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 =