perf(lint): avoid repeated source copies in shared-utils rule

oxlint-plugins/prefer-shared-utils.ts

@@ -108,27 +108,31 @@ const SHARED_UTILITY_DECLARATION =
         "g",
       );
 
-function maskRange(source: string, start: number, end: number) {
-  return `${source.slice(0, start)}${" ".repeat(end - start)}${source.slice(end)}`;
-}
-
 function maskCommentsAndQuotedStrings(source: string, options = { scanTemplateBodies: true }) {
-  let masked = source;
+  const parts: string[] = [];
+  let unmaskedStart = 0;
+  // Scan the original source and join once. Rebuilding the whole string for
+  // each span makes subsequent indexing repeatedly flatten it on large files.
+  function maskRange(start: number, end: number) {
+    parts.push(source.slice(unmaskedStart, start), " ".repeat(end - start));
+    unmaskedStart = end;
+  }
+
   let i = 0;
-  while (i < masked.length) {
-    const current = masked[i];
-    const next = masked[i + 1];
+  while (i < source.length) {
+    const current = source[i];
+    const next = source[i + 1];
     if (current === "/" && next === "/") {
-      const end = masked.indexOf("\n", i + 2);
-      const rangeEnd = end === -1 ? masked.length : end;
-      masked = maskRange(masked, i, rangeEnd);
+      const end = source.indexOf("\n", i + 2);
+      const rangeEnd = end === -1 ? source.length : end;
+      maskRange(i, rangeEnd);
       i = rangeEnd;
       continue;
     }
     if (current === "/" && next === "*") {
-      const end = masked.indexOf("*/", i + 2);
-      const rangeEnd = end === -1 ? masked.length : end + 2;
-      masked = maskRange(masked, i, rangeEnd);
+      const end = source.indexOf("*/", i + 2);
+      const rangeEnd = end === -1 ? source.length : end + 2;
+      maskRange(i, rangeEnd);
       i = rangeEnd;
       continue;
     }
@@ -138,44 +142,46 @@ function maskCommentsAndQuotedStrings(source: string, options = { scanTemplateBo
       // fixtures as template strings; mask those like ordinary strings so the
       // lint rule validates the test file itself, not its fixture payloads.
       let end = i + 1;
-      while (end < masked.length) {
-        if (masked[end] === "\\") {
+      while (end < source.length) {
+        if (source[end] === "\\") {
           end += 2;
           continue;
         }
-        if (masked[end] === "`") {
+        if (source[end] === "`") {
           end += 1;
           break;
         }
         end += 1;
       }
       if (!options.scanTemplateBodies) {
-        masked = maskRange(masked, i, end);
+        maskRange(i, end);
       }
       i = end;
       continue;
     }
     if (current === '"' || current === "'") {
       const quote = current;
       let end = i + 1;
-      while (end < masked.length) {
-        if (masked[end] === "\\") {
+      while (end < source.length) {
+        if (source[end] === "\\") {
           end += 2;
           continue;
         }
-        if (masked[end] === quote) {
+        if (source[end] === quote) {
           end += 1;
           break;
         }
         end += 1;
       }
-      masked = maskRange(masked, i, end);
+      maskRange(i, end);
       i = end;
       continue;
     }
     i += 1;
   }
-  return masked;
+  if (parts.length === 0) return source;
+  parts.push(source.slice(unmaskedStart));
+  return parts.join("");
 }
 
 function isCanonicalDefinitionFile(filename: string, modulePath: string) {

tests/oxlint-prefer-shared-utils.test.ts

@@ -176,6 +176,63 @@ void isUnknownRecord({});
     expect(result.output).toContain("Use shared isUnknownRecord");
   }, 30_000);
 
+  it("reports each helper once after many comments and escaped strings", () => {
+    const file = writeFixture(
+      "many-masked-spans.ts",
+      [
+        "export const docs = [",
+        ...Array.from(
+          { length: 2_000 },
+          () => String.raw`/* 😀 */ "escaped \" function isPromiseLike() {}", 'user\'s // docs',`,
+        ),
+        "].join('');",
+        "/* adjacent *//* comments */ function /* separator */ isPromiseLike(value: unknown) {",
+        "  return value !== null;",
+        "}",
+        "export const generatedSource = `",
+        "The user's output contains /* comments */ and // text.",
+        "An escaped backtick: \\`",
+        "function isPromiseLike(value) { return value !== null; }",
+        "`;",
+        "// A declaration after the generated template must still be checked.",
+        "export const compareStrings = (left: string, right: string) => left.localeCompare(right);",
+        "void isPromiseLike(null);",
+      ].join("\n"),
+    );
+
+    const result = runLint([file]);
+
+    expect(result.status).not.toBe(0);
+    expect(result.output.match(/Use shared isPromiseLike from/g)).toHaveLength(1);
+    expect(result.output.match(/Use shared compareStrings from/g)).toHaveLength(1);
+  }, 30_000);
+
+  it("masks test templates and escaped quotes without hiding later declarations", () => {
+    const file = writeTestFixture(
+      "escaped-templates.test.ts",
+      [
+        "export const fixture = `",
+        "The user's fixture contains /* and // and an escaped backtick: \\`",
+        "function isPromiseLike(value) { return value !== null; }",
+        "`;",
+        String.raw`export const doubleQuoted = "escaped \" function compareStrings() {}";`,
+        String.raw`export const singleQuoted = 'escaped \' function findFileWithExts() {}';`,
+        String.raw`export const backslash = "\\";`,
+        "// function isPromiseLike() {}",
+        "/* function compareStrings() {} */",
+        "export function isUnknownRecord(value: unknown) { return value !== null; }",
+      ].join("\r\n"),
+    );
+
+    const result = runLint([file]);
+
+    expect(result.status).not.toBe(0);
+    expect(result.output.match(/Use shared isUnknownRecord from/g)).toHaveLength(1);
+    expect(result.output).not.toContain("Use shared isPromiseLike");
+    expect(result.output).not.toContain("Use shared compareStrings");
+    expect(result.output).not.toContain("Use shared findFileWithExts");
+  }, 30_000);
+
   it("allows canonical modules, comments, and ordinary strings", () => {
     const commentsAndStringsFile = writeFixture(
       "comments-and-strings.ts",