it doesn't make sense to have more than the 255'th derivative in a single direction

src/expr/linkplan.rs

@@ -19,29 +19,29 @@ struct PreHasseNode {
    * split the derivation-vectors into levels (sum of their components)
  */
 
-fn sum(drvs: &[u16]) -> usize {
+fn sum(drvs: &[u8]) -> usize {
     drvs.iter().copied().map(usize::from).sum::<usize>()
 }
 
-fn parents(drvs: &[u16]) -> impl Iterator<Item = Box<[u16]>> + '_ {
+fn parents(drvs: &[u8]) -> impl Iterator<Item = Box<[u8]>> + '_ {
     drvs.iter()
         .enumerate()
         .filter_map(|(n, i)| i.checked_sub(1).map(|i2| (n, i2)))
         .map(|(n, j)| {
-            let mut i2: Box<[u16]> = drvs.to_owned().into_boxed_slice();
+            let mut i2: Box<[u8]> = drvs.to_owned().into_boxed_slice();
             i2[n] = j;
             i2
         })
 }
 
 /// calculate votes for each possible parent
-fn calculate_votes(mut que: Vec<Box<[u16]>>) -> Vec<BTreeMap<Box<[u16]>, u16>> {
+fn calculate_votes(mut que: Vec<Box<[u8]>>) -> Vec<BTreeMap<Box<[u8]>, u16>> {
     assert!(que.len() <= usize::from(u16::MAX));
 
     // split que into levels
     let mut levels = core::iter::repeat_with(BTreeMap::new)
         .take(que.iter().map(|i| sum(&i[..])).max().map(|i| i.checked_add(1).unwrap()).unwrap_or(0))
-        .collect::<Vec<BTreeMap<Box<[u16]>, u16>>>();
+        .collect::<Vec<BTreeMap<Box<[u8]>, u16>>>();
 
     for i in core::mem::replace(&mut que, Vec::new()) {
         let mut l = &mut levels[sum(&i[..])];

src/expr/mod.rs

@@ -12,7 +12,7 @@ use yz_string_utils::StrLexerBase;
 
 /// A reference to a PDE variable or derivative of such
 #[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct Link(pub u16, pub Box<[u16]>);
+pub struct Link(pub u16, pub Box<[u8]>);
 
 #[derive(Clone, Debug, PartialEq)]
 pub enum Expr<L> {
@@ -167,7 +167,7 @@ impl ParseLink for Link {
                 if dr.is_empty() {
                     break;
                 }
-                let dr = dr.parse::<u16>()?;
+                let dr = dr.parse::<u8>()?;
                 derivatives.push(dr);
             }
 
@@ -259,11 +259,11 @@ pub fn parse_expr_final<L: Clone + ParseLink>(slb: &mut StrLexerBase) -> Result<
 ```
 %vs
 # u_t + 0.11 u_x = 0
-0; prod(-0.11, 0_0_)
+0; prod(-0.11, 0_1)
 
 %mons
 # monitor u @ 1 + sin(t)
-0_; sum(1.0, sin(.))
+0_0; sum(1.0, sin(.))
 ```
 
 # Syntax:
@@ -374,11 +374,11 @@ mod tests {
         println!("{:?}", parse_system("
 %vs
 # u_t + 0.11 u_x = 0
-0; prod(-0.11, 0_0_)
+0; prod(-0.11, 0_1_)
 
 %mons
 # monitor u @ 1 + sin(t)
-0_; sum(1.0, sin(.))
+0_0; sum(1.0, sin(.))
         ").unwrap());
     }
 }