twoSum

Returns the exact result of adding two doubles.

• the resulting array is the reverse of the standard twoSum in the literature.

Theorem 7 (Knuth): Let a and b be p-bit floating-point numbers. Then the following algorithm will produce a nonoverlapping expansion x + y such that a + b = x + y, where x is an approximation to a + b and y is the roundoff error in the calculation of x.

See https://people.eecs.berkeley.edu/~jrs/papers/robustr.pdf