Implement strict/lax verification for definition scope

Author: szuend

src/decode/decode.test.ts

@@ -14,7 +14,7 @@ import {
 import { encodeSigned, encodeUnsigned } from "../vlq.ts";
 import { decode, DecodeMode } from "./decode.ts";
 import type { SourceMapJson } from "../scopes.d.ts";
-import { OriginalScopeFlags, Tag } from "../codec.ts";
+import { GeneratedRangeFlags, OriginalScopeFlags, Tag } from "../codec.ts";
 
 class ItemEncoder {
   #encodedItems: string[] = [];
@@ -377,4 +377,35 @@ describe("decode", () => {
 
     assertEquals(info.ranges[0]?.values, [""]);
   });
+
+  it("throws if GENERATED_RANGE_START.definition is not a valid original scope in strict mode", () => {
+    const encoder = new ItemEncoder();
+    encoder.addUnsignedVLQs(
+      Tag.GENERATED_RANGE_START,
+      GeneratedRangeFlags.HAS_DEFINITION,
+      0,
+      1,
+    ).finishItem();
+    encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_END, 2).finishItem();
+    const map = createMap(encoder.encode(), []);
+
+    assertThrows(() => decode(map, { mode: DecodeMode.STRICT }));
+  });
+
+  it("ignores if GENERATED_RANGE_START.definition is not a valid original scope in lax mode", () => {
+    const encoder = new ItemEncoder();
+    encoder.addUnsignedVLQs(
+      Tag.GENERATED_RANGE_START,
+      GeneratedRangeFlags.HAS_DEFINITION,
+      0,
+      1,
+    ).finishItem();
+    encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_END, 2).finishItem();
+    const map = createMap(encoder.encode(), []);
+
+    const info = decode(map, { mode: DecodeMode.LAX });
+
+    assertExists(info.ranges[0]);
+    assertStrictEquals(info.ranges[0].originalScope, undefined);
+  });
 });

src/decode/decode.ts

@@ -299,9 +299,15 @@ class Decoder {
 
     if (item.definitionIdx !== undefined) {
       this.#rangeState.defScopeIdx += item.definitionIdx;
-      range.originalScope =
-        this.#flatOriginalScopes[this.#rangeState.defScopeIdx];
-      // TODO: Maybe throw if the idx is invalid?
+      if (
+        this.#rangeState.defScopeIdx < 0 ||
+        this.#rangeState.defScopeIdx >= this.#flatOriginalScopes.length
+      ) {
+        this.#throwInStrictMode("Invalid definition scope index");
+      } else {
+        range.originalScope =
+          this.#flatOriginalScopes[this.#rangeState.defScopeIdx];
+      }
     }
 
     this.#rangeStack.push(range);