util: improve isSafeAddition.

lib/utils/util.js

@@ -200,14 +200,6 @@ if (Object.assign)
 
 util.MAX_SAFE_INTEGER = 0x1fffffffffffff;
 
-/**
- * Max safe addition (52 bits).
- * @const {Number}
- * @default
- */
-
-util.MAX_SAFE_ADDITION = 0xfffffffffffff;
-
 /**
  * Test whether a number is below MAX_SAFE_INTEGER.
  * @param {Number} value
@@ -226,9 +218,44 @@ util.isSafeInteger = function isSafeInteger(value) {
  */
 
 util.isSafeAddition = function isSafeAddition(a, b) {
+  var hi, lo, ahi, alo, bhi, blo;
+  var as, bs, s, c;
+
+  // We only work on positive numbers.
   assert(a >= 0);
   assert(b >= 0);
-  return a <= util.MAX_SAFE_ADDITION && b <= util.MAX_SAFE_ADDITION;
+
+  // Fast case.
+  if (a <= 0xfffffffffffff && b <= 0xfffffffffffff)
+    return true;
+
+  // Do a 64 bit addition and check the top 11 bits.
+  ahi = (a * (1 / 0x100000000)) | 0;
+  alo = a | 0;
+
+  bhi = (b * (1 / 0x100000000)) | 0;
+  blo = b | 0;
+
+  // Credit to @indutny for this method.
+  lo = (alo + blo) | 0;
+
+  s = lo >> 31;
+  as = alo >> 31;
+  bs = blo >> 31;
+
+  c = ((as & bs) | (~s & (as ^ bs))) & 1;
+
+  hi = (((ahi + bhi) | 0) + c) | 0;
+
+  hi >>>= 0;
+  ahi >>>= 0;
+  bhi >>>= 0;
+
+  // Overflow?
+  if (hi < ahi || hi < bhi)
+    return false;
+
+  return (hi & 0xffe00000) === 0;
 };
 
 /**