implement prefixed_length strings

src/string/prefixed_length.ts

@@ -0,0 +1,65 @@
+import { UnsizedType, u8 } from "../mod.ts";
+import { Options } from "../types/_common.ts";
+import { TEXT_DECODER, TEXT_ENCODER } from "./_common.ts";
+
+export class PrefixedLengthString extends UnsizedType<string> {
+  #prefixCodec: UnsizedType<number>;
+
+  constructor(prefixCodec: UnsizedType<number> = u8) {
+    super(1);
+    this.#prefixCodec = prefixCodec;
+  }
+
+  writePacked(value: string, dt: DataView, options: Options = { byteOffset: 0 }): void {
+    this.#prefixCodec.writePacked(value.length, dt, options);
+
+    const view = new Uint8Array(
+      dt.buffer,
+      dt.byteOffset + options.byteOffset,
+      value.length
+    );
+
+    TEXT_ENCODER.encodeInto(value, view);
+    super.incrementOffset(options, value.length);
+  }
+
+  write(value: string, dt: DataView, options: Options = { byteOffset: 0 }): void {
+    this.#prefixCodec.write(value.length, dt, options);
+    super.alignOffset(options);
+
+    const view = new Uint8Array(
+      dt.buffer,
+      dt.byteLength + options.byteOffset,
+      value.length,
+    );
+
+    TEXT_ENCODER.encodeInto(value, view);
+    super.incrementOffset(options, value.length);
+  }
+
+  readPacked(dt: DataView, options: Options = { byteOffset: 0 }): string {
+    const length = this.#prefixCodec.readPacked(dt, options);
+    const view = new Uint8Array(
+      dt.buffer,
+      dt.byteOffset + options.byteOffset,
+      length,
+    );
+
+    super.incrementOffset(options, length);
+    return TEXT_DECODER.decode(view);
+  }
+
+  read(dt: DataView, options: Options = { byteOffset: 0 }): string {
+    const length = this.#prefixCodec.read(dt, options);
+    super.alignOffset(options);
+
+    const view = new Uint8Array(
+      dt.buffer,
+      dt.byteOffset + options.byteOffset,
+      length,
+    );
+
+    super.incrementOffset(options, length);
+    return TEXT_DECODER.decode(view);
+  }
+}