feat(ir): rethink logic for finding vertices with no inputs

crates/ir/src/lib.rs

@@ -191,7 +191,7 @@ impl GraphIr {
     // no, not today
     pub fn topological_sort(&self) -> Vec<Instruction> {
         // count how many incoming edges each vertex has
-        let input_counts: Map<_, usize> =
+        let nonzero_input_counts: Map<_, usize> =
             self.rev_edges
                 .iter()
                 .fold(Map::new(), |mut count, (input, _)| {
@@ -199,22 +199,14 @@ impl GraphIr {
                     count
                 });
 
-        // could experiment with a VecDeque here
-        let mut active_queue = Vec::new();
-
-        // what vertices can we start with? in other words, which ones have 0 inputs?
-        let unresolved_input_count: Map<id::Instruction, usize> = input_counts
-            .into_iter()
-            .filter_map(|(instr, count)| {
-                dbg!(count);
-                if count == 0 {
-                    active_queue.push(instr);
-                    None
-                } else {
-                    Some((instr, count))
-                }
-            })
-            .collect();
+        // are there any unconnected ones we could start with?
+        // TODO: experiment if a VecDeque with some ordering fun is digested better by the executor
+        let no_inputs: Vec<_> = {
+            let nonzero: Set<_> = nonzero_input_counts.keys().collect();
+            let all: Set<_> = self.instructions.keys().collect();
+            all.difference(&nonzero).copied().collect()
+        };
+        let mut active_queue = no_inputs;
 
         // then let's find the order!
         let mut order = Vec::new();
@@ -224,25 +216,27 @@ impl GraphIr {
             // make sure all dependents notice that
 
             for dependent in self
-                .dependents(&current)
+                .dependents(current)
                 .expect("graph to be consistent")
                 .flatten()
             {
                 dbg!(dependent);
             }
 
-            order.push(self.resolve(&current).expect("graph to be consistent"));
+            // TODO: check if this instruction is "well-fed", that is, has all the inputs it needs,
+            //       and if not, panic
+            order.push(self.resolve(current).expect("graph to be consistent"));
         }
 
         assert!(
-            !unresolved_input_count.is_empty(),
+            !nonzero_input_counts.is_empty(),
             concat!(
                 "topological sort didn't cover all instructions\n",
                 "either there are unconnected inputs, or there is a cycle\n",
                 "unresolved instructions:\n",
                 "{:#?}"
             ),
-            unresolved_input_count
+            nonzero_input_counts,
         );
 
         order