Audience: SG6; WG14
S. Davis Herring <>
Los Alamos National Laboratory
October 16, 2017

Introduction

The simple problem of computing a value between two other values is surprisingly subtle in general. This paper proposes library functions to compute the midpoint between two integer, floating-point, or pointer values, as well as a more general routine that interpolates (or extrapolates) between two floating-point values.

These utilities are a pure library extension. With the exception of the pointer versions, they would be valuable and straightforward to implement in C (perhaps via type-generic macros); it would be appropriate to consult with WG14 about including them in the C standard library.

Midpoint

Integers

Computing the (integer) midpoint of two integer values via

(a+b)/2

can cause overflow for signed or unsigned integers as well as for floating-point values. Java's binary search implementation had this integer overflow bug for nearly a decade, and Mozilla had the same issue in its JavaScript implementation.

The standard alternative

a+(b-a)/2

works for unsigned integers (even if b<a). On a typical two's complement implementation where conversion from unsigned to signed preserves bit patterns, the library can provide the simple implementation

Integer mid(Integer a, Integer b) {
using U = make_unsigned_t<Integer>;
return Integer(U(a)+(U(b)-U(a))/2);
}

that works for signed or unsigned Integer. Note that when b==a+1 or b==a-1 (without overflow), the result is a because of truncating division. This can be exploited to round half-integers up or down by supplying a>=b or a<=b respectively. (The simple (a+b)/2 always truncates half-integers towards 0, yielding min(a,b) when they differ by 1.)

Pointers

When a binary search over an array is implemented using pointers, the array size must not exceed PTRDIFF_MAX to avoid undefined behavior ([expr.add]/5). The library can also provide a function template

template<class T>
T* mid(T *a, T *b);

which is straightforward on common architectures but, it seems, cannot be implemented portably and efficiently in cpp. As with integers, when the midpoint lies between two pointer values the one closer to a is chosen; for the usual case of a<b, this is compatible with the usual half-open ranges by selecting a when [a,b) is [a,a+1).

Floating-point types

Each of the midpoint formulas above can cause overflow for floating-point types; the latter can also produce results that are not correctly rounded (by rounding in the subtraction and the addition). A third choice

a/2+b/2

prevents overflow but is not correctly rounded for subnormal inputs (due to rounding each separately). The library can easily provide overloads of mid for floating-point types that is correctly rounded for all inputs by switching between these strategies:

Float mid(Float a, Float b)
{return isnormal(a) && isnormal(b) ? a/2+b/2 : (a+b)/2;}

Naming

The name mean is superior for the floating-point case, but for the other types (and the common application to binary search) the name mid used above is more appropriate. It would be possible to use both names (despite the possible confusion with the use of mean in [rand.dist]), but as it aims to replace the simple expression (a+b)/2 regardless of type, a single name seems best.

Linear interpolation

Both obvious approaches used in published implementations of floating-point linear interpolation have issues:

1. a+t*(b-a) does not in general reproduce b when t==1, and can overflow if a and b have the largest exponent and opposite signs.
2. t*b+(1-t)*a is not monotonic in general (unless a*b<=0).

Lacking an obvious, efficient means of obtaining a correctly rounded overall result, the goal is instead to guarantee the basic properties of

1. exactness: lint(a,b,0)==a && lint(a,b,1)==b
2. monotonicity: (lint(a,b,t2)-lint(a,b,t1)) * (t2-t1) * (b-a) >= 0
3. boundedness: t<0 || t>1 || isfinite(lint(a,b,t))
4. consistency: lint(a,a,t)==a

given that each argument isfinite (for monotonicity, t1 and t2 may also be infinite if a!=b). These properties are useful in proofs of correctness of algorithms based on lint. When interpolating, consistency follows from the other properties, but it and monotonicity apply even when extrapolating.

To demonstrate the feasibility of satisfying these properties, a simple implementation that provides all of them is given here:

Float lint(Float a, Float b, Float t) {
// Exact, monotonic, bounded, and (for a=b=0) consistent:
if(a*b <= 0) return t*b + (1-t)*a;

if(t==1) return b;                        // exact
// Exact at t=0, monotonic except near t=1, bounded, and consistent:
const Float x = a + t*(b-a);
return t>1 == b>a ? max(b,x) : min(b,x);  // monotonic near t=1
}

Putting b first in the min/max prefers it to another equal value (i.e., -0. vs. +0.), which avoids returning -0. for t==1 but +0. for other nearby values of t.

Whether it uses this implementation or not, the library can provide a function satisfying these mathematical properties.

Naming

The name lerp is commonly seen in the context of computer graphics, among other places. It is also sometimes called mix, but that name seems too generic. The name lint is associated with sampling between list elements; since it is the clearest abbreviation of linear interpolation, it is used here.

It would also be possible to make this function yet another overload of mid, but that would even more strongly imply a restriction to interpolation.