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Address PR review comments: cache, simplify_ite_rec, itos

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- Cache now indexes by (ele, r) pair using obj_pair_map
- Remove eval() function; operator()(ele, r) handles all cases
- Rewrite simplify_ite_rec with path vector of signed conditions
- Add range-based simplification: (lo <= x, false) + (x <= hi, false)
  eliminates ite(x = v, t, e) when v is outside [lo, hi]
- Add is_itos case in derive_to_re: guards on n >= 0, digit range,
  and first character match
- Port is_reverse normalization (previous commit)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
This commit is contained in:
Nikolaj Bjorner 2026-06-03 17:16:23 -07:00
parent 3a22994b80
commit 2e3dd32b90
3 changed files with 140 additions and 47 deletions
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166
src/ast/rewriter/seq_derive.cpp
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@ -66,14 +66,6 @@ namespace seq {
return (*this)(v, r);
}
expr_ref derive::eval(expr* ele, expr* d) {
expr_ref old_ele(m_ele, m);
m_ele = ele;
expr_ref result = simplify_ite(d);
m_ele = old_ele;
return result;
}
// -------------------------------------------------------
// Core derivative computation
// -------------------------------------------------------
@ -81,9 +73,9 @@ namespace seq {
expr_ref derive::derive_rec(expr* r) {
SASSERT(m_util.is_re(r));
// Check cache
// Check cache (indexed by both m_ele and r)
expr* cached = nullptr;
if (m_cache.find(r, cached))
if (m_cache.find(m_ele, r, cached))
return expr_ref(cached, m);
// Depth check
@ -96,7 +88,8 @@ namespace seq {
expr_ref result = derive_core(r);
// Cache the result
m_cache.insert(r, result);
m_cache.insert(m_ele, r, result);
m_trail.push_back(m_ele);
m_trail.push_back(r);
m_trail.push_back(result);
return result;
@ -261,6 +254,33 @@ namespace seq {
return mk_ite(cond, tail_re, empty);
}
// δ(to_re(itos(n))) - derivative of integer-to-string
// itos(n) produces digits '0'-'9' when n >= 0, empty when n < 0
expr* n = nullptr;
if (u().str.is_itos(s, n)) {
expr_ref empty(re().mk_empty(re_sort), m);
// Guard: n >= 0 and element is a digit and element = s[0]
expr_ref n_ge_0(m_autil.mk_ge(n, m_autil.mk_int(0)), m);
expr_ref char_0(m_util.mk_char('0'), m);
expr_ref char_9(m_util.mk_char('9'), m);
expr_ref ge_0(m_util.mk_le(char_0, m_ele), m);
expr_ref le_9(m_util.mk_le(m_ele, char_9), m);
expr_ref is_digit(m.mk_and(ge_0, le_9), m);
// First character of itos(n) matches ele
expr_ref zero_idx(m_autil.mk_int(0), m);
expr_ref first(u().str.mk_nth_i(s, zero_idx), m);
expr_ref eq_first(m.mk_eq(m_ele, first), m);
// Guard = n >= 0 && is_digit && ele = s[0]
expr_ref guard(m.mk_and(n_ge_0, m.mk_and(is_digit, eq_first)), m);
// Tail: to_re(substr(itos(n), 1, len(itos(n)) - 1))
expr_ref one(m_autil.mk_int(1), m);
expr_ref len(u().str.mk_length(s), m);
expr_ref rest_len(m_autil.mk_sub(len, one), m);
expr_ref rest(u().str.mk_substr(s, one, rest_len), m);
expr_ref rest_re(re().mk_to_re(rest), m);
return mk_ite(guard, rest_re, empty);
}
// Non-ground sequence: δ(to_re(s)) = ite(s ≠ "" ∧ ele = s[0], to_re(s[1:]), ∅)
expr_ref empty_seq(u().str.mk_empty(seq_sort), m);
expr_ref is_non_empty(m.mk_not(m.mk_eq(s, empty_seq)), m);
@ -807,44 +827,120 @@ namespace seq {
if (eval_cond(c, cond_val))
return simplify_ite(cond_val ? t : e);
// Simplify branches with knowledge of the condition's truth value
expr_ref st = simplify_ite_rec(c, false, t);
expr_ref se = simplify_ite_rec(c, true, e);
// Extract signed conditions from c for the true-branch path
path_t path_t_branch;
if (m.is_and(c)) {
for (expr* arg : *to_app(c))
path_t_branch.push_back({ arg, false });
} else {
path_t_branch.push_back({ c, false });
}
// Simplify the true branch under path knowledge
expr_ref st = simplify_ite_rec(path_t_branch, t);
// For the else branch, the whole condition is false
path_t path_e_branch;
path_e_branch.push_back({ c, true });
expr_ref se = simplify_ite_rec(path_e_branch, e);
return mk_ite(c, st, se);
}
expr_ref derive::simplify_ite_rec(expr* cond, bool sign, expr* d) {
expr_ref derive::simplify_ite_rec(path_t& path, expr* d) {
expr* c, * t, * e;
if (!m.is_ite(d, c, t, e))
return expr_ref(d, m);
// If the ITE condition matches cond directly
if (c == cond)
return sign ? simplify_ite(e) : simplify_ite(t);
// Check if c can be determined from the path
for (auto const& [cond, sign] : path) {
// Direct match: c == cond
if (c == cond)
return sign ? simplify_ite_rec(path, e) : simplify_ite_rec(path, t);
// If cond is (x = ch1) and c is (x = ch2) with ch1 != ch2:
// when sign is false (cond is true, i.e., x = ch1), then c must be false
expr* lhs1 = nullptr, * rhs1 = nullptr, * lhs2 = nullptr, * rhs2 = nullptr;
if (!sign && m.is_eq(cond, lhs1, rhs1) && m.is_eq(c, lhs2, rhs2)) {
if (u().is_const_char(lhs1)) std::swap(lhs1, rhs1);
if (u().is_const_char(lhs2)) std::swap(lhs2, rhs2);
unsigned ch1 = 0, ch2 = 0;
if (lhs1 == lhs2 && u().is_const_char(rhs1, ch1) && u().is_const_char(rhs2, ch2) && ch1 != ch2)
return simplify_ite_rec(cond, sign, e);
// c is (x = v), cond is (x = w) with sign=false (cond is true, so x=w)
// If v != w, then c is false → take else branch
expr* lhs1 = nullptr, * rhs1 = nullptr, * lhs2 = nullptr, * rhs2 = nullptr;
if (!sign && m.is_eq(cond, lhs1, rhs1) && m.is_eq(c, lhs2, rhs2)) {
if (m_util.is_const_char(lhs1)) std::swap(lhs1, rhs1);
if (m_util.is_const_char(lhs2)) std::swap(lhs2, rhs2);
if (lhs1 == lhs2 && m.are_distinct(rhs1, rhs2))
return simplify_ite_rec(path, e);
}
// Range constraint: cond is (lo <= x) or (x <= hi) with sign=false
// and c is (x = v). If v is outside the range, c is false.
unsigned v_val = 0, lo_val = 0, hi_val = 0;
if (!sign && m.is_eq(c, lhs2, rhs2)) {
if (m_util.is_const_char(lhs2)) std::swap(lhs2, rhs2);
if (m_util.is_const_char(rhs2, v_val)) {
// Check if cond is (lo <= x) where x == lhs2
expr* le_lhs = nullptr, * le_rhs = nullptr;
if (m_util.is_char_le(cond, le_lhs, le_rhs) && le_rhs == lhs2 &&
m_util.is_const_char(le_lhs, lo_val) && v_val < lo_val)
return simplify_ite_rec(path, e);
// Check if cond is (x <= hi) where x == lhs2
if (m_util.is_char_le(cond, le_lhs, le_rhs) && le_lhs == lhs2 &&
m_util.is_const_char(le_rhs, hi_val) && v_val > hi_val)
return simplify_ite_rec(path, e);
}
}
}
// General case: try to evaluate c given knowledge of cond
// Check if both range bounds are in path and c is (x = v) within range
expr* lhs_c = nullptr, * rhs_c = nullptr;
unsigned v_val = 0;
if (m.is_eq(c, lhs_c, rhs_c)) {
if (m_util.is_const_char(lhs_c)) std::swap(lhs_c, rhs_c);
if (m_util.is_const_char(rhs_c, v_val)) {
unsigned lo_bound = 0, hi_bound = UINT_MAX;
bool has_lo = false, has_hi = false;
for (auto const& [cond, sign] : path) {
if (sign) continue; // only use true conditions
expr* le_lhs = nullptr, * le_rhs = nullptr;
if (m_util.is_char_le(cond, le_lhs, le_rhs)) {
unsigned bound = 0;
if (le_rhs == lhs_c && m_util.is_const_char(le_lhs, bound)) {
lo_bound = bound; has_lo = true;
}
if (le_lhs == lhs_c && m_util.is_const_char(le_rhs, bound)) {
hi_bound = bound; has_hi = true;
}
}
}
if (has_lo && has_hi && lo_bound <= v_val && v_val <= hi_bound) {
// v is in range [lo, hi], so c is satisfiable
// Add (x = v, false) to path and simplify t
path.push_back({ c, false });
expr_ref st = simplify_ite_rec(path, t);
path.pop_back();
expr_ref se = simplify_ite_rec(path, e);
return mk_ite(c, st, se);
}
}
}
// Try to evaluate c directly
bool cond_val;
if (eval_cond(c, cond_val))
return simplify_ite_rec(cond, sign, cond_val ? t : e);
return simplify_ite_rec(path, cond_val ? t : e);
// Cannot simplify c: recurse into branches
expr_ref st = simplify_ite_rec(cond, sign, t);
expr_ref se = simplify_ite_rec(cond, sign, e);
// Cannot simplify c: recurse into branches with extended paths
// True branch: add conjuncts of c
path_t extended_path(path);
if (m.is_and(c)) {
for (expr* arg : *to_app(c))
extended_path.push_back({ arg, false });
} else {
extended_path.push_back({ c, false });
}
expr_ref st = simplify_ite_rec(extended_path, t);
// Else branch: add (c, true)
path.push_back({ c, true });
expr_ref se = simplify_ite_rec(path, e);
path.pop_back();
// Now also simplify c's branches with knowledge of c
st = simplify_ite_rec(c, false, st);
se = simplify_ite_rec(c, true, se);
return mk_ite(c, st, se);
}