diff --git a/.gitignore b/.gitignore
index 3ca43ae..ea8c4bf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,16 +1 @@
-# ---> Rust
-# Generated by Cargo
-# will have compiled files and executables
-debug/
-target/
-
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
-
-# These are backup files generated by rustfmt
-**/*.rs.bk
-
-# MSVC Windows builds of rustc generate these, which store debugging information
-*.pdb
-
+/target
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..33508fa
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "ecs"
+version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..914c6d3
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "ecs"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..dd0c22c
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,91 @@
+use std::fmt;
+
+// Error type
+struct SparseSetError;
+
+#[derive(Debug, Copy, Clone)]
+struct DenseItem<T> {
+    index: usize,
+    value: T
+}
+
+// TODO: Use arrays+slices instead of vecs
+#[derive(Debug)]
+struct SparseSet<T> {
+    sparse: Vec<Option<usize>>,
+    dense: Vec<DenseItem<T>>
+}
+
+impl<T> SparseSet<T> {
+    pub fn new() -> SparseSet<T> {
+        SparseSet {
+            sparse: vec![],
+            dense: vec![]
+        }
+    }
+
+    // Add an item to the dense vector and record its index in the sparse vector
+    fn insert(&mut self, value: T) {
+        let item = DenseItem {
+            index: self.dense.len(),
+            value: value
+        };
+        self.dense.push(item);
+        let dense_index = self.dense.len() - 1;
+        self.sparse.push(Some(dense_index));
+    }
+
+    // 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)
+        let target_dense_index = self.sparse[target_sparse_index].unwrap();
+
+        // 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();
+
+            // 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;
+        }
+
+        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);
+    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);
+    }
+}