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updated rewrite rules to propagate nullability over nonground regexes (#4663)

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* updated rewrite rules to propagate nullability over nonground regexes

* updated rewrite rules to propagate nullability over nonground regexes

* fixed incorrect rewrite status flag
This commit is contained in:
Margus Veanes 2020-08-26 00:26:17 -07:00 • committed by GitHub
parent ab10616b77
commit 78b88f761c
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2 changed files with 43 additions and 1 deletions
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42
src/ast/rewriter/seq_rewriter.cpp
View file

@ -2265,7 +2265,7 @@ expr_ref seq_rewriter::is_nullable_rec(expr* r) {
result = m().mk_ite(cond, is_nullable(r1), is_nullable(r2));
}
else if (m_util.is_re(r, seq_sort)) {
result = re().mk_in_re(str().mk_empty(seq_sort), r);
result = is_nullable_symbolic_regex(r, seq_sort);
}
else if (str().is_concat(r, r1, r2)) {
m_br.mk_and(is_nullable(r1), is_nullable(r2), result);
@ -2286,6 +2286,31 @@ expr_ref seq_rewriter::is_nullable_rec(expr* r) {
return result;
}
expr_ref seq_rewriter::is_nullable_symbolic_regex(expr* r, sort* seq_sort) {
SASSERT(m_util.is_re(r));
expr* elem = nullptr, * r2 = nullptr, * s = nullptr;
expr_ref elems(str().mk_empty(seq_sort), m());
expr_ref result(m());
while (re().is_derivative(r, elem, r2)) {
if (str().is_empty(elems))
elems = str().mk_unit(elem);
else
elems = str().mk_concat(str().mk_unit(elem), elems);
r = r2;
}
if (re().is_to_re(r, s)) {
// r is nullable
// iff after taking the derivatives the remaining sequence is empty
// iff the inner sequence equals to the sequence of derivative elements in reverse
result = m().mk_eq(elems, s);
return result;
}
// the default case when either r is not a derivative
// or when the nested derivatives are not applied to a sequence
result = re().mk_in_re(str().mk_empty(seq_sort), r);
return result;
}
/*
Push reverse inwards (whenever possible).
*/
@ -3203,6 +3228,11 @@ Disabled rewrite:
disjunctions that cover cases where s overlaps given that "ab" does
not overlap with any of the sequences.
It is disabled because the solver doesn't handle disjunctions of regexes well.
TBD:
Enable rewrite when R = R1|R2 and derivative cannot make progress: 's in R' ==> 's in R1' | 's in R2'
cannot make progress here means that R1 or R2 starts with an uninterpreted symbol
This will help propagate cases like "abc"X in opt(to_re(X)) to equalities.
*/
br_status seq_rewriter::mk_str_in_regexp(expr* a, expr* b, expr_ref& result) {
@ -3222,6 +3252,16 @@ br_status seq_rewriter::mk_str_in_regexp(expr* a, expr* b, expr_ref& result) {
result = m_br.mk_eq_rw(a, b1);
return BR_REWRITE1;
}
expr* eps = nullptr;
if (re().is_opt(b, b1) ||
(re().is_union(b, b1, eps) && re().is_epsilon(eps)) ||
(re().is_union(b, eps, b1) && re().is_epsilon(eps)))
{
result = m().mk_ite(m().mk_eq(str().mk_length(a), m_autil.mk_int(0)),
m().mk_true(),
re().mk_in_re(a, b1));
return BR_REWRITE_FULL;
}
if (str().is_empty(a)) {
result = is_nullable(b);
if (str().is_in_re(result))

2
src/ast/rewriter/seq_rewriter.h
View file

@ -193,6 +193,8 @@ class seq_rewriter {
expr_ref mk_der_cond(expr* cond, expr* ele, sort* seq_sort);
expr_ref mk_der_antimorov_union(expr* r1, expr* r2);
bool ite_bdds_compatabile(expr* a, expr* b);
/* if r has the form deriv(en..deriv(e1,to_re(s))..) returns 's = [e1..en]' else returns '() in r'*/
expr_ref is_nullable_symbolic_regex(expr* r, sort* seq_sort);
#ifdef Z3DEBUG
bool check_deriv_normal_form(expr* r, int level = 3);
#endif