Buggy attempt at using slices

src/main.rs

@@ -9,83 +9,79 @@ struct DenseItem<T> {
     value: T
 }
 
-// TODO: Use arrays+slices instead of vecs
+// 'sparse' is the sparse list tracking the indices of densely-stored items
+// 'dense' is the dense list containing the actual items
+// 'n' is the total number of densely-stored items
 #[derive(Debug)]
-struct SparseSet<T> {
-    sparse: Vec<Option<usize>>,
-    dense: Vec<DenseItem<T>>
+struct SparseSet<'s, 'd, T> {
+    sparse: &'s mut [Option<usize>],
+    dense: &'d mut [Option<DenseItem<T>>],
+    n: usize
 }
 
-impl<T> SparseSet<T> {
-    pub fn new() -> SparseSet<T> {
+impl<'s, 'd, T> SparseSet<'s, 'd, T> {
+    // A new sparse set can be created with two mutable slices: one for the sparse and one for the dense slices
+    pub fn new(sparse_slice: &'s mut [Option<usize>], dense_slice: &'d mut [Option<DenseItem<T>>], n: usize) -> SparseSet<'s, 'd, T> {
         SparseSet {
-            sparse: vec![],
-            dense: vec![]
+            sparse: sparse_slice,
+            dense: dense_slice,
+            n: n
         }
     }
 
-    // Add an item to the dense vector and record its index in the sparse vector
-    fn insert(&mut self, value: T) {
+    // Add an item to the set at a target sparse location
+    fn insert(&mut self, target_sparse_index: usize, value: T) {
+        // Create the item, first of all
         let item = DenseItem {
-            index: self.dense.len(),
+            index: target_sparse_index,
             value: value
         };
-        self.dense.push(item);
-        let dense_index = self.dense.len() - 1;
-        self.sparse.push(Some(dense_index));
+
+        // Insert the item into the dense slice at index N
+        self.dense[self.n] = Some(item);
+        
+        // Write N to the sparse slice at the target index
+        self.sparse[target_sparse_index] = Some(self.n);
+
+        // Increase N by one, lastly
+        self.n += 1;
     }
 
     // Remove an item at the supplied target index from the set
     // Replaces the selected target with the last item in the dense vector
     fn remove(&mut self, target_sparse_index: usize) -> Result<(), SparseSetError> {
-        // Find the dense index of the target (need to decide what to do from there)
+        // Get the last dense item and clear its former dense entry
+        let last_item = self.dense[self.n - 1].as_ref().unwrap();
+        let last_item_sparse_index = last_item.index;
+        self.dense[self.n - 1] = None;
+
+        // Replace the target dense item with the last dense item
         let target_dense_index = self.sparse[target_sparse_index].unwrap();
+        self.dense[target_dense_index] = Some(last_item);
 
-        // If the target points to the last item in the dense set, this'll be easier
-        if target_dense_index + 1 == self.dense.len() {
-            // Remove the target from the dense set
-            self.dense.pop();
+        // Update the (former) last dense item's corresponding dense index key to point to its new location (where the target used to be)
+        self.sparse[last_item_sparse_index] = Some(target_dense_index);
 
-            // Overwrite the target's sparse index
-            self.sparse[target_sparse_index] = None;
-        } else {
-            // Get the last item in the dense set as a source to switch with
-            // Change its index to the target sparse index, as it's replacing the target
-            let source_item = match self.dense.pop() {
-                Some(item) => { item },
-                None => { return Err(SparseSetError) }
-            };
-    
-            // Redirect the source item's sparse index to replace the target sparse index
-            self.sparse[source_item.index] = Some(target_sparse_index);
-    
-            // Assign the source item to the target's place in the dense set
-            self.dense[target_dense_index] = source_item;
-    
-            // Overwrite the target's sparse index
-            self.sparse[target_dense_index] = None;
-        }
+        // Clear the target's sparse entry
+        self.sparse[target_sparse_index] = None;
+
+        // Decrement N
+        self.n -= 1;
 
         Ok(())
     }
 }
 
 fn main() {
-    let mut test_set: SparseSet<f32> = SparseSet::new();
-    test_set.insert(69.0);
-    test_set.insert(420.0);
-    test_set.insert(9001.0);
+    // Create the arrays for our sparse set
+    let mut dense_array: [Option<DenseItem<f32>>; 8] = [None; 8];
+    let mut sparse_array: [Option<usize>; 8] = [None; 8];
+
+    let mut test_set: SparseSet<f32> = SparseSet::new(&mut sparse_array, &mut dense_array, 0);
+    test_set.insert(0, 69.0);
+    test_set.insert(4, 420.0);
+    test_set.insert(7, 9001.0);
     println!("{:?}", test_set);
     test_set.remove(1);
     println!("{:?}", test_set);
-
-    // Create a mutable array (contiguous memory!)
-    let mut array: [Option<DenseItem<f32>>; 8] = [None; 8];
-
-    // Create a mutable slice of that whole array
-    let slice: &[Option<DenseItem<f32>>] = &mut array[..];
-
-    for item in slice {
-        println!("{:?}", item);
-    }
 }