Give each element a serial number (#22)

Give each `Element` a serial number, which identifies it uniquely. The serial number is assigned by the `Element::new` constructor.

Because disallows potentially unsafe global state (at least without explicit `unsafe` blocks), the next serial number is stored in a thread-safe static atomic variable (`assembly::NEXT_ELEMENT_SERIAL`), as suggested in [this StackOverflow answer](https://stackoverflow.com/a/32936288). Since the overhead for keeping track of memory ordering should be minimal, we're using the strongest available ordering: [sequentially consistent](https://marabos.nl/atomics/memory-ordering.html#seqcst).

Resolves #20.

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Reviewed-on: #22
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>

app-proto/src/assembly.rs

@@ -1,7 +1,7 @@
 use nalgebra::{DMatrix, DVector};
 use rustc_hash::FxHashMap;
 use slab::Slab;
-use std::collections::BTreeSet;
+use std::{collections::BTreeSet, sync::atomic::{AtomicU64, Ordering}};
 use sycamore::prelude::*;
 use web_sys::{console, wasm_bindgen::JsValue}; /* DEBUG */
 
@@ -13,6 +13,13 @@ pub type ConstraintKey = usize;
 
 pub type ElementColor = [f32; 3];
 
+/* KLUDGE */
+// we should reconsider this design when we build a system for switching between
+// assemblies. at that point, we might want to switch to hierarchical keys,
+// where each each element has a key that identifies it within its assembly and
+// each assembly has a key that identifies it within the sesssion
+static NEXT_ELEMENT_SERIAL: AtomicU64 = AtomicU64::new(0);
+
 #[derive(Clone, PartialEq)]
 pub struct Element {
     pub id: String,
@@ -21,6 +28,10 @@ pub struct Element {
     pub representation: Signal<DVector<f64>>,
     pub constraints: Signal<BTreeSet<ConstraintKey>>,
 
+    // a serial number, assigned by `Element::new`, that uniquely identifies
+    // each element
+    pub serial: u64,
+    
     // the configuration matrix column index that was assigned to this element
     // last time the assembly was realized
     column_index: usize
@@ -33,12 +44,24 @@ impl Element {
         color: ElementColor,
         representation: DVector<f64>
     ) -> Element {
+        // take the next serial number, panicking if that was the last number we
+        // had left. the technique we use to panic on overflow is taken from
+        // _Rust Atomics and Locks_, by Mara Bos
+        //
+        //   https://marabos.nl/atomics/atomics.html#example-handle-overflow
+        //
+        let serial = NEXT_ELEMENT_SERIAL.fetch_update(
+            Ordering::SeqCst, Ordering::SeqCst,
+            |serial| serial.checked_add(1)
+        ).expect("Out of serial numbers for elements");
+        
         Element {
             id: id,
             label: label,
             color: color,
             representation: create_signal(representation),
             constraints: create_signal(BTreeSet::default()),
+            serial: serial,
             column_index: 0
         }
     }

app-proto/src/outline.rs

@@ -200,7 +200,7 @@ pub fn Outline() -> View {
                 view=|(key, elt)| view! {
                     ElementOutlineItem(key=key, element=elt)
                 },
-                key=|(key, _)| key.clone()
+                key=|(_, elt)| elt.serial
             )
         }
     }