buffer: use a branchless loop for atob

Author: chjj

lib/buffer.js

@@ -1246,9 +1246,9 @@ if (internalBinding('config').hasIntl) {
 }
 
 function btoa(input) {
-  // The implementation here has not been performance optimized in any way and
-  // should not be.
-  // Refs: https://github.com/nodejs/node/pull/38433#issuecomment-828426932
+  // The implementation here has been slightly performance optimized,
+  // but still not nearly as much as it should be.
+  // Refs: https://github.com/nodejs/node/pull/51670
   if (arguments.length === 0) {
     throw new ERR_MISSING_ARGS('input');
   }
@@ -1267,63 +1267,94 @@ function btoa(input) {
 // Refs: https://infra.spec.whatwg.org/#forgiving-base64-decode
 //       https://infra.spec.whatwg.org/#ascii-whitespace
 // Valid Characters: [\t\n\f\r +/0-9=A-Za-z]
-// Lookup table (-1 = invalid, 0 = valid)
+// Lookup table (-1 = invalid, 0 = whitespace, 1 = non-whitespace)
+// Note that `=` is set to `-1` as it is handled elsewhere.
 /* eslint-disable no-multi-spaces, indent */
 const kForgivingBase64AllowedChars = [
   -1, -1, -1, -1, -1, -1, -1, -1,
   -1,  0,  0, -1,  0,  0, -1, -1,
   -1, -1, -1, -1, -1, -1, -1, -1,
   -1, -1, -1, -1, -1, -1, -1, -1,
    0, -1, -1, -1, -1, -1, -1, -1,
-  -1, -1, -1,  0, -1, -1, -1,  0,
-   0,  0,  0,  0,  0,  0,  0,  0,
-   0,  0, -1, -1, -1,  0, -1, -1,
-  -1,  0,  0,  0,  0,  0,  0,  0,
-   0,  0,  0,  0,  0,  0,  0,  0,
-   0,  0,  0,  0,  0,  0,  0,  0,
-   0,  0,  0, -1, -1, -1, -1, -1,
-  -1,  0,  0,  0,  0,  0,  0,  0,
-   0,  0,  0,  0,  0,  0,  0,  0,
-   0,  0,  0,  0,  0,  0,  0,  0,
-   0,  0,  0, -1, -1, -1, -1, -1,
+  -1, -1, -1,  1, -1, -1, -1,  1,
+   1,  1,  1,  1,  1,  1,  1,  1,
+   1,  1, -1, -1, -1, -1, -1, -1,
+  -1,  1,  1,  1,  1,  1,  1,  1,
+   1,  1,  1,  1,  1,  1,  1,  1,
+   1,  1,  1,  1,  1,  1,  1,  1,
+   1,  1,  1, -1, -1, -1, -1, -1,
+  -1,  1,  1,  1,  1,  1,  1,  1,
+   1,  1,  1,  1,  1,  1,  1,  1,
+   1,  1,  1,  1,  1,  1,  1,  1,
+   1,  1,  1, -1, -1, -1, -1, -1,
 ];
 /* eslint-enable no-multi-spaces, indent */
 
 function atob(input) {
-  // The implementation here has not been performance optimized in any way and
-  // should not be.
-  // Refs: https://github.com/nodejs/node/pull/38433#issuecomment-828426932
+  // The implementation here has been slightly performance optimized,
+  // but still not nearly as much as it should be.
+  // Refs: https://github.com/nodejs/node/pull/51670
   if (arguments.length === 0) {
     throw new ERR_MISSING_ARGS('input');
   }
 
   input = `${input}`;
+
   let nonAsciiWhitespaceCharCount = 0;
   let equalCharCount = 0;
+  let length = input.length;
+
+  // We use an accumulator to track errors. If, at the end,
+  // any high bits are set in `acc`, an invalid character has
+  // been parsed.
+  //
+  // This works because invalid base64 characters in the lookup
+  // table are `-1` and any non-ascii character will be greater
+  // than 0x7f.
+  let acc = 0;
 
-  for (let n = 0; n < input.length; n++) {
-    const ch = StringPrototypeCharCodeAt(input, n);
-    const val = kForgivingBase64AllowedChars[ch & 0x7f];
+  // Right-trim whitespace and equal signs.
+  while (length > 0) {
+    const ch = StringPrototypeCharCodeAt(input, length - 1);
 
-    if ((ch | val) & ~0x7f) {
-      throw lazyDOMException('Invalid character', 'InvalidCharacterError');
+    // Possibly-valid whitespace.
+    if (ch <= 0x20) {
+      acc |= kForgivingBase64AllowedChars[ch];
+      length--;
+      continue;
     }
 
-    if (ch > 0x20) {
+    // Equals sign.
+    if (ch === 0x3d) {
       nonAsciiWhitespaceCharCount++;
+      equalCharCount++;
+      length--;
+      continue;
+    }
 
-      if (ch === 0x3d) {
-        equalCharCount++;
-      } else if (equalCharCount) {
-        // The `=` char is only allowed at the end.
-        throw lazyDOMException('Invalid character', 'InvalidCharacterError');
-      }
+    break;
+  }
 
-      if (equalCharCount > 2) {
-        // Only one more `=` is permitted after the first equal sign.
-        throw lazyDOMException('Invalid character', 'InvalidCharacterError');
-      }
-    }
+  // Parse optimistically. Check for errors after.
+  // Equal signs are considered errors at this point (value = -1).
+  for (let n = 0; n < length; n++) {
+    const ch = StringPrototypeCharCodeAt(input, n);
+    const value = kForgivingBase64AllowedChars[ch & 0x7f];
+
+    acc |= ch | value;
+
+    // Valid non-whitespace has a value of `1`.
+    nonAsciiWhitespaceCharCount += value;
+  }
+
+  if (acc & ~0x7f) {
+    // We parsed an invalid character at some point in one of the loops.
+    throw lazyDOMException('Invalid character', 'InvalidCharacterError');
+  }
+
+  if (equalCharCount > 2) {
+    // Only two equal signs are permitted.
+    throw lazyDOMException('Invalid character', 'InvalidCharacterError');
   }
 
   let reminder = nonAsciiWhitespaceCharCount % 4;

test/parallel/test-btoa-atob.js

@@ -66,6 +66,17 @@ for (let i = 0; i <= 0xffff; i++) {
   throws(() => atob(ch + 'aaa'), invalidChar);
 }
 
+strictEqual(atob('YQ'), 'a');
+strictEqual(atob('YQ '), 'a');
+strictEqual(atob('Y Q'), 'a');
+strictEqual(atob('Y Q\t'), 'a');
+strictEqual(atob('Y Q=\t= '), 'a');
+strictEqual(atob('Y Q = = '), 'a');
+throws(() => atob('YQ='), invalidChar);
+throws(() => atob('YQ==='), invalidChar);
+throws(() => atob('Y=Q'), invalidChar);
+throws(() => atob('YQ\v'), invalidChar);
+
 throws(() => btoa('abcd\ufeffx'), invalidChar);
 
 const charset =