Saturday 26 September 2020

The range a sum cannot fall within

Throughout this post, the variables A, B, and R are used, with R defined as R = A + B, and A ≤ B. Arithmetic in this post is unsigned and modulo 2k. Note that A ≤ B is not a restriction on the input, it is a choice to label the smaller input as A and the larger input as B. Addition is commutative, so this choice can be made without loss of generality.

R < A || R ≥ B

The sum is less than A iff the addition wraps (1), otherwise it has to be at least B (2).

  1. B cannot be so high that the addition can wrap all the way up to or past A. To make A + B add up to A, B would have had to be 2k, which is one beyond the maximum value it can be. R = A is possible only if B is zero, in which case R ≥ B holds instead.
  2. Since A is at least zero, in the absence of wrapping there is no way to reduce the value below the inputs.

Perhaps that all looks obvious, but this has a useful application: if the carry-out of the addition is not available, it can be computed via carry = (x + y) < x, which is a relatively well-known trick. It does not matter which of x or y is the smaller or larger input, the sum cannot fall within the "forbidden zone" between them. The occasionally seen carry = (x + y) < max(x, y) adds an unnecessary complication.

R < (A & B) || R ≥ (A | B)

This is a stronger statement, because A & B is usually smaller than A and A | B is usually greater than B.

If no wrapping occurs, then R ≥ (A | B). This can be seen for example by splitting the addition into a XOR and adding the carries separately, (A + B) = (A ^ B) + (A & B) * 2, while bitwise OR can be decomposed similarly into (A | B) = (A ^ B) + (A & B)(see below). Since there is no wrapping (by assumption), (A & B) * 2 ≥ (A & B) and therefore (A + B) ≥ (A | B). Or, with less algebra: addition sometimes produces a zero where the bitwise OR produces a one, but then addition compensates doubly for it by carrying into the next position.

For the case in which wrapping occurs I will take a bit-by-bit view. In order to wrap, the carry out of bit k-1 must be 1. In order for the sum to be greater than or equal to A & B, bit k-1 of the sum must be greater than or equal to bit k-1 of A & B. That combination means that the carry into bit k-1 of the sum must have been 1 as well. Furthermore, bit k-1 of the sum can't be greater than bit k-1 of A & B, at most it can be equal, which means bit k-2 must be examined as well. The same argument applies to bit k-2 and so on, until finally for the least-significant bit it becomes impossible for it to be carried into, so the whole thing falls down: by contradiction, A + B must be less than A & B when the sum wraps.

What about (A | B) = (A ^ B) + (A & B) though?

The more obvious version is (A | B) = (A ^ B) | (A & B), compensating for the bits reset by the XOR by ORing exactly those bits back in. Adding them back in also works, because the set bits in A ^ B and A & B are disjoint: a bit being set in the XOR means that exactly one of the input bits was set, which makes their AND zero.