introduce tags, structs, and reticles

this was meant to be split into smaller changes, but I realised I edited my existing revision too late.

crates/haku/src/trampoline.rs

@@ -0,0 +1,78 @@
+use tiny_skia::Pixmap;
+
+use crate::{
+    render::{Renderer, RendererLimits},
+    system::System,
+    value::{Ref, Reticle, Value, Vec2},
+    vm::{Exception, Vm},
+};
+
+#[derive(Debug, Clone, Copy)]
+pub struct Trampoline {
+    pub value: Value,
+}
+
+// NOTE: This must be kept in sync with haku.js.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum Cont {
+    Scribble,
+    Dotter,
+}
+
+impl Trampoline {
+    pub fn new(init: Value) -> Self {
+        Self { value: init }
+    }
+
+    pub fn cont(&self, vm: &Vm) -> Cont {
+        let Some((_, refv)) = vm.get_ref_value(self.value) else {
+            return Cont::Scribble;
+        };
+
+        match refv {
+            Ref::Reticle(_) => Cont::Dotter,
+            _ => Cont::Scribble,
+        }
+    }
+
+    pub fn scribble(
+        &mut self,
+        vm: &Vm,
+        pixmap: &mut Pixmap,
+        translation: Vec2,
+        limits: &RendererLimits,
+    ) -> Result<(), Exception> {
+        let mut renderer = Renderer::new(&mut *pixmap, limits);
+        renderer.translate(translation.x, translation.y);
+        renderer.render(vm, self.value)
+    }
+
+    pub fn dotter(
+        &mut self,
+        vm: &mut Vm,
+        system: &System,
+        from: Vec2,
+        to: Vec2,
+        num: f32,
+    ) -> Result<(), Exception> {
+        let (_, vref) = vm.get_ref_value(self.value).expect("value must be a ref");
+        let &Ref::Reticle(Reticle::Dotter {
+            draw: Value::Ref(draw_id),
+        }) = vref
+        else {
+            panic!("value must be a dotter reticle");
+        };
+
+        let dotter = vm.create_ref(Ref::Closure(system.create_dotter(
+            from.into(),
+            to.into(),
+            num,
+        )))?;
+
+        let value = vm.run(system, draw_id, &[Value::Ref(dotter)])?;
+        self.value = value;
+
+        Ok(())
+    }
+}