General Information on the PPL

0.8

The Main Features

The Parma Polyhedra Library (PPL) is a modern C++ library for the manipulation of numerical information that can be represented by points in some $n$-dimensional vector space. For instance, one of the key domains the PPL supports is that of rational convex polyhedra (Section Convex Polyhedra). Such domains are employed in several systems for the analysis and verification of hardware and software components, with applications spanning imperative, functional and logic programming languages, synchronous languages and synchronization protocols, real-time and hybrid systems. Even though the PPL library is not meant to target a particular problem, the design of its interface has been largely influenced by the needs of the above class of applications. That is the reason why the library implements a few operators that are more or less specific to static analysis applications, while lacking some other operators that might be useful when working, e.g., in the field of computational geometry.

The main features of the library are the following:

In addition to the basic domains, we also provide generic support for constructing new domains from pre-existing domains. The following domains and domain constructors are provided by the PPL:

In the following sections we describe these domains and domain constructors together with their representations and operations that are available to the PPL user.

In the final section of this chapter (Section Using the Library), we provide some additional advice on the use of the library.

Convex Polyhedra

In this section we introduce convex polyhedra, as considered by the library, in more detail. For more information about the definitions and results stated here see [BRZH02b], [Fuk98], [NW88], and [Wil93].

Vectors, Matrices and Scalar Products

We denote by $\Rset^n$ the $n$-dimensional vector space on the field of real numbers $\Rset$, endowed with the standard topology. The set of all non-negative reals is denoted by $\nonnegRset$. For each $i \in \{0, \ldots, n-1\}$, $v_i$ denotes the $i$-th component of the (column) vector $\vect{v} = (v_0, \ldots, v_{n-1})^\transpose \in \Rset^n$. We denote by $\vect{0}$ the vector of $\Rset^n$, called the origin, having all components equal to zero. A vector $\vect{v} \in \Rset^n$ can be also interpreted as a matrix in $\Rset^{n \times 1}$ and manipulated accordingly using the usual definitions for addition, multiplication (both by a scalar and by another matrix), and transposition, denoted by $\vect{v}^\transpose$.

The scalar product of $\vect{v},\vect{w} \in \Rset^n$, denoted $\langle \vect{v}, \vect{w} \rangle$, is the real number

\[ \vect{v}^\transpose \vect{w} = \sum_{i=0}^{n-1} v_i w_i. \]

For any $S_1, S_2 \sseq \Rset^n$, the Minkowski's sum of $S_1$ and $S_2$ is: $S_1 + S_2 = \{\, \vect{v}_1 + \vect{v}_2 \mid \vect{v}_1 \in S_1, \vect{v}_2 \in S_2 \,\}.$

Affine Hyperplanes and Half-spaces

For each vector $\vect{a} \in \Rset^n$ and scalar $b \in \Rset$, where $\vect{a} \neq \vect{0}$, and for each relation symbol $\mathord{\relsym} \in \{ =, \geq, > \}$, the linear constraint $\langle \vect{a}, \vect{x} \rangle \relsym b$ defines:

Note that each hyperplane $\langle \vect{a}, \vect{x} \rangle = b$ can be defined as the intersection of the two closed affine half-spaces $\langle \vect{a}, \vect{x} \rangle \geq b$ and $\langle -\vect{a}, \vect{x} \rangle \geq -b$. Also note that, when $\vect{a} = \vect{0}$, the constraint $\langle \vect{0}, \vect{x} \rangle \relsym b$ is either a tautology (i.e., always true) or inconsistent (i.e., always false), so that it defines either the whole vector space $\Rset^n$ or the empty set $\emptyset$.

Convex Polyhedra

The set $\cP \sseq \Rset^n$ is a not necessarily closed convex polyhedron (NNC polyhedron, for short) if and only if either $\cP$ can be expressed as the intersection of a finite number of (open or closed) affine half-spaces of $\Rset^n$ or $n = 0$ and $\cP = \emptyset$. The set of all NNC polyhedra on the vector space $\Rset^n$ is denoted $\Pset_n$.

The set $\cP \in \Pset_n$ is a closed convex polyhedron (closed polyhedron, for short) if and only if either $\cP$ can be expressed as the intersection of a finite number of closed affine half-spaces of $\Rset^n$ or $n = 0$ and $\cP = \emptyset$. The set of all closed polyhedra on the vector space $\Rset^n$ is denoted $\CPset_n$.

When ordering NNC polyhedra by the set inclusion relation, the empty set $\emptyset$ and the vector space $\Rset^n$ are, respectively, the smallest and the biggest elements of both $\Pset_n$ and $\CPset_n$. The vector space $\Rset^n$ is also called the universe polyhedron.

In theoretical terms, $\Pset_n$ is a lattice under set inclusion and $\CPset_n$ is a sub-lattice of $\Pset_n$.

Note:
In the following, we will usually specify operators on the domain $\Pset_n$ of NNC polyhedra. Unless an explicit distinction is made, these operators are provided with the same specification when applied to the domain $\CPset_n$ of topologically closed polyhedra. The implementation maintains a clearer separation between the two domains of polyhedra (see Topologies and Topological-compatibility): while computing polyhedra in $\Pset_n$ may provide more precise results, polyhedra in $\CPset_n$ can be represented and manipulated more efficiently. As a rule of thumb, if your application will only manipulate polyhedra that are topologically closed, then it should use the simpler domain $\CPset_n$. Using NNC polyhedra is only recommended if you are going to actually benefit from the increased accuracy.

Bounded Polyhedra

An NNC polyhedron $\cP \in \Pset_n$ is bounded if there exists a $\lambda \in \nonnegRset$ such that

\[ \cP \sseq \bigl\{\, \vect{x} \in \Rset^n \bigm| - \lambda \leq x_j \leq \lambda \text{ for } j = 0, \ldots, n-1 \,\bigr\}. \]

A bounded polyhedron is also called a polytope.

Representations of Convex Polyhedra

NNC polyhedra can be specified by using two possible representations, the constraints (or implicit) representation and the generators (or parametric) representation.

Constraints Representation

In the sequel, we will simply write ``equality'' and ``inequality'' to mean ``linear equality'' and ``linear inequality'', respectively; also, we will refer to either an equality or an inequality as a constraint.

By definition, each polyhedron $\cP \in \Pset_n$ is the set of solutions to a constraint system, i.e., a finite number of constraints. By using matrix notation, we have

\[ \cP \defeq \{\, \vect{x} \in \Rset^n \mid A_1 \vect{x} = \vect{b}_1, A_2 \vect{x} \geq \vect{b}_2, A_3 \vect{x} > \vect{b}_3 \,\}, \]

where, for all $i \in \{1, 2, 3\}$, $A_i \in \Rset^{m_i} \times \Rset^n$ and $\vect{b}_i \in \Rset^{m_i}$, and $m_1, m_2, m_3 \in \Nset$ are the number of equalities, the number of non-strict inequalities, and the number of strict inequalities, respectively.

Combinations and Hulls

Let $S = \{ \vect{x}_1, \ldots, \vect{x}_k \} \sseq \Rset^n$ be a finite set of vectors. For all scalars $\lambda_1, \ldots, \lambda_k \in \Rset$, the vector $\vect{v} = \sum_{j=1}^k \lambda_j \vect{x}_j$ is said to be a linear combination of the vectors in $S$. Such a combination is said to be

We denote by $\linearhull(S)$ (resp., $\conichull(S)$, $\affinehull(S)$, $\convexhull(S)$) the set of all the linear (resp., positive, affine, convex) combinations of the vectors in $S$.

Let $P, C \sseq \Rset^n$, where $P \union C = S$. We denote by $\NNChull(P, C)$ the set of all convex combinations of the vectors in $S$ such that $\lambda_j > 0$ for some $\vect{x}_j \in P$ (informally, we say that there exists a vector of $P$ that plays an active role in the convex combination). Note that $\NNChull(P, C) = \NNChull(P, P \union C)$ so that, if $C \sseq P$,

\[ \convexhull(P) = \NNChull(P, \emptyset) = \NNChull(P, P) = \NNChull(P, C). \]

It can be observed that $\linearhull(S)$ is an affine space, $\conichull(S)$ is a topologically closed convex cone, $\convexhull(S)$ is a topologically closed polytope, and $\NNChull(P, C)$ is an NNC polytope.

Points, Closure Points, Rays and Lines

Let $\cP \in \Pset_n$ be an NNC polyhedron. Then

A point of an NNC polyhedron $\cP \in \Pset_n$ is a vertex if and only if it cannot be expressed as a convex combination of any other pair of distinct points in $\cP$. A ray $\vect{r}$ of a polyhedron $\cP$ is an extreme ray if and only if it cannot be expressed as a positive combination of any other pair $\vect{r}_1$ and $\vect{r}_2$ of rays of $\cP$, where $\vect{r} \neq \lambda \vect{r}_1$, $\vect{r} \neq \lambda \vect{r}_2$ and $\vect{r}_1 \neq \lambda \vect{r}_2$ for all $\lambda \in \nonnegRset$ (i.e., rays differing by a positive scalar factor are considered to be the same ray).

Generators Representation

Each NNC polyhedron $\cP \in \Pset_n$ can be represented by finite sets of lines $L$, rays $R$, points $P$ and closure points $C$ of $\cP$. The 4-tuple $\cG = (L, R, P, C)$ is said to be a generator system for $\cP$, in the sense that

\[ \cP = \linearhull(L) + \conichull(R) + \NNChull(P, C), \]

where the symbol '$+$' denotes the Minkowski's sum.

When $\cP \in \CPset_n$ is a closed polyhedron, then it can be represented by finite sets of lines $L$, rays $R$ and points $P$ of $\cP$. In this case, the 3-tuple $\cG = (L, R, P)$ is said to be a generator system for $\cP$ since we have

\[ \cP = \linearhull(L) + \conichull(R) + \convexhull(P). \]

Thus, in this case, every closure point of $\cP$ is a point of $\cP$.

For any $\cP \in \Pset_n$ and generator system $\cG = (L, R, P, C)$ for $\cP$, we have $\cP = \emptyset$ if and only if $P = \emptyset$. Also $P$ must contain all the vertices of $\cP$ although $\cP$ can be non-empty and have no vertices. In this case, as $P$ is necessarily non-empty, it must contain points of $\cP$ that are not vertices. For instance, the half-space of $\Rset^2$ corresponding to the single constraint $y \geq 0$ can be represented by the generator system $\cG = (L, R, P, C)$ such that $L = \bigl\{ (1, 0)^\transpose \bigr\}$, $R = \bigl\{ (0, 1)^\transpose \bigr\}$, $P = \bigl\{ (0, 0)^\transpose \bigr\}$, and $C = \emptyset$. It is also worth noting that the only ray in $R$ is not an extreme ray of $\cP$.

Minimized Representations

A constraints system $\cC$ for an NNC polyhedron $\cP \in \Pset_n$ is said to be minimized if no proper subset of $\cC$ is a constraint system for $\cP$.

Similarly, a generator system $\cG = (L, R, P, C)$ for an NNC polyhedron $\cP \in \Pset_n$ is said to be minimized if there does not exist a generator system $\cG' = (L', R', P', C') \neq \cG$ for $\cP$ such that $L' \sseq L$, $R' \sseq R$, $P' \sseq P$ and $C' \sseq C$.

Double Description

Any NNC polyhedron $\cP$ can be described by using a constraint system $\cC$, a generator system $\cG$, or both by means of the double description pair (DD pair) $(\cC, \cG)$. The double description method is a collection of well-known as well as novel theoretical results showing that, given one kind of representation, there are algorithms for computing a representation of the other kind and for minimizing both representations by removing redundant constraints/generators.

Such changes of representation form a key step in the implementation of many operators on NNC polyhedra: this is because some operators, such as intersections and poly-hulls, are provided with a natural and efficient implementation when using one of the representations in a DD pair, while being rather cumbersome when using the other.

Topologies and Topological-compatibility

As indicated above, when an NNC polyhedron $\cP$ is necessarily closed, we can ignore the closure points contained in its generator system $\cG = (L, R, P, C)$ (as every closure point is also a point) and represent $\cP$ by the triple $(L, R, P)$. Similarly, $\cP$ can be represented by a constraint system that has no strict inequalities. Thus a necessarily closed polyhedron can have a smaller representation than one that is not necessarily closed. Moreover, operators restricted to work on closed polyhedra only can be implemented more efficiently. For this reason the library provides two alternative ``topological kinds'' for a polyhedron, NNC and C. We shall abuse terminology by referring to the topological kind of a polyhedron as its topology.

In the library, the topology of each polyhedron object is fixed once for all at the time of its creation and must be respected when performing operations on the polyhedron.

Unless it is otherwise stated, all the polyhedra, constraints and/or generators in any library operation must obey the following topological-compatibility rules:

Wherever possible, the library provides methods that, starting from a polyhedron of a given topology, build the corresponding polyhedron having the other topology.

Space Dimensions and Dimension Compatibility

The space dimension of an NNC polyhedron $\cP \in \Pset_n$ (resp., a C polyhedron $\cP \in \CPset_n$) is the dimension $n \in \Nset$ of the corresponding vector space $\Rset^n$. The space dimension of constraints, generators and other objects of the library is defined similarly.

Unless it is otherwise stated, all the polyhedra, constraints and/or generators in any library operation must obey the following (space) dimension-compatibility rules:

While the space dimension of a constraint, a generator or a system thereof is automatically adjusted when needed, the space dimension of a polyhedron can only be changed by explicit calls to operators provided for that purpose.

Affine Independence and Affine Dimension

A finite set of points $\{ \vect{x}_1, \ldots, \vect{x}_k \} \sseq \Rset^n$ is affinely independent if, for all $\lambda_1, \ldots, \lambda_k \in \Rset$, the system of equations

\[ \sum_{i = 1}^k \lambda_i \vect{x}_i = \vect{0}, \quad \sum_{i = 1}^k \lambda_i = 0 \]

implies that, for each $i = 1, \ldots, k$, $\lambda_i = 0$.

The maximum number of affinely independent points in $\Rset^n$ is $n + 1$.

A non-empty NNC polyhedron $\cP \in \Pset_n$ has affine dimension $k \in \Nset$, denoted by $\pdim(\cP) = k$, if the maximum number of affinely independent points in $\cP$ is $k + 1$.

We remark that the above definition only applies to polyhedra that are not empty, so that $0 \leq \pdim(\cP) \leq n$. By convention, the affine dimension of an empty polyhedron is 0 (even though the ``natural'' generalization of the definition above would imply that the affine dimension of an empty polyhedron is $-1$).

Note:
The affine dimension $k \leq n$ of an NNC polyhedron $\cP \in \Pset_n$ must not be confused with the space dimension $n$ of $\cP$, which is the dimension of the enclosing vector space $\Rset^n$. In particular, we can have $\pdim(\cP) \neq \pdim(\cQ)$ even though $\cP$ and $\cQ$ are dimension-compatible; and vice versa, $\cP$ and $\cQ$ may be dimension-incompatible polyhedra even though $\pdim(\cP) = \pdim(\cQ)$.

Rational Polyhedra

An NNC polyhedron is called rational if it can be represented by a constraint system where all the constraints have rational coefficients. It has been shown that an NNC polyhedron is rational if and only if it can be represented by a generator system where all the generators have rational coefficients.

The library only supports rational polyhedra. The restriction to rational numbers applies not only to polyhedra, but also to the other numeric arguments that may be required by the operators considered, such as the coefficients defining (rational) affine transformations and (rational) bounding boxes.

Operations on Convex Polyhedra

In this section we briefly describe operations on NNC polyhedra that are provided by the library.

Intersection and Convex Polyhedral Hull

For any pair of NNC polyhedra $\cP_1, \cP_2 \in \Pset_n$, the intersection of $\cP_1$ and $\cP_2$, defined as the set intersection $\cP_1 \inters \cP_2$, is the biggest NNC polyhedron included in both $\cP_1$ and $\cP_2$; similarly, the convex polyhedral hull (or poly-hull) of $\cP_1$ and $\cP_2$, denoted by $\cP_1 \uplus \cP_2$, is the smallest NNC polyhedron that includes both $\cP_1$ and $\cP_2$. The intersection and poly-hull of any pair of closed polyhedra in $\CPset_n$ is also closed.

In theoretical terms, the intersection and poly-hull operators defined above are the binary meet and the binary join operators on the lattices $\Pset_n$ and $\CPset_n$.

Convex Polyhedral Difference

For any pair of NNC polyhedra $\cP_1, \cP_2 \in \Pset_n$, the convex polyhedral difference (or poly-difference) of $\cP_1$ and $\cP_2$ is defined as the smallest convex polyhedron containing the set-theoretic difference of $\cP_1$ and $\cP_2$.

In general, even though $\cP_1, \cP_2 \in \CPset_n$ are topologically closed polyhedra, their poly-difference may be a convex polyhedron that is not topologically closed. For this reason, when computing the poly-difference of two C polyhedra, the library will enforce the topological closure of the result.

Concatenating Polyhedra

Viewing a polyhedron as a set of tuples (its points), it is sometimes useful to consider the set of tuples obtained by concatenating an ordered pair of polyhedra. Formally, the concatenation of the polyhedra $\cP \in \Pset_n$ and $\cQ \in \Pset_m$ (taken in this order) is the polyhedron $\cR \in \Pset_{n+m}$ such that

\[ \cR \defeq \Bigl\{\, (x_0, \ldots, x_{n-1}, y_0, \ldots, y_{m-1})^\transpose \in \Rset^{n+m} \Bigm| (x_0, \ldots, x_{n-1})^\transpose \in \cP, (y_0, \ldots, y_{m-1})^\transpose \in \cQ \,\Bigl\}. \]

Another way of seeing it is as follows: first embed polyhedron $\cP$ into a vector space of dimension $n+m$ and then add a suitably renamed-apart version of the constraints defining $\cQ$.

Adding New Dimensions to the Vector Space

The library provides two operators for adding a number $i$ of space dimensions to an NNC polyhedron $\cP \in \Pset_n$, therefore transforming it into a new NNC polyhedron $\cQ \in \Pset_{n+i}$. In both cases, the added dimensions of the vector space are those having the highest indices.

The operator add_space_dimensions_and_embed embeds the polyhedron $\cP$ into the new vector space of dimension $i+n$ and returns the polyhedron $\cQ$ defined by all and only the constraints defining $\cP$ (the variables corresponding to the added dimensions are unconstrained). For instance, when starting from a polyhedron $\cP \sseq \Rset^2$ and adding a third space dimension, the result will be the polyhedron

\[ \cQ = \bigl\{\, (x_0, x_1, x_2)^\transpose \in \Rset^3 \bigm| (x_0, x_1)^\transpose \in \cP \,\bigr\}. \]

In contrast, the operator add_space_dimensions_and_project projects the polyhedron $\cP$ into the new vector space of dimension $i+n$ and returns the polyhedron $\cQ$ whose constraint system, besides the constraints defining $\cP$, will include additional constraints on the added dimensions. Namely, the corresponding variables are all constrained to be equal to 0. For instance, when starting from a polyhedron $\cP \sseq \Rset^2$ and adding a third space dimension, the result will be the polyhedron

\[ \cQ = \bigl\{\, (x_0, x_1, 0)^\transpose \in \Rset^3 \bigm| (x_0, x_1)^\transpose \in \cP \,\bigr\}. \]

Removing Dimensions from the Vector Space

The library provides two operators for removing space dimensions from an NNC polyhedron $\cP \in \Pset_n$, therefore transforming it into a new NNC polyhedron $\cQ \in \Pset_m$ where $m \leq n$.

Given a set of variables, the operator remove_space_dimensions removes all the space dimensions specified by the variables in the set. For instance, letting $\cP \in \Pset_4$ be the singleton set $\bigl\{ (3, 1, 0, 2)^\transpose \bigr\} \sseq \Rset^4$, then after invoking this operator with the set of variables $\{x_1, x_2\}$ the resulting polyhedron is

\[ \cQ = \bigl\{ (3, 2)^\transpose \bigr\} \sseq \Rset^2. \]

Given a space dimension $m$ less than or equal to that of the polyhedron, the operator remove_higher_space_dimensions removes the space dimensions having indices greater than or equal to $m$. For instance, letting $\cP \in \Pset_4$ defined as before, by invoking this operator with $m = 2$ the resulting polyhedron will be

\[ \cQ = \bigl\{ (3, 1)^\transpose \bigr\} \sseq \Rset^2. \]

Mapping the Dimensions of the Vector Space

The operator map_space_dimensions provided by the library maps the dimensions of the vector space $\Rset^n$ according to a partial injective function $\pard{\rho}{\{0, \ldots, n-1\}}{\Nset}$ such that $\rho\bigl(\{0, \ldots, n-1\}\bigr) = \{0, \ldots, m-1\}$ with $m \leq n$. Dimensions corresponding to indices that are not mapped by $\rho$ are removed.

If $m = 0$, i.e., if the function $\rho$ is undefined everywhere, then the operator projects the argument polyhedron $\cP \in \Pset_n$ onto the zero-dimension space $\Rset^0$; otherwise the result is $\cQ \in \Pset_m$ given by

\[ \cQ \defeq \Bigl\{\, \bigl(v_{\rho^{-1}(0)}, \ldots, v_{\rho^{-1}(m-1)}\bigr)^\transpose \Bigm| (v_0, \ldots, v_{n-1})^\transpose \in \cP \,\Bigr\}. \]

Expanding One Dimension of the Vector Space to Multiple Dimensions

The operator expand_space_dimension provided by the library adds $m$ new space dimensions to a polyhedron $\cP \in \Pset_n$, with $n > 0$, so that dimensions $n$, $n+1$, $\ldots$, $n+m-1$ of the result $\cQ$ are exact copies of the $i$-th space dimension of $\cP$. More formally,

\[ \cQ \defeq \sset{ \vect{u} \in \Rset^{n+m} }{ \exists \vect{v}, \vect{w} \in \cP \st u_i = v_i \\ \qquad \mathord{} \land \forall j = n, n+1, \ldots, n+m-1 \itc u_j = w_i \\ \qquad \mathord{} \land \forall k = 0, \ldots, n-1 \itc k \neq i \implies u_k = v_k = w_k }. \]

This operation has been proposed in [GDMDRS04].

Folding Multiple Dimensions of the Vector Space into One Dimension

The operator fold_space_dimensions provided by the library, given a polyhedron $\cP \in \Pset_n$, with $n > 0$, folds a set of space dimensions $J = \{ j_0, \ldots, j_{m-1} \}$, with $m < n$ and $j < n$ for each $j \in J$, into space dimension $i < n$, where $i \notin J$. The result is given by

\[ \cQ \defeq \biguplus_{d = 0}^m \cQ_d \]

where

\[ \cQ_m \defeq \sset{ \vect{u} \in \Rset^{n-m} }{ \exists \vect{v} \in \cP \st u_{i'} = v_i \\ \qquad \mathord{} \land \forall k = 0, \ldots, n-1 \itc k \neq i \implies u_{k'} = v_k } \]

and, for $ d = 0 $, $ \ldots $, $ m-1 $,

\[ \cQ_d \defeq \sset{ \vect{u} \in \Rset^{n-m} }{ \exists \vect{v} \in \cP \st u_{i'} = v_{j_d} \\ \qquad \mathord{} \land \forall k = 0, \ldots, n-1 \itc k \neq i \implies u_{k'} = v_k }, \]

and, finally, for $ k = 0 $, $ \ldots $, $ n-1 $,

\[ k' \defeq k - \card \{\, j \in J \mid k > j \,\}, \]

($\card S$ denotes the cardinality of the finite set $S$).

This operation has been proposed in [GDMDRS04].

Images and Preimages of Affine Transfer Relations

For each relation $\reld{\phi}{\Rset^n}{\Rset^m}$, we denote by $\phi(S) \sseq \Rset^m$ the image under $\phi$ of the set $S \sseq \Rset^n$; formally,

\[ \phi(S) \defeq \bigl\{\, \vect{w} \in \Rset^m \bigm| \exists \vect{v} \in S \st (\vect{v}, \vect{w}) \in \phi \,\bigr\}. \]

Similarly, we denote by $\phi^{-1}(S') \sseq \Rset^n$ the preimage under $\phi$ of $S' \sseq \Rset^m$, that is

\[ \phi^{-1}(S') \defeq \bigl\{\, \vect{v} \in \Rset^n \bigm| \exists \vect{w} \in S' \st (\vect{v}, \vect{w}) \in \phi \,\bigr\}. \]

If $n = m$, then the relation $\phi$ is said to be space dimension preserving.

The relation $\reld{\phi}{\Rset^n}{\Rset^m}$ is said to be an affine relation if there exists $\ell \in \Nset$ such that

\[ \forall \vect{v} \in \Rset^n, \vect{w} \in \Rset^m \itc (\vect{v}, \vect{w}) \in \phi \iff \bigland_{i=1}^{\ell} \bigl( \langle \vect{c}_i, \vect{w} \rangle \relsym_i \langle \vect{a}_i, \vect{v} \rangle + b_i \bigr), \]

where $\vect{a}_i \in \Rset^n$, $\vect{c}_i \in \Rset^m$, $b_i \in \Rset$ and $\mathord{\relsym}_i \in \{ <, \leq, =, \geq, > \}$, for each $i = 1, \ldots, \ell$.

As a special case, the relation $\reld{\phi}{\Rset^n}{\Rset^m}$ is an affine function if and only if there exist a matrix $A \in \Rset^m \times \Rset^n$ and a vector $\vect{b} \in \Rset^m$ such that,

\[ \forall \vect{v} \in \Rset^n, \vect{w} \in \Rset^m \itc (\vect{v}, \vect{w}) \in \phi \iff \vect{w} = A\vect{v} + \vect{b}. \]

The set $\Pset_n$ of NNC polyhedra is closed under the application of images and preimages of any space dimension preserving affine relation. The same property holds for the set $\CPset_n$ of closed polyhedra, provided the affine relation makes no use of the strict relation symbols $<$ and $>$. Images and preimages of affine relations can be used to model several kinds of transition relations, including deterministic assignments of affine expressions, (affinely constrained) nondeterministic assignments and affine conditional guards.

A space dimension preserving relation $\reld{\phi}{\Rset^n}{\Rset^n}$ can be specified by means of a shorthand notation:

As an example, assuming $\reld{\phi}{\Rset^3}{\Rset^3}$, the notation $x'_0 - x'_2 \geq 2 x_0 - x_1$, where the primed variable $x'_1$ does not occur, is meant to specify the affine relation defined by

\[ \forall \vect{v} \in \Rset^3, \vect{w} \in \Rset^3 \itc (\vect{v}, \vect{w}) \in \phi \iff (w_0 + w_2 \geq 2 v_0 - v_1) \land (w_1 = v_1). \]

The same relation is specified by $x'_0 + 0 \cdot x'_1 - x'_2 \geq 2 x_0 - x_1$, since $x'_1$ occurs with coefficient 0.

The library allows for the computation of images and preimages of polyhedra under restricted subclasses of space dimension preserving affine relations, as described in the following.

Single-Update Affine Functions.

Given a primed variable $x'_k$ and an unprimed affine expression $\langle \vect{a}, \vect{x} \rangle + b$, the affine function $\fund{\phi = \bigl(x'_k = \langle \vect{a}, \vect{x} \rangle + b\bigr)} {\Rset^n}{\Rset^n}$ is defined by

\[ \forall \vect{v} \in \Rset^n \itc \phi(\vect{v}) = A\vect{v} + \vect{b}, \]

where

\[ A = \begin{pmatrix} 1 & & 0 & 0 & \cdots & \cdots & 0 \\ & \ddots & & \vdots & & & \vdots \\ 0 & & 1 & 0 & \cdots & \cdots & 0 \\ a_0 & \cdots & a_{k-1} & a_k & a_{k+1} & \cdots & a_{n-1} \\ 0 & \cdots & \cdots & 0 & 1 & & 0 \\ \vdots & & & \vdots & & \ddots & \\ 0 & \cdots & \cdots & 0 & 0 & & 1 \end{pmatrix}, \qquad \vect{b} = \begin{pmatrix} 0 \\ \vdots \\ 0 \\ b \\ 0 \\ \vdots \\ 0 \end{pmatrix} \]

and the $a_i$ (resp., $b$) occur in the $(k+1)$st row in $A$ (resp., position in $\vect{b}$). Thus function $\phi$ maps any vector $(v_0, \ldots, v_{n-1})^\transpose$ to

\[ \Bigl(v_0, \ldots, \bigl(\textstyle{\sum_{i=0}^{n-1}} a_i v_i + b\bigr), \ldots, v_{n-1}\Bigr)^\transpose. \]

The affine image operator computes the affine image of a polyhedron $\cP$ under $x'_k = \langle \vect{a}, \vect{x} \rangle + b$. For instance, suppose the polyhedron $\cP$ to be transformed is the square in $\Rset^2$ generated by the set of points $\bigl\{ (0, 0)^\transpose, (0, 3)^\transpose, (3, 0)^\transpose, (3, 3)^\transpose \bigr\}$. Then, if the primed variable is $x_0$ and the affine expression is $x_0 + 2 x_1 + 4$ (so that $k = 0$, $a_0 = 1, a_1 = 2, b = 4$), the affine image operator will translate $\cP$ to the parallelogram $\cP_1$ generated by the set of points $\bigl\{ (4, 0)^\transpose, (10, 3)^\transpose, (7, 0)^\transpose, (13, 3)^\transpose \bigr\}$ with height equal to the side of the square and oblique sides parallel to the line $x_0 - 2 x_1$. If the primed variable is as before (i.e., $k = 0$) but the affine expression is $x_1$ (so that $a_0 = 0, a_1 = 1, b = 0$), then the resulting polyhedron $\cP_2$ is the positive diagonal of the square.

The affine preimage operator computes the affine preimage of a polyhedron $\cP$ under $x'_k = \langle \vect{a}, \vect{x} \rangle + b$. For instance, suppose now that we apply the affine preimage operator as given in the first example using primed variable $x_0$ and affine expression $x_0 + 2 x_1 + 4$ to the parallelogram $\cP_1$; then we get the original square $\cP$ back. If, on the other hand, we apply the affine preimage operator as given in the second example using primed variable $x_0$ and affine expression $x_1$ to $\cP_2$, then the resulting polyhedron is the stripe obtained by adding the line $(1, 0)^\transpose$ to polyhedron $\cP_2$.

Observe that provided the coefficient $a_k$ of the considered variable in the affine expression is non-zero, the affine function is invertible.

Single-Update Bounded Affine Relations.

Given a primed variable $x'_k$ and two unprimed affine expressions $\mathrm{lb} = \langle \vect{a}, \vect{x} \rangle + b$ and $\mathrm{ub} = \langle \vect{c}, \vect{x} \rangle + d$, the bounded affine relation $\phi = (\mathrm{lb} \leq x'_k \leq \mathrm{ub})$ is defined as

\[ \forall \vect{v} \in \Rset^n, \vect{w} \in \Rset^n \itc (\vect{v}, \vect{w}) \in \phi \iff \bigl( \langle \vect{a}, \vect{v} \rangle + b \leq w_k \leq \langle \vect{c}, \vect{v} \rangle + d \bigr) \land \Bigl( \bigland_{0 \leq i < n, i \neq k} w_i = v_i \Bigr). \]

Generalized Affine Relations.

Similarly, the generalized affine relation $\phi = (\mathrm{lhs}' \relsym \mathrm{rhs})$, where $\mathrm{lhs} = \langle \vect{c}, \vect{x} \rangle + d$ and $\mathrm{rhs} = \langle \vect{a}, \vect{x} \rangle + b$ are affine expressions and $\mathord{\relsym} \in \{ <, \leq, =, \geq, > \}$ is a relation symbol, is defined as

\[ \forall \vect{v} \in \Rset^n, \vect{w} \in \Rset^n \itc (\vect{v}, \vect{w}) \in \phi \iff \bigl( \langle \vect{c}, \vect{w} \rangle + d \relsym \langle \vect{a}, \vect{v} \rangle + b \bigr) \land \Bigl( \bigland_{0 \leq i < n, c_i = 0} w_i = v_i \Bigr). \]

When $\mathrm{lhs} = x_k$ and $\mathord{\relsym} \in \{ = \}$, then the above affine relation becomes equivalent to the single-update affine function $x'_k = \mathrm{rhs}$ (hence the name given to this operator). It is worth stressing that the notation is not symmetric, because the variables occurring in expression $\mathrm{lhs}$ are interpreted as primed variables, whereas those occurring in $\mathrm{rhs}$ are unprimed; for instance, the transfer relations $\mathrm{lhs}' \leq \mathrm{rhs}$ and $\mathrm{rhs}' \geq \mathrm{lhs}$ are not equivalent in general.

Time-Elapse Operator

The time-elapse operator has been defined in [HPR97]. Actually, the time-elapse operator provided by the library is a slight generalization of that one, since it also works on NNC polyhedra. For any two NNC polyhedra $\cP, \cQ \in \Pset_n$, the time-elapse between $\cP$ and $\cQ$, denoted $ \cP \nearrow \cQ$, is the smallest NNC polyhedron containing the set

\[ \bigl\{\, \vect{p} + \lambda \vect{q} \in \Rset^n \bigm| \vect{p} \in \cP, \vect{q} \in \cQ, \lambda \in \nonnegRset \,\bigr\}. \]

Note that, if $\cP,\cQ \in \CPset_n$ are closed polyhedra, the above set is also a closed polyhedron. In contrast, when $\cQ$ is not topologically closed, the above set might not be an NNC polyhedron.

Relation-With Operators

The library provides operators for checking the relation holding between an NNC polyhedron and either a constraint or a generator.

Suppose $\cP$ is an NNC polyhedron and $\cC$ an arbitrary constraint system representing $\cP$. Suppose also that $ c = \bigl( \langle \vect{a}, \vect{x} \rangle \relsym b \bigr) $ is a constraint with $\mathord{\relsym} \in \{ =, \geq, > \}$ and $\cQ$ the set of points that satisfy $c$. The possible relations between $\cP$ and $c$ are as follows.

The polyhedron $\cP$ subsumes the generator $g$ if adding $g$ to any generator system representing $\cP$ does not change $\cP$.

Intervals, Boxes and Bounding Boxes

An interval in $\Rset$ is a pair of bounds, called lower and upper. Each bound can be either (1) closed and bounded, (2) open and bounded, or (3) open and unbounded. If the bound is bounded, then it has a value in $\Rset$. An $n$-dimensional box $\cB$ in $\Rset^n$ is a sequence of $n$ intervals in $\Rset$.

The polyhedron $\cP$ represents a box $\cB$ in $\Rset^n$ if $\cP$ is described by a constraint system in $\Rset^n$ that consists of one constraint for each bounded bound (lower and upper) in an interval in $\cB$: Letting $\vect{e}_i = (0, \ldots, 1, \ldots, 0)^\transpose$ be the vector in $\Rset^n$ with 1 in the $i$'th position and zeroes in every other position; if the lower bound of the $i$'th interval in $\cB$ is bounded, the corresponding constraint is defined as $\langle \vect{e}_i, \vect{x} \rangle \relsym b$, where $b$ is the value of the bound and $\mathord{\relsym}$ is $\mathord{\geq}$ if it is a closed bound and $\mathord{>}$ if it is an open bound. Similarly, if the upper bound of the $i$'th interval in $\cB$ is bounded, the corresponding constraint is defined as $\langle\vect{e}_i,\vect{x}\rangle \relsym b$, where $b$ is the value of the bound and $\mathord{\relsym}$ is $\mathord{\leq}$ if it is a closed bound and $\mathord{<}$ if it is an open bound.

If every bound in the intervals defining a box $\cB$ is either closed and bounded or open and unbounded, then $\cB$ represents a closed polyhedron.

The bounding box of an NNC polyhedron $\cP$ is the smallest $n$-dimensional box containing $\cP$.

The library provides operations for computing the bounding box of an NNC polyhedron and conversely, for obtaining the NNC polyhedron representing a given bounding box.

Widening Operators

The library provides two widening operators for the domain of polyhedra. The first one, that we call H79-widening, mainly follows the specification provided in the PhD thesis of N. Halbwachs [Hal79], also described in [HPR97]. Note that in the computation of the H79-widening $\cP \widen \cQ$ of two polyhedra $\cP, \cQ \in \CPset_n$ it is required as a precondition that $\cP \sseq \cQ$ (the same assumption was implicitly present in the cited papers).

The second widening operator, that we call BHRZ03-widening, is an instance of the specification provided in [BHRZ03a]. This operator also requires as a precondition that $\cP \sseq \cQ$ and it is guaranteed to provide a result which is at least as precise as the H79-widening.

Both widening operators can be applied to NNC polyhedra. The user is warned that, in such a case, the results may not closely match the geometric intuition which is at the base of the specification of the two widenings. The reason is that, in the current implementation, the widenings are not directly applied to the NNC polyhedra, but rather to their internal representations. Implementation work is in progress and future versions of the library may provide an even better integration of the two widenings with the domain of NNC polyhedra.

Note:
As is the case for the other operators on polyhedra, the implementation overwrites one of the two polyhedra arguments with the result of the widening application. To avoid trivial misunderstandings, it is worth stressing that if polyhedra $\cP$ and $\cQ$ (where $\cP \sseq \cQ$) are identified by program variables p and q, respectively, then the call q.H79_widening_assign(p) will assign the polyhedron $\cP \widen \cQ$ to variable q. Namely, it is the bigger polyhedron $\cQ$ which is overwritten by the result of the widening. The smaller polyhedron is not modified, so as to lead to an easier coding of the usual convergence test ($\cP \Sseq \cP \widen \cQ$ can be coded as p.contains(q)). Note that, in the above context, a call such as p.H79_widening_assign(q) is likely to result in undefined behavior, since the precondition $\cQ \sseq \cP$ will be missed (unless it happens that $\cP = \cQ$). The same observation holds for all flavors of widenings and extrapolation operators that are implemented in the library and for all the foreign language interfaces.

Widening with Tokens

When approximating a fixpoint computation using widening operators, a common tactic to improve the precision of the final result is to delay the application of widening operators. The usual approach is to fix a parameter $k$ and only apply widenings starting from the $k$-th iteration.

The library also supports an improved widening delay strategy, that we call widening with tokens [BHRZ03a]. A token is a sort of wildcard allowing for the replacement of the widening application by the exact upper bound computation: the token is used (and thus consumed) only when the widening would have resulted in an actual precision loss (as opposed to the potential precision loss of the classical delay strategy). Thus, all widening operators can be supplied with an optional argument, recording the number of available tokens, which is decremented when tokens are used. The approximated fixpoint computation will start with a fixed number $k$ of tokens, which will be used if and when needed. When there are no tokens left, the widening is always applied.

Extrapolation Operators

Besides the two widening operators, the library also implements several extrapolation operators, which differ from widenings in that their use along an upper iteration sequence does not ensure convergence in a finite number of steps.

In particular, for each of the two widenings there is a corresponding limited extrapolation operator, which can be used to implement the widening ``up to'' technique as described in [HPR97]. Each limited extrapolation operator takes a constraint system as an additional parameter and uses it to improve the approximation yielded by the corresponding widening operator. Note that a convergence guarantee can only be obtained by suitably restricting the set of constraints that can occur in this additional parameter. For instance, in [HPR97] this set is fixed once and for all before starting the computation of the upward iteration sequence.

The bounded extrapolation operators further enhance each one of the limited extrapolation operators described above, by ensuring that their results cannot be worse than the smallest bounding box enclosing the two argument polyhedra.

Bounded Difference Shapes

For each vector $\vect{a} \in \Rset^n$ and scalar $b \in \Rset$, and for each relation symbol $\mathord{\relsym} \in \{ =, \geq\}$, the linear constraint $\langle \vect{a}, \vect{x} \rangle \relsym b$ is said to be a bounded difference if there exist two indices $i, j \in \{ 0, \ldots, n-1 \}$ such that:

A convex polyhedron $\cP \in \CPset_n$ is said to be a bounded difference shape (BDS, for short) if and only if either $\cP$ can be expressed as the intersection of a finite number of bounded difference constraints or $n = 0$ and $\cP = \emptyset$.

By construction, a BDS is always topologically closed. Under the usual set inclusion ordering, the set of all BDSs on the vector space $\Rset^n$ is a lattice having the empty set $\emptyset$ and the universe $\Rset^n$ as the smallest and the biggest elements, respectively. In theoretical terms, it is a meet sub-lattice of $\CPset_n$, meaning that the intersection of a finite set of BDSs is still a BDS; on the other hand, in general the poly-hull of two BDSs is not a BDS. The smallest BDS containing a finite set of BDSs is said to be their bds-hull.

The PPL provides support for computations on the domain of rational bounded difference shapes that, in selected contexts, can achieve a better precision/efficiency ratio with respect to the corresponding computations on a domain of convex polyhedra. As far as the representation of the rational inhomogeneous term of each bounded difference is concerned, several rounding-aware implementation choices are available, including:

The user interface for BDSs is meant to be as similar as possible to the one developed for the domain of closed polyhedra: in particular, all operators on polyhedra are also available for the domain of BDSs, even though they are typically characterized by a lower degree of precision.

Widening and Extrapolation Operators on BD Shapes

For the domain of BDSs, the library provides a variant of the widening operator for convex polyhedra defined in [CH78]. The implementation follows the specification in [BHMZ05], resulting in an operator which is well-defined on the domain of BDSs (i.e., it does not depend on the internal representation of BDSs), while still ensuring convergence in a finite number of steps.

The library also implements an extension of the widening operator for intervals as defined in [CC76]. The reader is warned that such an extension, even though being well-defined on the domain of BDSs, is not provided with a convergence guarantee and is therefore an extrapolation operator.

The Powerset Construction

The PPL provides the finite powerset construction; this takes a pre-existing domain and upgrades it to one that can represent disjunctive information (by using a finite number of disjuncts). The construction follows the approach described in [Bag98], also summarized in [BHZ04] where there is an account of generic widenings for the powerset domain (some of which are supported in the instantiation of this construction by the domain of convex polyhedra and described in Section The Polyhedra Powerset Domain).

The Powerset Domain

The domain is built from a pre-existing base-level domain $D$ which must include an entailment relation `$\mathord{\entails}$', meet operation `$\mathord{\meet}$', a top element `$\true$' and bottom element `$\false$'.

A set $\cS \in \wp(D)$ is called non-redundant with respect to `$\mathord{\entails}$' if and only if $\false \notin \cS$ and $\forall d_1, d_2 \in \cS \itc d_1 \entails d_2 \implies d_1 = d_2$. The set of finite non-redundant subsets of $D$ (with respect to `$\mathord{\entails}$') is denoted by $\wpfn{D}{\entails}$. The function $\fund{\nonredmap}{\wpf(D)}{\wpfn{D}{\entails}}$, called Omega-reduction, maps a finite set into its non-redundant counterpart; it is defined, for each $\cS \in \wpf(D)$, by

\[ \nonredmap(\cS) \defeq \cS \setdiff \{\, d \in \cS \mid d = \false \text{ or } \exists d' \in \cS \st d \sentails d' \,\}. \]

where $d \sentails d'$ denotes $d \entails d' \land d \ne d'$.

As the intended semantics of a powerset domain element $\cS \in \wpf(D)$ is that of disjunction of the semantics of $D$, the finite set $\cS$ is semantically equivalent to the non-redundant set $\nonredmap(\cS)$; and elements of $\cS$ will be called disjuncts. The restriction to the finite subsets reflects the fact that here disjunctions are implemented by explicit collections of disjuncts. As a consequence of this restriction, for any $\cS \in \wpf(D)$ such that $\cS \neq \{ \false \}$, $\nonredmap(\cS)$ is the (finite) set of the maximal elements of $\cS$.

The finite powerset domain over a domain $D$ is the set of all finite non-redundant sets of $D$ and denoted by $D_{\smallP}$. The domain includes an approximation ordering `$\mathord{\entailsP}$' defined so that, for any $\cS_1$ and $\cS_2 \in D_{\smallP}$, $\cS_1 \entailsP \cS_2$ if and only if

\[ \forall d_1 \in \cS_1 \itc \exists d_2 \in \cS_2 \st d_1 \entails d_2. \]

Therefore the top element is $\{\true\}$ and the bottom element is the emptyset.

Note:
As far as Omega-reduction is concerned, the library adopts a lazy approach: an element of the powerset domain is represented by a potentially redundant sequence of disjuncts. Redundancies can be eliminated by explicitly invoking the operator omega_reduce(), e.g., before performing the output of a powerset element. Note that all the documented operators automatically perform Omega-reductions on their arguments, when needed or appropriate.

Operations on the Powerset Construction

In this section we briefly describe the generic operations on Powerset Domains that are provided by the library for any given base-level domain $D$.

Meet and Upper Bound

Given the sets $\cS_1$ and $\cS_2 \in D_{\smallP}$, the meet and upper bound operators provided by the library returns the set $ \nonredmap \bigl( \{\, d_1 \meet d_2 \mid d_1 \in \cS_1, d_2 \in \cS_2 \,\} \bigr) $ and Omega-reduced set union $\nonredmap(\cS_1 \union \cS_2)$ respectively.

Adding a Disjunct

Given the powerset element $\cS \in D_{\smallP}$ and the base-level element $d \in D$, the add disjunct operator provided by the library returns the powerset element $\nonredmap\bigl(\cS \union \{d\}\bigr)$.

Collapsing a Powerset Element

If the given powerset element is not empty, then the collapse operator returns the singleton powerset consisting of an upper-bound of all the disjuncts.

The Polyhedra Powerset Domain

The Polyhedra powerset domain $(\Pset_n)_{\smallP}$ provided by the PPL is the finite powerset domain (defined in Section The Powerset Construction) over the domain of NNC polyhedra $\Pset_n$.

In addition to the operations described for the generic powerset domain in Section Operations on the Powerset Construction, we provide some operations that are specific to this instantiation. Of these, most correspond to the application of the equivalent operation on each of the NNC polyhedra that are in the given set. Here we just describe those operations that are particular to the polyhedra powerset domain.

Geometric Comparisons

Given the sets $\cS_1, \cS_2 \in (\Pset_n)_{\smallP}$, then we say that $\cS_1$ geometrically covers $\cS_2$ if every point (in some element) in a polyhedron in $\cS_2$ is also a point in a polyhedron in $\cS_1$. If $\cS_1$ geometrically covers $\cS_2$ and $\cS_2$ geometrically covers $\cS_1$, then we say that they are geometrically equal.

Pairwise Merge

Given the powerset $\cS \in (\Pset_n)_{\smallP}$, then the pairwise merge operator takes pairs of distinct elements in $\cS$ whose poly-hull is the same as their set-theoretical union and replaces them by their union. This replacement is done recursively so that, for each pair $\cP, \cQ$ of distinct polyhedra in the result set, we have $\cP \uplus \cQ \neq \cP \union \cQ$.

Extrapolation Operators

The library implements a generalization of the extrapolation operator for powerset domains proposed in [BGP99]. The operator BGP99_extrapolation_assign is made parametric by allowing for the specification of a base-level extrapolation operator different from the H79 widening (e.g., the BHRZ03 widening can be used). Note that, in the general case, this operator cannot guarantee the convergence of the iteration sequence in a finite number of steps (for a counter-example, see [BHZ04]).

Certificate-Based Widenings

The PPL library provides support for the specification of proper widening operators on the powerset domain of convex polyhedra. In particular, this version of the library implements an instance of the certificate-based widening framework proposed in [BHZ03b].

A finite convergence certificate for an extrapolation operator is a formal way of ensuring that such an operator is indeed a widening on the considered domain. Given a widening operator on the base-level domain, together with the corresponding convergence certificate, the BHZ03 framework shows how it is possible to lift this widening so as to work on the finite powerset domain, while still ensuring convergence in a finite number of iterations.

Being highly parametric, the BHZ03 widening framework can be instantiated in many ways. The current implementation provides the templatic operator BHZ03_widening_assign<Certificate, Widening> which only exploits a fraction of this generality, by allowing the user to specify the base-level widening function and the corresponding certificate. The widening strategy is fixed and uses two extrapolation heuristics: first, the least upper bound is tried; second, the BGP99 extrapolation operator is tried, possibly applying pairwise merging. If both heuristics fail to converge according to the convergence certificate, then an attempt is made to apply the base-level widening to the poly-hulls of the two arguments, possibly improving the result obtained by means of the poly-difference operator. For more details and a justification of the overall approach, see [BHZ03b] and [BHZ04].

The library provides two convergence certificates: while BHRZ03_Certificate is compatible with both the BHRZ03 and the H79 widenings, H79_Certificate is only compatible with the latter. Note that using different certificates will change the results obtained, even when using the same base-level widening operator. It is also worth stressing that it is up to the user to see that the widening operator is actually compatible with a given convergence certificate. If such a requirement is not met, then an extrapolation operator will be obtained.

Using the Library

A Note on the Implementation of the Operators

When adopting the double description method for the representation of convex polyhedra, the implementation of most of the operators may require an explicit conversion from one of the two representations into the other one, leading to algorithms having a worst-case exponential complexity. However, thanks to the adoption of lazy and incremental computation techniques, the library turns out to be rather efficient in many practical cases.

In earlier versions of the library, a number of operators were introduced in two flavors: a lazy version and an eager version, the latter having the operator name ending with _and_minimize. In principle, only the lazy versions should be used. The eager versions were added to help a knowledgeable user obtain better performance in particular cases. Basically, by invoking the eager version of an operator, the user is trading laziness to better exploit the incrementality of the inner library computations. Starting from version 0.5, the lazy and incremental computation techniques have been refined to achieve a better integration: as a consequence, the lazy versions of the operators are now almost always more efficient than the eager versions.

One of the cases when an eager computation still makes sense is when the well-known fail-first principle comes into play. For instance, if you have to compute the intersection of several polyhedra and you strongly suspect that the result will become empty after a few of these intersections, then you may obtain a better performance by calling the eager version of the intersection operator, since the minimization process also enforces an emptiness check. Note anyway that the same effect can be obtained by interleaving the calls of the lazy operator with explicit emptiness checks.

On Object-Orientation and Polymorphism: A Disclaimer

The PPL library is mainly a collection of so-called ``concrete data types'': while providing the user with a clean and friendly interface, these types are not meant to --- i.e., they should not --- be used polymorphically (since, e.g., most of the destructors are not declared virtual). In practice, this restriction means that the library types should not be used as public base classes to be derived from. A user willing to extend the library types, adding new functionalities, often can do so by using containment instead of inheritance; even when there is the need to override a protected method, non-public inheritance should suffice.

On Const-Correctness: A Warning about the Use of References and Iterators

Most operators of the library depend on one or more parameters that are declared ``const'', meaning that they will not be changed by the application of the considered operator. Due to the adoption of lazy computation techniques, in many cases such a const-correctness guarantee only holds at the semantic level, whereas it does not necessarily hold at the implementation level. For a typical example, consider the extraction from a polyhedron of its constraint system representation. While this operation is not going to change the polyhedron, it might actually invoke the internal conversion algorithm and modify the generators representation of the polyhedron object, e.g., by reordering the generators and removing those that are detected as redundant. Thus, any previously computed reference to the generators of the polyhedron (be it a direct reference object or an indirect one, such as an iterator) will no longer be valid. For this reason, code fragments such as the following should be avoided, as they may result in undefined behavior: 
// Find a reference to the first point of the non-empty polyhedron `ph'.
const Generator_System& gs = ph.generators();
Generator_System::const_iterator i = gs.begin();
for (Generator_System::const_iterator gs_end = gs.end(); i != gs_end; ++i)
  if (i->is_point())
    break;
const Generator& p = *i;
// Get the constraints of `ph'.
const Constraint_System& cs = ph.constraints();
// Both the const iterator `i' and the reference `p'
// are no longer valid at this point.
cout << p.divisor() << endl;  // Undefined behavior!
++i;                          // Undefined behavior!
As a rule of thumb, if a polyhedron plays any role in a computation (even as a const parameter), then any previously computed reference to parts of the polyhedron may have been invalidated. Note that, in the example above, the computation of the constraint system could have been placed after the uses of the iterator i and the reference p. Anyway, if really needed, it is always possible to take a copy of, instead of a reference to, the parts of interest of the polyhedron; in the case above, one may have taken a copy of the generator system by replacing the second line of code with the following: 
Generator_System gs = ph.generators();
The same observations, modulo syntactic sugar, apply to the operators defined in the C interface of the library.

Bibliography

[Bag98]
R. Bagnara. A hierarchy of constraint systems for data-flow analysis of constraint logic-based languages. Science of Computer Programming, 30(1-2):119-155, 1998.

[BGP99]
T. Bultan, R. Gerber, and W. Pugh. Model-checking concurrent systems with unbounded integer variables: Symbolic representations, approximations, and experimental results. ACM Transactions on Programming Languages and Systems, 21(4):747-789, 1999.

[BHMZ05]
R. Bagnara, P. M. Hill, E. Mazzi, and E. Zaffanella. Widening Operators for Weakly-Relational Numeric Abstractions. In C. Hankin and I. Silveroni, editors, Static Analysis: Proceedings of the 12th International Symposium, volume 3672 of Lecture Notes in Computer Science, pages 3-18, London, UK, 2005. Springer-Verlag, Berlin.

[BHRZ03a]
R. Bagnara, P. M. Hill, E. Ricci, and E. Zaffanella. Precise widening operators for convex polyhedra. In R. Cousot, editor, Static Analysis: Proceedings of the 10th International Symposium, volume 2694 of Lecture Notes in Computer Science, pages 337-354, San Diego, California, USA, 2003. Springer-Verlag, Berlin.

[BHRZ03b]
R. Bagnara, P. M. Hill, E. Ricci, and E. Zaffanella. Precise widening operators for convex polyhedra. Quaderno 312, Dipartimento di Matematica, Università di Parma, Italy, 2003. Available at http://www.cs.unipr.it/Publications/.

[BHZ02a]
R. Bagnara, P. M. Hill, and E. Zaffanella. A new encoding and implementation of not necessarily closed convex polyhedra. Quaderno 305, Dipartimento di Matematica, Università di Parma, Italy, 2002. Available at http://www.cs.unipr.it/Publications/.

[BHZ02b]
R. Bagnara, P. M. Hill, and E. Zaffanella. A new encoding of not necessarily closed convex polyhedra. In M. Carro, C. Vacheret, and K.-K. Lau, editors, Proceedings of the 1st CoLogNet Workshop on Component-based Software Development and Implementation Technology for Computational Logic Systems, pages 147-153, Madrid, Spain, 2002. Published as TR Number CLIP4/02.0, Universidad Politécnica de Madrid, Facultad de Informática.

[BHZ03a]
R. Bagnara, P. M. Hill, and E. Zaffanella. A new encoding and implementation of not necessarily closed convex polyhedra. In M. Leuschel, S. Gruner, and S. Lo Presti, editors, Proceedings of the 3rd Workshop on Automated Verification of Critical Systems, pages 161-176, Southampton, UK, 2003. Published as TR Number DSSE-TR-2003-2, University of Southampton.

[BHZ03b]
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