Composite is now generic over size in memory.

Previously the Composite trait accepted a Vec<Value> for reading from memory, and returned a Vec<Value> for writing to memory. This meant the caller might provide the wrong size of Vec (e.g. a Point3d requires 3 memory addresses, so the code has to check and handle if the user only passes in 2).

Now those methods accept/return a [Value; N] which is statically guaranteed to be the right size! This means there's no more need for runtime checks that the Vec is the right size -- because the array is guaranteed to be the right size.

This involves removing the SIZE constant and instead changing it into a SIZE const generic.

src/wasm-lib/execution-plan/src/composite.rs

@@ -1,36 +1,24 @@
-use crate::{ExecutionError, Value};
+use crate::{ExecutionError, NumericValue, Value};
 
 /// Types that can be written to or read from KCEP program memory,
 /// but require multiple values to store.
 /// They get laid out into multiple consecutive memory addresses.
-pub trait Composite: Sized {
-    /// How many memory addresses are required to store this value?
-    const SIZE: usize;
+pub trait Composite<const SIZE: usize>: Sized {
     /// Store the value in memory.
-    fn into_parts(self) -> Vec<Value>;
+    fn into_parts(self) -> [Value; SIZE];
     /// Read the value from memory.
-    fn from_parts(values: Vec<Value>) -> Result<Self, ExecutionError>;
+    fn from_parts(values: [Value; SIZE]) -> Result<Self, ExecutionError>;
 }
 
-impl Composite for kittycad::types::Point3D {
-    fn into_parts(self) -> Vec<Value> {
-        let points = [self.x, self.y, self.z];
-        points
-            .into_iter()
-            .map(|x| Value::NumericValue(crate::NumericValue::Float(x)))
-            .collect()
+impl Composite<3> for kittycad::types::Point3D {
+    fn into_parts(self) -> [Value; 3] {
+        [self.x, self.y, self.z]
+            .map(NumericValue::Float)
+            .map(Value::NumericValue)
     }
 
-    const SIZE: usize = 3;
-
-    fn from_parts(values: Vec<Value>) -> Result<Self, ExecutionError> {
-        let n = values.len();
-        let Ok([x, y, z]): Result<[Value; 3], _> = values.try_into() else {
-            return Err(ExecutionError::MemoryWrongSize {
-                actual: n,
-                expected: Self::SIZE,
-            });
-        };
+    fn from_parts(values: [Value; 3]) -> Result<Self, ExecutionError> {
+        let [x, y, z] = values;
         let x = x.try_into()?;
         let y = y.try_into()?;
         let z = z.try_into()?;

src/wasm-lib/execution-plan/src/lib.rs

@@ -6,9 +6,10 @@
 //! You can think of it as a domain-specific language for making KittyCAD API calls and using
 //! the results to make other API calls.
 
+use std::{collections::HashMap, fmt};
+
 use composite::Composite;
 use serde::{Deserialize, Serialize};
-use std::{collections::HashMap, fmt};
 
 mod composite;
 #[cfg(test)]
@@ -48,7 +49,7 @@ impl Memory {
 
     /// Store a composite value (i.e. a value which takes up multiple addresses in memory).
     /// Store its parts in consecutive memory addresses starting at `start`.
-    pub fn set_composite<T: Composite>(&mut self, composite_value: T, start: Address) {
+    pub fn set_composite<T: Composite<{ N }>, const N: usize>(&mut self, composite_value: T, start: Address) {
         let parts = composite_value.into_parts().into_iter();
         for (value, addr) in parts.zip(start.0..) {
             self.0.insert(addr, value);
@@ -57,17 +58,14 @@ impl Memory {
 
     /// Get a composite value (i.e. a value which takes up multiple addresses in memory).
     /// Its parts are stored in consecutive memory addresses starting at `start`.
-    pub fn get_composite<T: Composite>(&self, start: Address) -> Result<T, ExecutionError> {
-        let addrs = start.0..start.0 + T::SIZE;
-        let values: Vec<Value> = addrs
-            .into_iter()
-            .map(|a| {
-                let addr = Address(a);
-                self.get(&addr)
-                    .map(|x| x.to_owned())
-                    .ok_or(ExecutionError::MemoryEmpty { addr })
-            })
-            .collect::<Result<_, _>>()?;
+    pub fn get_composite<T: Composite<{ N }>, const N: usize>(&self, start: Address) -> Result<T, ExecutionError> {
+        let addrs: [Address; N] = core::array::from_fn(|i| Address(i + start.0));
+        let values: [Value; N] = arr_res_to_res_array(addrs.map(|addr| {
+            self.get(&addr)
+                .map(|x| x.to_owned())
+                .ok_or(ExecutionError::MemoryEmpty { addr })
+        }))?;
+
         T::from_parts(values)
     }
 }
@@ -273,6 +271,18 @@ pub enum ExecutionError {
     CannotApplyOperation { op: Operation, operands: Vec<Value> },
     #[error("Tried to read a '{expected}' from KCEP program memory, found an '{actual}' instead")]
     MemoryWrongType { expected: &'static str, actual: String },
-    #[error("Wrong size of memory trying to read value from KCEP program memory: got {actual} but wanted {expected}")]
-    MemoryWrongSize { expected: usize, actual: usize },
+}
+
+/// Take an array of result and return a result of array.
+/// If all members of the array are Ok(T), returns Ok with an array of the T values.
+/// If any member of the array was Err(E), return Err with the first E value.
+fn arr_res_to_res_array<T, E, const N: usize>(arr: [Result<T, E>; N]) -> Result<[T; N], E> {
+    let mut out = core::array::from_fn(|_| None);
+    for (i, res) in arr.into_iter().enumerate() {
+        out[i] = match res {
+            Ok(x) => Some(x),
+            Err(e) => return Err(e),
+        };
+    }
+    Ok(out.map(|opt| opt.unwrap()))
 }