feat: Implement small int types U4 and U2 (#28)

src/array/array.ts

@@ -39,7 +39,6 @@ export class ArrayType<T> extends UnsizedType<T[]> {
     if (value.length !== this.length) {
       throw new TypeError("T[].length !== ArrayType<T>.length");
     }
-    if (value.length === 0) return;
 
     const { type } = this;
     for (let i = 0; i < value.length; i++) {
@@ -56,7 +55,6 @@ export class ArrayType<T> extends UnsizedType<T[]> {
     if (value.length !== this.length) {
       throw new TypeError("T[].length !== ArrayType<T>.length");
     }
-    if (value.length === 0) return;
 
     const { type } = this;
     for (let i = 0; i < value.length; i++) {

src/small_number/mod.ts

@@ -0,0 +1,49 @@
+import { type Options, SizedType } from "../mod.ts";
+
+type U2Number = 0 | 1 | 2 | 3;
+type U4Number = U2Number | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15;
+
+export class U2 extends SizedType<U2Number> {
+  constructor() {
+    super(1, 1);
+  }
+
+  readPacked(dt: DataView, options: Options = { byteOffset: 0 }): U2Number {
+    const v = dt.getUint8(options.byteOffset) & 0b11;
+    super.incrementOffset(options);
+    return v as U2Number;
+  }
+
+  writePacked(
+    value: U2Number,
+    dt: DataView,
+    options: Options = { byteOffset: 0 },
+  ): void {
+    dt.setUint8(options.byteOffset, value & 0b11);
+    super.incrementOffset(options);
+  }
+}
+
+export class U4 extends SizedType<U4Number> {
+  constructor() {
+    super(1, 1);
+  }
+
+  readPacked(dt: DataView, options: Options = { byteOffset: 0 }): U4Number {
+    const v = dt.getUint8(options.byteOffset) & 0b1111;
+    super.incrementOffset(options);
+    return v as U4Number;
+  }
+
+  writePacked(
+    value: U4Number,
+    dt: DataView,
+    options: Options = { byteOffset: 0 },
+  ): void {
+    dt.setUint8(options.byteOffset, value & 0b1111);
+    super.incrementOffset(options);
+  }
+}
+
+export const u2 = new U2();
+export const u4 = new U4();