tor-browser

string-startswith-constant-string.js (2010B)

---

// Test case to cover String.prototype.startsWith with a constant search string.
//
// String.prototype.startsWith with a short (≤32 characters) constant string is
// optimised during lowering.

function* characters(...ranges) {
 for (let [start, end] of ranges) {
   for (let i = start; i <= end; ++i) {
     yield i;
   }
 }
}

const ascii = [...characters(
 [0x41, 0x5A], // A..Z
 [0x61, 0x7A], // a..z
 [0x30, 0x39], // 0..9
)];

const latin1 = [...characters(
 [0xC0, 0xFF], // À..ÿ
)];

const twoByte = [...characters(
 [0x100, 0x17E], // Ā..ž
)];

function toRope(s) {
 try {
   return newRope(s[0], s.substring(1));
 } catch {}
 // newRope can fail when |s| fits into an inline string. In that case simply
 // return the input.
 return s;
}

function atomize(s) {
 return Object.keys({[s]: 0})[0];
}

for (let i = 1; i <= 32; ++i) {
 let strings = [ascii, latin1, twoByte].flatMap(codePoints => [
   // Same string as the input.
   String.fromCodePoint(...codePoints.slice(0, i)),

   // Same length as the input, but a different string.
   String.fromCodePoint(...codePoints.slice(1, i + 1)),

   // Shorter string than the input.
   String.fromCodePoint(...codePoints.slice(0, i - 1)),

   // Longer string than the input.
   String.fromCodePoint(...codePoints.slice(0, i + 1)),
 ]).flatMap(x => [
   x,
   toRope(x),
   newString(x, {twoByte: true}),
   atomize(x),
 ]);

 for (let codePoints of [ascii, latin1, twoByte]) {
   let str = String.fromCodePoint(...codePoints.slice(0, i));

   let fn = Function("strings", `
     const expected = strings.map(x => {
       // Prevent Warp compilation when computing the expected results.
       with ({}) ;
       return x.startsWith("${str}");
     });

     for (let i = 0; i < 250; ++i) {
       let idx = i % strings.length;
       let str = strings[idx];

       let actual = str.startsWith("${str}");
       if (actual !== expected[idx]) throw new Error();
     }
   `);
   fn(strings);
 }
}