Add theory_seq_len: partial axiom instantiation for sequence length constraints

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/params/smt_params.cpp

@@ -188,9 +188,9 @@ void smt_params::display(std::ostream & out) const {
 }
 
 void smt_params::validate_string_solver(symbol const& s) const {
-    if (s == "z3str3" || s == "seq" || s == "empty" || s == "auto" || s == "none" || s == "nseq")
+    if (s == "z3str3" || s == "seq" || s == "empty" || s == "auto" || s == "none" || s == "nseq" || s == "seq_len")
         return;
-    throw default_exception("Invalid string solver value. Legal values are z3str3, seq, empty, auto, none, nseq");
+    throw default_exception("Invalid string solver value. Legal values are z3str3, seq, empty, auto, none, nseq, seq_len");
 }
 
 void smt_params::setup_QF_UF() {

src/smt/CMakeLists.txt

@@ -72,6 +72,7 @@ z3_add_component(smt
     theory_pb.cpp
     theory_recfun.cpp
     theory_seq.cpp
+    theory_seq_len.cpp
     theory_sls.cpp
     theory_special_relations.cpp    
     theory_user_propagator.cpp

src/smt/smt_setup.cpp

@@ -36,6 +36,7 @@ Revision History:
 #include "smt/theory_seq.h"
 #include "smt/theory_char.h"
 #include "smt/theory_nseq.h"
+#include "smt/theory_seq_len.h"
 #include "smt/theory_special_relations.h"
 #include "smt/theory_sls.h"
 #include "smt/theory_pb.h"
@@ -760,11 +761,14 @@ namespace smt {
         else if (m_params.m_string_solver == "nseq") {
             setup_nseq();
         }
+        else if (m_params.m_string_solver == "seq_len") {
+            setup_seq_len();
+        }
         else if (m_params.m_string_solver == "auto") {            
                 setup_seq();
         } 
         else {
-            throw default_exception("invalid parameter for smt.string_solver, valid options are 'seq', 'auto', 'nseq'");
+            throw default_exception("invalid parameter for smt.string_solver, valid options are 'seq', 'auto', 'nseq', 'seq_len'");
         }
     }
 
@@ -796,6 +800,11 @@ namespace smt {
         setup_char();
     }
 
+    void setup::setup_seq_len() {
+        m_context.register_plugin(alloc(smt::theory_seq_len, m_context));
+        setup_char();
+    }
+
     void setup::setup_finite_set() {
         m_context.register_plugin(alloc(smt::theory_finite_set, m_context));
     }

src/smt/smt_setup.h

@@ -103,6 +103,7 @@ namespace smt {
         void setup_seq();
         void setup_char();
         void setup_nseq();
+        void setup_seq_len();
         void setup_finite_set();
         void setup_card();
         void setup_sls();

src/smt/theory_seq_len.cpp

@@ -0,0 +1,400 @@
+/*++
+Copyright (c) 2026 Microsoft Corporation
+
+Module Name:
+
+    theory_seq_len.cpp
+
+Abstract:
+
+    Theory solver for sequence length constraints using partial axiom
+    instantiation. See theory_seq_len.h for details.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2026-03-10
+
+--*/
+#include "smt/theory_seq_len.h"
+#include "smt/smt_context.h"
+#include "ast/ast_pp.h"
+
+namespace smt {
+
+    theory_seq_len::theory_seq_len(context& ctx)
+        : theory(ctx, ctx.get_manager().mk_family_id("seq")),
+          m_util(ctx.get_manager()),
+          m_autil(ctx.get_manager()) {}
+
+    // -----------------------------------------------------------------------
+    // Internalization
+    // -----------------------------------------------------------------------
+
+    bool theory_seq_len::internalize_atom(app* atom, bool /*gate_ctx*/) {
+        expr* s = nullptr, *r = nullptr;
+
+        // Handle (str.in_re s r): register a bool_var so assign_eh fires.
+        if (m_util.str.is_in_re(atom, s, r)) {
+            // Ensure the string argument has an enode.
+            if (!ctx.e_internalized(s))
+                ctx.internalize(s, false);
+            if (!ctx.b_internalized(atom)) {
+                bool_var bv = ctx.mk_bool_var(atom);
+                ctx.set_var_theory(bv, get_id());
+                ctx.mark_as_relevant(bv);
+            }
+            // Ensure the string has a theory variable so length is accessible.
+            mk_var(ctx.get_enode(s));
+            return true;
+        }
+        return internalize_term(atom);
+    }
+
+    bool theory_seq_len::internalize_term(app* term) {
+        // Internalize all children first.
+        for (expr* arg : *term) {
+            if (!ctx.e_internalized(arg))
+                ctx.internalize(arg, false);
+        }
+
+        if (!ctx.e_internalized(term))
+            ctx.mk_enode(term, false, false, true);
+
+        enode* n = ctx.get_enode(term);
+        if (!is_attached_to_var(n))
+            mk_var(n);
+
+        // If this is a seq.len term, assert non-negativity immediately.
+        expr* inner = nullptr;
+        if (m_util.str.is_length(term, inner))
+            add_length_non_neg(inner);
+
+        return true;
+    }
+
+    // -----------------------------------------------------------------------
+    // assign_eh: fires when a bool_var (e.g. str.in_re) is assigned.
+    // -----------------------------------------------------------------------
+
+    void theory_seq_len::assign_eh(bool_var v, bool is_true) {
+        if (!is_true)
+            return; // only axiomatize positive memberships
+        expr* e = ctx.bool_var2expr(v);
+        expr* s = nullptr, *r = nullptr;
+        if (!m_util.str.is_in_re(e, s, r))
+            return;
+        literal lit(v, false); // positive literal for the membership
+        add_regex_length_axioms(s, r, lit);
+    }
+
+    // -----------------------------------------------------------------------
+    // Model building
+    // -----------------------------------------------------------------------
+
+    void theory_seq_len::init_model(model_generator& mg) {
+        mg.register_factory(alloc(seq_factory, m, get_family_id(), mg.get_model()));
+    }
+
+    // -----------------------------------------------------------------------
+    // mk_fresh: create a copy of this theory for a fresh context.
+    // -----------------------------------------------------------------------
+
+    theory* theory_seq_len::mk_fresh(context* new_ctx) {
+        return alloc(theory_seq_len, *new_ctx);
+    }
+
+    // -----------------------------------------------------------------------
+    // display
+    // -----------------------------------------------------------------------
+
+    void theory_seq_len::display(std::ostream& out) const {
+        out << "theory_seq_len\n";
+    }
+
+    // -----------------------------------------------------------------------
+    // add_length_non_neg: assert (seq.len e) >= 0
+    // -----------------------------------------------------------------------
+
+    void theory_seq_len::add_length_non_neg(expr* e) {
+        expr_ref len(m_util.str.mk_length(e), m);
+        expr_ref ge0(m_autil.mk_ge(len, m_autil.mk_int(0)), m);
+        if (!ctx.b_internalized(ge0))
+            ctx.internalize(ge0, true);
+        literal lit = ctx.get_literal(ge0);
+        ctx.mark_as_relevant(lit);
+        literal_vector lits;
+        lits.push_back(lit);
+        assert_axiom(lits);
+    }
+
+    // -----------------------------------------------------------------------
+    // get_length_constraint: extract a semi-linear length constraint from r.
+    //
+    // On success, sets:
+    //   period == 0: lengths in r are exactly {base}
+    //   period > 0:  lengths in r are {base + k*period : k >= 0}
+    //
+    // Returns false if no useful constraint can be extracted.
+    // -----------------------------------------------------------------------
+
+    bool theory_seq_len::has_fixed_length(expr* r, unsigned& len) const {
+        unsigned mn = m_util.re.min_length(r);
+        unsigned mx = m_util.re.max_length(r);
+        if (mn == mx && mx != UINT_MAX) {
+            len = mn;
+            return true;
+        }
+        return false;
+    }
+
+    bool theory_seq_len::get_length_constraint(expr* r, unsigned& base, unsigned& period) const {
+        expr* r1 = nullptr, *r2 = nullptr;
+        unsigned lo = 0, hi = 0;
+
+        // Empty regex: no string can match -> handled by conflict, not length.
+        if (m_util.re.is_empty(r))
+            return false;
+
+        // Full sequence: any length is possible.
+        if (m_util.re.is_full_seq(r))
+            return false;
+
+        // Fixed-length regex (min == max).
+        {
+            unsigned flen = 0;
+            if (has_fixed_length(r, flen)) {
+                base   = flen;
+                period = 0;
+                return true;
+            }
+        }
+
+        // (R1)* where R1 has fixed length n: lengths = {k*n : k >= 0}
+        if (m_util.re.is_star(r, r1)) {
+            unsigned n = 0;
+            if (has_fixed_length(r1, n) && n > 0) {
+                base   = 0;
+                period = n;
+                return true;
+            }
+            return false;
+        }
+
+        // (R1)+ where R1 has fixed length n: lengths = {k*n : k >= 1} = {n + k*n : k >= 0}
+        if (m_util.re.is_plus(r, r1)) {
+            unsigned n = 0;
+            if (has_fixed_length(r1, n) && n > 0) {
+                base   = n;
+                period = n;
+                return true;
+            }
+            return false;
+        }
+
+        // Loop {lo, hi} where R1 has fixed length n:
+        // lengths = {lo*n, (lo+1)*n, ..., hi*n} (a range, not necessarily simple period)
+        // For now, just assert the lower bound as "base" and give min length as constraint.
+        if (m_util.re.is_loop(r, r1, lo, hi)) {
+            unsigned n = 0;
+            if (has_fixed_length(r1, n)) {
+                // Lengths are exactly multiples of n between lo*n and hi*n.
+                // We express this as a range: the constraint len = lo*n + k*n for 0<=k<=hi-lo.
+                // Since our schema only handles one arithmetic progression, just use
+                // base=lo*n, period=n (and an upper bound will be added separately).
+                base   = lo * n;
+                period = (hi > lo) ? n : 0;
+                return true;
+            }
+            return false;
+        }
+
+        // Loop with lower bound only (no upper bound).
+        if (m_util.re.is_loop(r, r1, lo)) {
+            unsigned n = 0;
+            if (has_fixed_length(r1, n) && n > 0) {
+                base   = lo * n;
+                period = n;
+                return true;
+            }
+            return false;
+        }
+
+        // Concatenation R1 ++ R2: if both parts have constraints, combine.
+        if (m_util.re.is_concat(r, r1, r2)) {
+            unsigned b1 = 0, p1 = 0, b2 = 0, p2 = 0;
+            bool ok1 = get_length_constraint(r1, b1, p1);
+            bool ok2 = get_length_constraint(r2, b2, p2);
+            if (ok1 && ok2) {
+                // If both are fixed length:
+                if (p1 == 0 && p2 == 0) {
+                    base   = b1 + b2;
+                    period = 0;
+                    return true;
+                }
+                // If one is fixed and the other periodic:
+                if (p1 == 0) {
+                    base   = b1 + b2;
+                    period = p2;
+                    return true;
+                }
+                if (p2 == 0) {
+                    base   = b1 + b2;
+                    period = p1;
+                    return true;
+                }
+                // Both periodic: only combine if same period.
+                if (p1 == p2) {
+                    base   = b1 + b2;
+                    period = p1;
+                    return true;
+                }
+                // Incompatible periods: fall through.
+            }
+            // Partial: at least assert minimum length.
+            return false;
+        }
+
+        // Option (R1)?: lengths = {0, n} where n is fixed length of R1.
+        if (m_util.re.is_opt(r, r1)) {
+            unsigned n = 0;
+            if (has_fixed_length(r1, n)) {
+                // lengths are 0 or n
+                base   = 0;
+                period = (n > 0) ? n : 0;
+                return true;
+            }
+            return false;
+        }
+
+        return false;
+    }
+
+    // -----------------------------------------------------------------------
+    // add_regex_length_axioms
+    // -----------------------------------------------------------------------
+
+    void theory_seq_len::add_regex_length_axioms(expr* s, expr* r, literal lit) {
+        expr_ref len(m_util.str.mk_length(s), m);
+
+        // Always assert the minimum length constraint:
+        // (s in R) => |s| >= min_length(R)
+        unsigned min_len = m_util.re.min_length(r);
+        if (min_len > 0) {
+            expr_ref ge_min(m_autil.mk_ge(len, m_autil.mk_int(min_len)), m);
+            if (!ctx.b_internalized(ge_min))
+                ctx.internalize(ge_min, true);
+            literal ge_min_lit = ctx.get_literal(ge_min);
+            ctx.mark_as_relevant(ge_min_lit);
+            literal_vector lits;
+            lits.push_back(~lit);
+            lits.push_back(ge_min_lit);
+            assert_axiom(lits);
+        }
+        else {
+            // At minimum assert non-negativity.
+            add_length_non_neg(s);
+        }
+
+        // Assert maximum length constraint if finite:
+        // (s in R) => |s| <= max_length(R)
+        unsigned max_len = m_util.re.max_length(r);
+        if (max_len != UINT_MAX) {
+            expr_ref le_max(m_autil.mk_le(len, m_autil.mk_int(max_len)), m);
+            if (!ctx.b_internalized(le_max))
+                ctx.internalize(le_max, true);
+            literal le_max_lit = ctx.get_literal(le_max);
+            ctx.mark_as_relevant(le_max_lit);
+            literal_vector lits;
+            lits.push_back(~lit);
+            lits.push_back(le_max_lit);
+            assert_axiom(lits);
+        }
+
+        // Extract semi-linear length constraint.
+        unsigned base = 0, period = 0;
+        if (!get_length_constraint(r, base, period))
+            return;
+
+        if (period == 0) {
+            // Fixed length: (s in R) => |s| = base
+            expr_ref eq_base(m.mk_eq(len, m_autil.mk_int(base)), m);
+            if (!ctx.b_internalized(eq_base))
+                ctx.internalize(eq_base, true);
+            literal eq_lit = ctx.get_literal(eq_base);
+            ctx.mark_as_relevant(eq_lit);
+            literal_vector lits;
+            lits.push_back(~lit);
+            lits.push_back(eq_lit);
+            assert_axiom(lits);
+        }
+        else {
+            // Semi-linear: lengths are {base + k * period : k >= 0}
+            // Introduce a fresh variable (non-negative integer) and assert:
+            //   (s in R) => |s| = base + period * k
+            //   (s in R) => k >= 0
+            // mk_fresh_const appends a unique numeric suffix, so each call
+            // produces a distinct variable even with the same prefix.
+            sort* int_sort = m_autil.mk_int();
+            app* period_multiplier = m.mk_fresh_const("seq_len_k", int_sort, false);
+
+            // k >= 0
+            expr_ref x_ge0(m_autil.mk_ge(period_multiplier, m_autil.mk_int(0)), m);
+            if (!ctx.b_internalized(x_ge0))
+                ctx.internalize(x_ge0, true);
+            literal x_ge0_lit = ctx.get_literal(x_ge0);
+            ctx.mark_as_relevant(x_ge0_lit);
+            {
+                literal_vector lits;
+                lits.push_back(~lit);
+                lits.push_back(x_ge0_lit);
+                assert_axiom(lits);
+            }
+
+            // |s| = base + period * k
+            expr_ref period_times_x(m_autil.mk_mul(m_autil.mk_int(period), period_multiplier), m);
+            expr_ref rhs(m_autil.mk_add(m_autil.mk_int(base), period_times_x), m);
+            expr_ref eq_len(m.mk_eq(len, rhs), m);
+            if (!ctx.b_internalized(eq_len))
+                ctx.internalize(eq_len, true);
+            literal eq_lit = ctx.get_literal(eq_len);
+            ctx.mark_as_relevant(eq_lit);
+            {
+                literal_vector lits;
+                lits.push_back(~lit);
+                lits.push_back(eq_lit);
+                assert_axiom(lits);
+            }
+
+            TRACE(seq, tout << "seq_len: (s in R) => |s| = " << base << " + "
+                            << period << " * k for s=" << mk_pp(s, m)
+                            << " r=" << mk_pp(r, m) << "\n";);
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Helper: mk_literal
+    // -----------------------------------------------------------------------
+
+    literal theory_seq_len::mk_literal(expr* e) {
+        if (!ctx.b_internalized(e))
+            ctx.internalize(e, true);
+        literal lit = ctx.get_literal(e);
+        ctx.mark_as_relevant(lit);
+        return lit;
+    }
+
+    // -----------------------------------------------------------------------
+    // Helper: assert_axiom (th_axiom in the SMT context)
+    // -----------------------------------------------------------------------
+
+    void theory_seq_len::assert_axiom(literal_vector& lits) {
+        // Skip if already satisfied.
+        for (literal l : lits)
+            if (ctx.get_assignment(l) == l_true)
+                return;
+        for (literal l : lits)
+            ctx.mark_as_relevant(l);
+        ctx.mk_th_axiom(get_id(), lits.size(), lits.data());
+    }
+
+}

src/smt/theory_seq_len.h

@@ -0,0 +1,91 @@
+/*++
+Copyright (c) 2026 Microsoft Corporation
+
+Module Name:
+
+    theory_seq_len.h
+
+Abstract:
+
+    Theory solver for sequence length constraints.
+
+    Handles the theory of sequences (seq_decl_plugin) using partial axiom
+    instantiation:
+    - Axiomatizes (seq.len S) >= 0 for all sequence variables S.
+    - Derives stronger lower/upper bounds on seq.len based on the structure of S.
+    - Axiomatizes S in RegEx by extracting semi-linear length constraints from
+      the regex. For example, if RegEx is (aa)*, it asserts (seq.len S) = 2*X_s
+      where X_s >= 0 is a fresh integer variable.
+    - Returns FC_DONE in final_check (incomplete but sound partial solver).
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2026-03-10
+
+--*/
+#pragma once
+
+#include "ast/seq_decl_plugin.h"
+#include "ast/arith_decl_plugin.h"
+#include "model/seq_factory.h"
+#include "smt/smt_theory.h"
+#include "smt/smt_model_generator.h"
+
+namespace smt {
+
+    class theory_seq_len : public theory {
+        seq_util        m_util;
+        arith_util      m_autil;
+
+        // -----------------------------------------------------------------------
+        // Virtual overrides required by theory base class
+        // -----------------------------------------------------------------------
+        bool internalize_atom(app* atom, bool gate_ctx) override;
+        bool internalize_term(app* term) override;
+        void new_eq_eh(theory_var, theory_var) override {}
+        void new_diseq_eh(theory_var, theory_var) override {}
+        theory* mk_fresh(context* new_ctx) override;
+        char const* get_name() const override { return "seq-len"; }
+        void display(std::ostream& out) const override;
+
+        // -----------------------------------------------------------------------
+        // Optional overrides
+        // -----------------------------------------------------------------------
+        void assign_eh(bool_var v, bool is_true) override;
+        final_check_status final_check_eh(unsigned) override { return FC_DONE; }
+        void push_scope_eh() override { theory::push_scope_eh(); }
+        void pop_scope_eh(unsigned num_scopes) override { theory::pop_scope_eh(num_scopes); }
+        void init_model(model_generator& mg) override;
+
+        // -----------------------------------------------------------------------
+        // Axiom generation helpers
+        // -----------------------------------------------------------------------
+
+        // Assert (seq.len e) >= 0.
+        void add_length_non_neg(expr* e);
+
+        // Assert length axioms implied by S in R (when the membership is true).
+        // lit is the literal for (S in R).
+        void add_regex_length_axioms(expr* s, expr* r, literal lit);
+
+        // Returns true if the regex r has a fixed length (min==max).
+        bool has_fixed_length(expr* r, unsigned& len) const;
+
+        // Extract semi-linear length constraint for a regex.
+        // Returns true if a useful constraint can be extracted.
+        // If so, sets:
+        //   period > 0: lengths are of the form (base + period*k), k >= 0
+        //   period == 0: length is exactly base
+        bool get_length_constraint(expr* r, unsigned& base, unsigned& period) const;
+
+        // Helper: internalize and get literal for an expr.
+        literal mk_literal(expr* e);
+
+        // Helper: assert a clause (disjunction of literals).
+        void assert_axiom(literal_vector& lits);
+
+    public:
+        theory_seq_len(context& ctx);
+    };
+
+}