builtin scripts

kiln-core/src/builtin_scripts.rs

@@ -0,0 +1,46 @@
+//! Built-in scripts: archetypes that are really scripted objects.
+//!
+//! Some archetypes (currently the `pusher_*` family) have behavior that an
+//! ordinary scripted object can express exactly. Rather than special-case them in
+//! the engine, the map loader *expands* such an archetype into an [`ObjectDef`]
+//! carrying an embedded Rhai script (see [`ObjectDef::builtin_script`]) plus a
+//! `BUILTIN_<archetype>` tag. The script reads that tag for any per-instance
+//! parameter (the pusher reads it for its direction), and [`crate::map_file`]'s
+//! save path uses the same tag to collapse the object back into its archetype
+//! keyword so maps round-trip.
+//!
+//! This module is the single place to register a new script-backed archetype:
+//! add an arm to [`archetype_script`].
+
+use crate::archetype::Archetype;
+
+/// Prefix for the tag that marks an object as an expanded built-in archetype and
+/// names which archetype it came from (e.g. `"BUILTIN_pusher_east"`).
+pub(crate) const BUILTIN_TAG_PREFIX: &str = "BUILTIN_";
+
+/// The pusher behavior, embedded into the binary. Shared by every direction —
+/// direction comes from the object's tag, so all pushers share one compiled AST.
+const PUSHER: &str = include_str!("scripts/pusher.rhai");
+
+/// The tag identifying an object as the expanded form of `arch`, e.g.
+/// `"BUILTIN_pusher_east"`.
+pub(crate) fn builtin_tag(arch: Archetype) -> String {
+    format!("{BUILTIN_TAG_PREFIX}{}", arch.name())
+}
+
+/// Recovers the archetype a `BUILTIN_*` tag came from, or `None` if `tag` is not a
+/// built-in tag naming a known archetype. Used by the save path to round-trip.
+pub(crate) fn archetype_from_builtin_tag(tag: &str) -> Option<Archetype> {
+    let name = tag.strip_prefix(BUILTIN_TAG_PREFIX)?;
+    Archetype::try_from(name).ok()
+}
+
+/// Returns the embedded script implementing `arch` as an object, or `None` if the
+/// archetype is a plain terrain cell. The one place script-backed archetypes are
+/// declared.
+pub(crate) fn archetype_script(arch: Archetype) -> Option<&'static str> {
+    match arch {
+        Archetype::Pusher(_) => Some(PUSHER),
+        _ => None,
+    }
+}