says so - `module ludic_clock uses ludic_base`
   - and the compiler refuses a reference into any other module, a package's included. Its tests are one program: `ludic.base`, the mechanic, and a fake for each port.
2. **Ports for questions.** What a mechanic needs to ASK the world is a `port` it owns
   (`export port FishingWorld { is_water: fn(float, float) -> bool }`), in primitive and
   `ludic.base` types only. The game binds it once, as a declaration (`bind FishingWorld {
   is_water: fn lake }`), and the compiler refuses a program that uses a port with a required
   member and never binds it. A member with a default may be left out.
3. **Queues for facts.** What HAPPENED is pushed onto a `Queue<T>` the mechanic owns (`caught`)
   and drained by the game in a later phase. A mechanic never acts on another's behalf.
4. **Verbs for changes.** A mechanic's data is its `state` (`ToyFishingState`), handed to its
   functions as a parameter, and it changes only through its exported functions (`fishing_cast`,
   `pack_add`). Nothing writes another module's state; a function value (`fn fishing_tick`) is
   called with its states supplied, so a system never passes them itself.
5. **Phases for order.** A system names its phase; within a phase the game's list is the order.
   No system says "after fishing".
6. **Its own save section.** Each system saves under its own key with its own version and
   migrates its own old versions in `load`. A missing section is a reset.
7. **Its own dice.** A mechanic is handed an `Rng`, never draws from `Random.*`, so moving it
   cannot shift anyone else's rolls.

## The API

| | |
| --- | --- |
| `Tick { dt, frame, hours }`, `tick_new(dt, frame, hours)` | what a system's tick receives |
| `PH_INPUT`, `PH_SENSE`, `PH_SIMULATE`, `PH_RESOLVE`, `PH_COMMIT`, `PH_PRESENT`, `PH_COUNT` | the phases, in the order they run |
| `Queue<T>`, `queue_new<T>(name)`, `q_push(q, v)`, `q_drain(q) -> []T`, `q_len(q)`, `q_clear(q)` | facts, first in first out (`queue_new`, not `q_new`: that name is render3d's quaternion). A queue keeps its own count, so its verbs take only the queue: a mechanic's verbs take only the mechanic's own state |
| `q_tag(q) -> QueueTag`, `core_undrained(tags) -> []string` | the names of the given queues still holding facts; ask it at the end of a frame |
| `Rng`, `rng_new(seed)`, `rng_seed(r, seed)`, `rng_next(r)`, `rng_float(r)`, `rng_between(r, lo, hi)`, `rng_span(r, lo, hi)` | a xorshift32 stream of its own (`rng_between` is inclusive; `rng_range` is taken by `Random.range`) |
| `SaveNode { found, version, data }`, `save_tree()`, `save_section(root, key, version, v)`, `load_section(root, key) -> SaveNode`, `save_encode`, `save_decode` | the save tree: `{"fishing": {"v": 3, ...}}`; `v` is reserved in a section, and a non-object value rides under `data` |
| `sv_int`, `sv_float`, `sv_bool`, `sv_str` (key, fallback), `sv_ints` | typed reads with a fallback; a float is kept as thousandths |
| `sv_put_int`, `sv_put_float`, `sv_put_bool`, `sv_put_str`, `sv_put_ints` | the matching writes |
| `System { key, phase, version, init, reset, tick, save, load }`, `system_new(key, phase)` | a system; a null function is a verb it does not have |
| `core_add(s)`, `core_clear()`, `core_count()` | the game's system list (a key may appear once) |
| `def Systems key { ... }`, `SYS_<KEY>`, `SYS_COUNT` | a system declared from any module; the list starts from these |
| `core_init_all()`, `core_reset_all()`, `core_tick_all(t)`, `core_save_all() -> Val`, `core_load_all(v)` | the runner: in the order added, tick phase by phase |

## Entities: `Table<T>` and its indexes

A mechanic that keeps many of something (Things on the ground, animals, drops) keeps them as rows
of a `Table<T>` in its state rather than as a list it scans. The table is data-oriented and
allocates nothing per query:

- **Rows are dense.** Hot data is columns - `tb.f[c]` (floats) and `tb.i[c]` (ints), one value per
  row - and `tb.rec[row]` is a record `T` for everything cold. A removal moves the last row into
  the gap (`tb_remove`), so a sweep over `0 .. tb_len(tb)` touches contiguous memory.
- **Handles go stale.** `tb_add` returns a handle: a 22-bit slot and a 9-bit generation. `tb_row(tb,
  h)` is -1 once the entity is removed, even after the slot is reused. Keep handles, never rows.
- **Indexes are kept by the setters.** `tb_set_f`, `tb_set_xz`, `tb_set_i` (or writing `tb.f` /
  `tb.i` directly and then `tb_refile`) stamp the row's change tick and refile it in every index
  that reads that column, in O(1). An index is never a frame behind.
  - `tb_grid(tb, cx, cz, gate, cell)` - a spatial hash over two float columns, gated by an int
    column (a row is filed while it is non-zero: "active"). `tb_nearest(tb, g, x, z, maxr, mc, mv)`
    searches rings outward and stops at the first ring that cannot hold anything nearer;
    `tb_within(..., out)` fills a caller's `words`. Buckets double as the rows grow.
  - `tb_index(tb, col, gate)` - a cached query: the rows holding each value of an int column (a
    kind). `ix_rows(ix, v)`, `ix_count(ix, v)`, `ix_first(ix, v)`.
  - `tb_nearest_of` / `tb_within_of` plan between the two: a rare kind is scanned from its own
    list, a common one searched by rings.
- **Change detection.** `tb_advance(tb)` moves the table to its next tick; `tb_changed_since(tb,
  tick, out)` and `tb_added_since` name the rows written since - what a save or a message needs to
  send a delta instead of everything.
- **Observers.** `tb_on_add(tb, fn f)` / `tb_on_remove(tb, fn f)`: `f(handle)` (its states supplied,
  like a system's) is told each handle as its row is made and before it goes.
- **Groups and chunks.** `tb_remove_all(tb, ix, v)` removes every row an index files under one
  value - a kind, or a chunk of the world a stream lets go (`chunk_of(x, z, size)` packs a chunk's
  cell into an int). An index keeps a small value (below 1024) in its own slot and gives a larger,
  sparse one (a chunk's packed cell) a slot through a map, so each value costs one list.
- **Change detection is per 64-row block too**: `tb_changed_since` skips a block nobody wrote
  since. A row moved into a removal's gap keeps its own tick; the removal is the observer's.
- **Parallel work reads, never writes, a state.** `Job.parallel_for(n, fn work, ctx)` over a
  table's columns runs on every core; the compiler refuses a worker that takes a state as `mut`
  (a result goes into `ctx`, a shared count through a `Sync` handle kept in the state).
- **Stable ids.** `IntMap` (`imap_new`, `imap_put`, `imap_get(m, k, none)`, `imap_del`) maps an
  id kept in a save or a message to a handle without a scan.

Ludic frees nothing a safe program allocates, so a query that built a list per call leaked every
frame. Every question here writes into a buffer the caller keeps. What each costs, against a
`[]Record` list scanned (`M4 Pro`, one thread):

| | 10 000 | 100 000 | 1 000 000 |
| --- | --- | --- | --- |
| nearest, any | 0.37 us (list 30) | 1.5 us (list 307) | 7.2 us (list 3075) |
| nearest of a kind (1 in 40) | 1.4 us (list 5.8) | 3.3 us (list 56) | 12 us (list 864) |
| by id | 0.1 us (list 1.7) | 0.09 us (list 17) | 1.5 us (list 324) |
| within 30 m | 0.9 us | 1.5 us | 6.9 us |
| a move, refiled | 11 ns | 14 ns | 44 ns |

`tests/ecs_fuzz_test.ludic` holds the grid and the kind index against a scan through thousands of
random adds, removes, moves, kind changes and gate flips.

## A toy mechanic

```ludic
module toy_fishing uses ludic_base
numbers float
import "ludic.base"
export property Caught { species: int = 0, weight: float = 0.0 }
export port FishingWorld { is_water: fn(float, float) -> bool }           # its port
export state ToyFishingState {
  dice: Rng = null
  casts: int = 0
  caught: Queue<Caught> = null                                   # its facts
}
export function fishing_cast(toy_fishing_st: mut ToyFishingState, x: float, z: float) -> bool {                 # its verb
  if not FishingWorld.is_water(x, z) { return false }
  toy_fishing_st.casts += 1
  return true
}
function fishing_reset(toy_fishing_st: mut ToyFishingState) -> void {
  toy_fishing_st.casts = 0
  toy_fishing_st.dice = rng_new(7)
  toy_fishing_st.caught = queue_new("fishing.caught")
}
function fishing_tick(toy_fishing_st: mut ToyFishingState, t: Tick) -> void {
  while toy_fishing_st.casts > 0 {
    let c = new Caught
    c.species = rng_between(toy_fishing_st.dice, 0, 2)
    c.weight = 0.5 + rng_float(toy_fishing_st.dice)
    q_push(toy_fishing_st.caught, c)
    toy_fishing_st.casts -= 1
  }
}
export function fishing_system() -> System {
  let s = system_new("fishing", PH_SIMULATE)
  s.reset = fn fishing_reset
  s.tick = fn fishing_tick
  return s
}
```

## A game wiring two of them

`toy_pack` is the same shape: verbs `pack_add(kind, n)` / `pack_count(kind)` and a system in
`PH_COMMIT` that saves its counts. Neither knows the other; the game binds fishing's port, writes
the route, and orders the three.

```ludic
import "toys/fishing"
import "toys/pack"
import "ludic.base"
program Game {
  numbers float
  function lake(x: float, z: float) -> bool { return x < 100.0 }
  bind FishingWorld { is_water: fn lake }

  # the route: a landed fish goes into the pack
  function route_fishing_pack(fishing: ToyFishingState, pack: mut ToyPackState, t: Tick) -> void {
    let fish = q_drain(fishing.caught)
    for i in 0 .. len(fish) { pack_add(pack, fish[i].species, 1) }
  }

  function game_start(base_st: mut BaseState) -> void {
    core_add(base_st, fishing_system())
    let r = system_new("route.fishing_pack", PH_RESOLVE)
    r.tick = fn route_fishing_pack           # a fn(Tick) -> void: its states are supplied
    core_add(base_st, r)
    core_add(base_st, pack_system())
    core_reset_all(base_st)
  }
}
```

Both are compiled and run by `tests/route_test.ludic` (the mechanics are `tests/toys/`).

## Tests

Each piece has a program under `tests/`, and `ludic test` runs them all, every test block in a
process of its own:

```bash
ludic test packages/ludic.base
```

`queue_test`, `rng_test`, `save_test`, `system_test`, `registry_test` and `route_test`. They were
written before a generic call inside a `test` body resolved and before each test had a fresh
state, so the queue cases are functions a test calls and the runner's cases start with
`core_clear()`; neither is needed any more.

## Declared systems

- `Systems` is an `open registry`, so a system can be declared instead of added: `def Systems
  fishing { phase: PH_SIMULATE, tick: fn fishing_tick }` from any module. The runner starts from
  the declared systems - the order the compiler gives an open registry: ludic.base's own (none),
  then each other module's by module name, each module's in the order it is read - and `core_add`
  appends after them. A game that wants to decide the order itself writes the `def`s in its own
  files, or keeps calling `core_add` in one place.