# Sets, chains and rules - part I

In this post, I will develop the process through which the MathOptSetDistances.jl package has been created and evolved. In the second one, I will go over the differentiation part.

# MathOptInterface and the motivation

MathOptInterface.jl or MOI for short is a Julia package to unify structured constrained optimization problems. The abstract representation of problems MOI addresses is as follows:

\begin{align} \min_{x}\,\, & F(x) \\\\ \text{s.t.}\,\, & G_k(x) \in \mathcal{S}_k \,\, \forall k \\\\ & x \in \mathcal{X}. \end{align}

$\mathcal{X}$ is the domain of the decision variables, $F$ is the objective function, mapping values of the variables to the real line. The constrained aspect comes from the constraints $G_k(x) \in \mathcal{S}_k$, some mappings of the variables $G_k$ have to belong to a certain set $\mathcal{S}_k$. See this recent paper on MOI for more information on this representation.

The structured aspect comes from the fact that a specific form of $F$, $G$ and $\mathcal{S}$ is known in advance by the modeller. In other words, MOI does not deal with arbitrary unknown functions or black-box sets. For such cases, other tools are more adapted.

From a given problem in this representation, two operations can be of interest within a solution algorithm or from a user perspective:

1. Given a value for $x$, evaluating a function $F(x)$ or $G(x)$,
2. Given a value $v$ in the co-domain of $G_k$, asserting whether $v \in S_k$.

The first point is addressed by the function eval_variables in the MOI.Utilities submodule (documentation).

The second point appears as simple (or at least it did to me) but is trickier. What tolerance should be set? Most solvers include a numerical tolerance on constraint violations, should this be propagated from user choices, and how?

The deceivingly simple feature ended up opening one of the longest discussions in the MOI repository.

Fairly straightforward[…]

Optimistic me, beginning of the PR, February 2020

A more meaningful query for solvers is, given a value $v$, what is the distance from $v$ to the set $\mathcal{S}$:

\begin{align} (\text{δ(v, s)})\,\,\min_{v_p}\,\, & \text{dist}(v_p, v) \\\\ \text{s.t.}\,\, & v_p \in \mathcal{S}. \end{align}

The optimal value of the problem above noted $δ(v, s)$ depends on the notion of the distance taken between two values in the domain $\mathcal{V}$, noted $dist(\cdot,\cdot)$ here. In terms of implementation, the signature is roughly:

distance_to_set(v::V, s::S) -> Real


Aside: this is an example where multiple dispatch brings great value to the design: the implementation of distance_to_set depends on both the value type V and the type of set S. See why it’s useful in the Bonus section.

If $\mathcal{S}$ was a generic set, computing this distance would be as hard as solving an optimization problem with constraints $v \in \mathcal{S}$ but since we are dealing with structured optimization, many particular sets have closed-form solutions for the problem above.

# Examples

$\|\cdot\|$ will denote the $l_2-$norm if not specified.

The distance computation problem defined by the following data:

\begin{align} & v \in \mathcal{V} = \mathbb{R}^n,\\ & \mathcal{S} = \mathbb{Z}^n,\\ & dist(a, b) = \|a - b\| \end{align}

consists of rounding element-wise to the closest integer.

The following data:

\begin{align} & v \in \mathcal{V} = \mathbb{R}^n,\\ & \mathcal{S} = \mathbb{R}^n_+,\\ & dist(a, b) = \|a - b\| \end{align}

find the closest point in the positive orthant, with a result:

$$v_{p}\left[i\right] = \text{max}(v\left[i\right], 0) \,\, \forall i \in \{1..n\}.$$

# Set projections

The distance from a point to a set tells us how far a given candidate is from respecting a constraint. But for many algorithms, the quantity of interest is the projection itself:

$$\Pi_{\mathcal{S}}(v) \equiv \text{arg} \min_{v_p \in \mathcal{S}} \text{dist}(v, v_p).$$

Like the optimal distance, the best projection onto a set can often be defined in closed form i.e. without using generic optimization methods.

We also keep the convention that the projection of a point already in the set is always itself: $$δ(v, \mathcal{S}) = 0 \,\, \Leftrightarrow \,\, v \in \mathcal{S} \,\, \Leftrightarrow \,\, \Pi_{\mathcal{S}}(v) = v.$$

The interesting thing about projections is that once obtained, a distance can be computed easily, although only computing the distance can be slightly more efficient, since we do not need to allocate the projected point.

# User-defined distance notions

Imagine a set defined using two functions: \begin{align} \mathcal{S} = \{v \in \mathcal{V}\,|\, f(v) \leq 0, g(v)\leq 0 \}. \end{align}

The distance must be evaluated with respect to two values: $$(max(f(v), 0), max(g(v), 0)).$$

Here, the choice boils down to a norm, but hard-coding it seems harsh and rigid for users. Even if we plan correctly and add most norms people would expect, someone will end up with new exotic problems on sets, complex numbers or function spaces.

The solution that came up after discussions is adding a type to dispatch on, specifying the notion of distance used:

function distance_to_set(d::D, v::V, s::S)
where {D <: AbstractDistance, V, S <: MOI.AbstractSet}
# ...
end


which can for instance encode a p-norm or anything else. In many cases, there is no ambiguity, and the package defines DefaultDistance() exactly for this.

# Bonus

If you are coming from a class-based object-oriented background, a common design choice is to define a Set abstract class with a method project_on_set(v::V) to implement. This would work for most situations, since a set often implies a domain V. What about the following:

# Projecting onto the reals (no-op)
project_on_set(v::AbstractVector{T}, s::Reals) where {T <: Real}

# Projecting onto the reals (actual work)
project_on_set(v::AbstractVector{T}, s::Reals) where {T <: Complex}


Which “class” should own the implementation in that case? From what I observed, libraries end up with either an enumeration:

if typeof(v) == AbstractVector{<:Reals}
# ...
elseif # ...
end


or when the number of possible domains is expected to be low, with several methods:

# in the set class Reals
function project_real(v::AbstractVector{T}) where {T <: Real}
end

function project_complex(v::AbstractVector{T}) where {T <: Complex}
end

function project_scalar(v::T) where {T <: Real}
end


As a last remark, one may wonder why would one define trivial sets as the MOI.Reals or the MOI.Zeros. A good example where this is needed is the polyhedral cone: $$A x = 0$$ with $x$ a vector. This makes more sense to define $Ax$ as the function and
MOI.Zeros as the set.

##### Mathieu Besançon
###### Postdoctoral researcher, mathematical optimization

Mathematical optimization, scientific programming and related.