[lib] Add keyed array

Author: x0k

packages/form/src/lib/keyed-array.svelte.ts

@@ -0,0 +1,57 @@
+export interface KeyedArray<T> {
+  key(index: number): number;
+  push(value: T): void;
+  swap(a: number, b: number): void;
+  insert(index: number, value: T): void;
+  remove(index: number): void;
+}
+
+const EMPTY: any[] = [];
+
+export function createKeyedArray<T>(
+  array: () => T[]
+): KeyedArray<T> {
+  let arrayRef: WeakRef<T[]> = new WeakRef(EMPTY);
+  let lastKeys: number[] = EMPTY;
+  let lastKey = -1;
+  const keys = $derived.by(() => {
+    const arr = array();
+    if (arrayRef.deref() === arr) {
+      return lastKeys;
+    }
+    arrayRef = new WeakRef(arr);
+    lastKeys = new Array(arr.length);
+    for (let i = 0; i < arr.length; i++) {
+      // NOTE: there is no `wrap-around` behavior
+      // But i think `Infinity` is unreachable here
+      lastKeys[i] = ++lastKey;
+    }
+    return lastKeys;
+  });
+  return {
+    key(index: number) {
+      return keys[index]!;
+    },
+    push(value) {
+      lastKeys.push(++lastKey);
+      array().push(value);
+    },
+    swap(a, b) {
+      const arr = array();
+      const key = lastKeys[a];
+      lastKeys[a] = lastKeys[b]!;
+      lastKeys[b] = key!;
+      const tmp = arr[a]!;
+      arr[a] = arr[b]!;
+      arr[b] = tmp;
+    },
+    insert(index, value) {
+      lastKeys.splice(index, 0, ++lastKey);
+      array().splice(index, 0, value);
+    },
+    remove(index) {
+      lastKeys.splice(index, 1);
+      array().splice(index, 1);
+    },
+  };
+}