add empty BitArray implementation

src/bitarray.ts

@@ -1,5 +1,5 @@
 import { join } from "./collector.js";
-import { Enumerable, arrayLike, sequence } from "./sync.js";
+import { arrayLike, sequence } from "./sync.js";
 import { asArray, isDefined } from "./utils.js";
 
 export interface BitArray extends Iterable<boolean> {
@@ -81,28 +81,6 @@ function* getBytes(bits: Iterable<boolean>) {
   }
 }
 
-export namespace BitArray {
-  export function create(length: number): BitArray {
-    return new BitArrayImpl(length);
-  }
-
-  export function from(bits: Iterable<boolean>): BitArray {
-    const arr = asArray(bits);
-    const result = create(arr.length);
-
-    let i = 0;
-    for (const bit of arr) {
-      result.set(i++, bit);
-    }
-
-    return result;
-  }
-
-  export function of(...bits: boolean[]): BitArray {
-    return from(bits);
-  }
-}
-
 class BitArrayImpl implements BitArray {
   readonly #length: number;
   readonly #bits: Uint8Array;
@@ -281,6 +259,100 @@ class BitArrayImpl implements BitArray {
   }
 }
 
+class EmptyBitArray implements BitArray {
+  [Symbol.iterator](): Iterator<boolean, any, undefined> {
+    return {
+      next: () => ({ done: true, value: undefined })
+    };
+  }
+
+  get length() {
+    return 0;
+  }
+
+  isFull() {
+    return false;
+  }
+
+  isEmpty() {
+    return false;
+  }
+
+  get(index: number): boolean {
+    throw new Error("BitArray has zero length");
+  }
+
+  set(index: number, value: boolean) {
+    throw new Error("BitArray has zero length");
+  }
+
+  fill(value: boolean) { }
+
+  and(other: BitArray) {
+    if (other.length !== 0) {
+      throw new Error("The other BitArray does not have zero length");
+    }
+
+    return this;
+  }
+
+  or(other: BitArray) {
+    if (other.length !== 0) {
+      throw new Error("The other BitArray does not have zero length");
+    }
+
+    return this;
+  }
+
+  xor(other: BitArray) {
+    if (other.length !== 0) {
+      throw new Error("The other BitArray does not have zero length");
+    }
+
+    return this;
+  }
+
+  not() {
+    return this;
+  }
+
+  contains(other: BitArray) {
+    return false;
+  }
+
+  intersects(other: BitArray) {
+    return false;
+  }
+
+  slice(offset: number, length: number) {
+    if (offset > 0) {
+      throw new Error("offset > 0");
+    }
+
+    if (length > 0) {
+      throw new Error("length > 0");
+    }
+
+    return this;
+  }
+
+  copy() {
+    return this;
+  }
+
+  toArray(): boolean[] {
+    return [];
+  }
+
+  equals(other: BitArray) {
+    return other === this || other && other.length === 0;
+  }
+
+  toString() {
+    return "";
+  }
+}
+
 class BitArraySlice implements BitArray {
   readonly #parent: BitArray;
   readonly #offset: number;
@@ -428,3 +500,32 @@ class BitArraySlice implements BitArray {
     return sequence(this).select(bit => bit ? '1' : '0').collect(join());
   }
 }
+
+const emptyBitArray = new EmptyBitArray();
+
+export namespace BitArray {
+  export const EMPTY = emptyBitArray;
+
+  export function create(length: number): BitArray {
+    if (length < 0) {
+      throw new Error("length < 0");
+    }
+
+    return length === 0 ? EMPTY : new BitArrayImpl(length);
+  }
+
+  export function from(bits: Iterable<boolean>): BitArray {
+    const arr = asArray(bits);
+    const result = create(arr.length);
+
+    for (let i = 0; i < arr.length; i++) {
+      result.set(i, arr[i]);
+    }
+
+    return result;
+  }
+
+  export function of(...bits: boolean[]): BitArray {
+    return from(bits);
+  }
+}