feat(etag): add etag serializer

etag.ts

@@ -0,0 +1,14 @@
+import { ETag } from "./types.ts";
+import { quote, weakPrefix } from "./utils.ts";
+
+export type WeakETagFormat = `W/"${string}"`;
+export type StrongETagFormat = `"${string}"`;
+export type ETagFormat = StrongETagFormat | WeakETagFormat;
+
+/** Serialize {@link ETag} into string. */
+export function stringify(etag: ETag): ETagFormat {
+  const opaqueTag = quote(etag.tag);
+  const etagFormat = etag.weak ? weakPrefix(opaqueTag) : opaqueTag;
+
+  return etagFormat;
+}

etag_test.ts

@@ -0,0 +1,29 @@
+import { stringify } from "./etag.ts";
+import { ETag } from "./types.ts";
+import { assertEquals, describe, it } from "./_dev_deps.ts";
+
+describe("stringify", () => {
+  it("return weak etag format", () => {
+    const table: [ETag, string][] = [
+      [{ weak: true, tag: "" }, `W/""`],
+      [{ weak: true, tag: "abc" }, `W/"abc"`],
+      [{ weak: true, tag: `"abc"` }, `W/""abc""`],
+    ];
+
+    table.forEach(([etag, expected]) => {
+      assertEquals(stringify(etag), expected);
+    });
+  });
+
+  it("return strong etag format", () => {
+    const table: [ETag, string][] = [
+      [{ weak: false, tag: "" }, `""`],
+      [{ weak: false, tag: "abc" }, `"abc"`],
+      [{ weak: false, tag: `"abc"` }, `""abc""`],
+    ];
+
+    table.forEach(([etag, expected]) => {
+      assertEquals(stringify(etag), expected);
+    });
+  });
+});