feat: Nil top-level original scopes use an empty tag now

Author: szuend

src/codec.ts

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 export const enum Tag {
+  EMPTY = 0x0,
   ORIGINAL_SCOPE_START = 0x1,
   ORIGINAL_SCOPE_END = 0x2,
   ORIGINAL_SCOPE_VARIABLES = 0x3,
@@ -14,6 +15,7 @@ export const enum Tag {
 }
 
 export const enum EncodedTag {
+  EMPTY = "A", // 0x0
   ORIGINAL_SCOPE_START = "B", // 0x1
   ORIGINAL_SCOPE_END = "C", // 0x2
   ORIGINAL_SCOPE_VARIABLES = "D", // 0x3

src/decode/decode.ts

@@ -141,14 +141,11 @@ class Decoder {
     const iter = new TokenIterator(this.#encodedScopes);
 
     while (iter.hasNext()) {
-      if (iter.peek() === ",") {
-        iter.nextChar(); // Consume ",".
-        this.#scopes.push(null); // Add an EmptyItem;
-        continue;
-      }
-
       const tag = iter.nextUnsignedVLQ();
       switch (tag) {
+        case Tag.EMPTY:
+          this.#scopes.push(null);
+          break;
         case Tag.ORIGINAL_SCOPE_START: {
           const item: OriginalScopeStartItem = {
             flags: iter.nextUnsignedVLQ(),
@@ -255,11 +252,6 @@ class Decoder {
       if (iter.hasNext()) iter.nextChar();
     }
 
-    if (iter.currentChar() === ",") {
-      // Handle trailing EmptyItem.
-      this.#scopes.push(null);
-    }
-
     if (this.#scopeStack.length > 0) {
       this.#throwInStrictMode(
         "Encountered ORIGINAL_SCOPE_START without matching END!",

src/encode/encode.test.ts

@@ -48,7 +48,7 @@ describe("encode", () => {
   it("encodes null OriginalScopes correctly", () => {
     const info = builder.addNullScope().addNullScope().addNullScope().build();
 
-    assertStrictEquals(encode(info).scopes, ",,");
+    assertStrictEquals(encode(info).scopes, "A,A,A");
   });
 
   it("throws when a child scope' start is not nested properly within its parent", () => {

src/encode/encoder.ts

@@ -66,7 +66,7 @@ export class Encoder {
 
   #encodeOriginalScope(scope: OriginalScope | null): void {
     if (scope === null) {
-      this.#encodedItems.push("");
+      this.#encodedItems.push(EncodedTag.EMPTY);
       return;
     }