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/*
* richard-atp-engine: A saturation-based automated theorem prover for first-order logic.
* Copyright (C) 2026 Richard Qin
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
#include "atp/infer/substitution.h"
#include "atp/core/term.h"
#include "atp/core/term_bank.h"
#include "atp/core/types.h"
namespace atp {
bool Substitution::bind(TermId var, TermId term) {
auto [it, inserted] = bindings_.emplace(var, term);
if (!inserted) {
// Already bound — succeed only if bound to the same term
return it->second == term;
}
return true;
}
TermId Substitution::lookup(TermId var) const {
auto it = bindings_.find(var);
if (it != bindings_.end()) {
return it->second;
}
return kInvalidId;
}
TermId Substitution::resolve(TermId id, const TermBank& bank) const {
// Walk the chain: X → Y → Z → f(a)
// Stop when we hit a non-variable or an unbound variable.
while (bank.isVariable(id)) {
auto it = bindings_.find(id);
if (it == bindings_.end()) {
break; // Unbound variable — stop here
}
id = it->second;
}
return id;
}
TermId Substitution::apply(TermId term, TermBank& bank) const {
// First, resolve through variable chain
term = resolve(term, bank);
if (bank.isVariable(term)) {
// Free variable (unbound) — leave as-is
return term;
}
// IMPORTANT: Copy symbol_id and args BEFORE recursing.
// Recursive apply() calls may trigger bank.makeTerm() → realloc → dangling ref.
const Term& t = bank.getTerm(term);
SymbolId sym = t.symbol_id;
std::vector<TermId> old_args(t.args); // copy!
if (old_args.empty()) {
// Constant or 0-arity — no substitution needed
return term;
}
// Recursively apply to all arguments
std::vector<TermId> new_args;
new_args.reserve(old_args.size());
bool changed = false;
for (TermId arg : old_args) {
TermId new_arg = apply(arg, bank);
new_args.push_back(new_arg);
if (new_arg != arg) {
changed = true;
}
}
if (!changed) {
return term;
}
return bank.makeTerm(sym, new_args);
}
bool Substitution::occursIn(TermId var, TermId term, const TermBank& bank) const {
term = resolve(term, bank);
if (var == term) {
return true;
}
if (bank.isVariable(term)) {
return false; // Different unbound variable
}
const Term& t = bank.getTerm(term);
for (TermId arg : t.args) {
if (occursIn(var, arg, bank)) {
return true;
}
}
return false;
}
void Substitution::clear() {
bindings_.clear();
}
} // namespace atp
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