tor-browser

fromcharcode-charcodeat-zero.js (2630B)

---

function toCharCodes(str) {
 return [...str].map(s => s.length === 1 ? [s.charCodeAt(0)] : [s.charCodeAt(0), s.charCodeAt(1)])
                .flat();
}

function test() {
 // [Ascii; Latin-1, non-Ascii; non-Latin-1; non-BMP]
 const constant = "AaÁáĀā𝐀𝐚";
 const charCodes = toCharCodes(constant);

 // Create a linear, but non-constant string with the same contents.
 const linear = String.fromCharCode(...charCodes);
 assertEq(linear, constant);

 // Optimisation applies for |MCharCodeAt(MFromCharCode(MCharCodeAt(str, idx)), 0|.
 for (let i = 0; i < 500; ++i) {
   let idx = i & 3;

   // str[idx] is compiled to |MFromCharCode(MCharCodeAt(str, idx))|.
   assertEq(constant[idx].charCodeAt(0), charCodes[idx]);
   assertEq(linear[idx].charCodeAt(0), charCodes[idx]);

   // str.charAt(idx) is compiled to |MFromCharCode(MCharCodeAt(str, idx))|.
   assertEq(constant.charAt(idx).charCodeAt(0), charCodes[idx]);
   assertEq(linear.charAt(idx).charCodeAt(0), charCodes[idx]);
 }
}
for (let i = 0; i < 4; ++i) {
 test();
}

function testNonConstantIndex() {
 // [Ascii; Latin-1, non-Ascii; non-Latin-1; non-BMP]
 const constant = "AaÁáĀā𝐀𝐚";
 const charCodes = toCharCodes(constant);

 // Create a linear, but non-constant string with the same contents.
 const linear = String.fromCharCode(...charCodes);
 assertEq(linear, constant);

 // No optimisation when the index isn't a constant zero.
 let indices = [0, 1, 2, 3];
 for (let i = 0; i < 500; ++i) {
   let idx = i & 3;

   // Always zero, but too complicated to infer at compile time.
   let zero = indices[idx] - idx;

   assertEq(constant[idx].charCodeAt(zero), charCodes[idx]);
   assertEq(linear[idx].charCodeAt(zero), charCodes[idx]);

   assertEq(constant.charAt(idx).charCodeAt(zero), charCodes[idx]);
   assertEq(linear.charAt(idx).charCodeAt(zero), charCodes[idx]);
 }
}
for (let i = 0; i < 4; ++i) {
 testNonConstantIndex();
}

function testOOB() {
 // [Ascii; Latin-1, non-Ascii; non-Latin-1; non-BMP]
 const constant = "AaÁáĀā𝐀𝐚";
 const charCodes = toCharCodes(constant);

 // Create a linear, but non-constant string with the same contents.
 const linear = String.fromCharCode(...charCodes);
 assertEq(linear, constant);

 // No optimisation when the index is out-of-bounds.
 for (let i = 0; i < 500; ++i) {
   let idx = i & 3;

   assertEq(constant[idx].charCodeAt(1), NaN);
   assertEq(linear[idx].charCodeAt(1), NaN);

   assertEq(constant.charAt(idx).charCodeAt(1), NaN);
   assertEq(linear.charAt(idx).charCodeAt(1), NaN);
 }
}
for (let i = 0; i < 4; ++i) {
 testOOB();
}