perf(wire): pre-size effect batch encoding

hemx-core/src/lib.rs

@@ -459,9 +459,18 @@ pub struct EffectBatch {
 }
 
 impl EffectBatch {
+    /// Return the exact number of bytes produced by [`Self::to_wire`].
+    pub fn encoded_len(&self) -> usize {
+        batch_wire_len(self)
+    }
+
     pub fn to_wire(&self) -> Vec<u8> {
-        let mut out = Vec::new();
+        let encoded_len = self.encoded_len();
+        let mut out = Vec::with_capacity(encoded_len);
+        let initial_capacity = out.capacity();
         write_batch(self, &mut out);
+        debug_assert_eq!(out.len(), encoded_len);
+        debug_assert_eq!(out.capacity(), initial_capacity);
         out
     }
 
@@ -492,6 +501,72 @@ pub enum WireError {
 }
 
 const WIRE_MAGIC: &[u8; 4] = b"HEMX";
+const WIRE_BATCH_HEADER_LEN: usize = WIRE_MAGIC.len() + 4 + 8 + 4;
+
+fn batch_wire_len(batch: &EffectBatch) -> usize {
+    WIRE_BATCH_HEADER_LEN + batch.ops.iter().map(effect_wire_len).sum::<usize>()
+}
+
+fn effect_wire_len(effect: &Effect) -> usize {
+    1 + match effect {
+        Effect::Put { target, payload } => ref_wire_len(target) + payload_wire_len(payload),
+        Effect::Insert {
+            target,
+            key,
+            payload,
+        }
+        | Effect::Prepend {
+            target,
+            key,
+            payload,
+        } => ref_wire_len(target) + str_wire_len(key) + payload_wire_len(payload),
+        Effect::Remove { target, key } => {
+            ref_wire_len(target) + option_str_wire_len(key.as_deref())
+        }
+        Effect::Move {
+            target,
+            key,
+            before,
+        } => ref_wire_len(target) + str_wire_len(key) + option_str_wire_len(before.as_deref()),
+        Effect::Focus { target } => ref_wire_len(target),
+        Effect::Navigate {
+            url, scroll, title, ..
+        } => {
+            str_wire_len(url) + 1 + scroll_wire_len(scroll) + option_str_wire_len(title.as_deref())
+        }
+        Effect::Emit { name, payload } => str_wire_len(name) + str_wire_len(payload),
+    }
+}
+
+fn ref_wire_len(reference: &ResourceRef) -> usize {
+    1 + 4
+        + 1
+        + match &reference.scope {
+            None => 0,
+            Some(ScopeKey::KeyValue(value) | ScopeKey::Field(value)) => str_wire_len(value),
+        }
+}
+
+fn payload_wire_len(payload: &Payload) -> usize {
+    1 + match payload {
+        Payload::Text(value) | Payload::Html(value) => str_wire_len(value),
+    }
+}
+
+fn scroll_wire_len(scroll: &ScrollBehavior) -> usize {
+    1 + match scroll {
+        ScrollBehavior::Preserve | ScrollBehavior::Top => 0,
+        ScrollBehavior::Element(target) => ref_wire_len(target),
+    }
+}
+
+fn option_str_wire_len(value: Option<&str>) -> usize {
+    1 + value.map_or(0, str_wire_len)
+}
+
+const fn str_wire_len(value: &str) -> usize {
+    4 + value.len()
+}
 
 fn write_batch(batch: &EffectBatch, out: &mut Vec<u8>) {
     out.extend_from_slice(WIRE_MAGIC);