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//! Recursive Clause Minimization
//!
//! This module implements recursive minimization of learned clauses.
//! The basic idea is to recursively check if a literal in a learned clause
//! can be removed by examining its reason clause.
//!
//! Reference: "Minimizing Learned Clauses" by Niklas Sörensson and Armin Biere
use crate::clause::ClauseDatabase;
use crate::literal::Lit;
#[allow(unused_imports)]
use crate::prelude::*;
use crate::trail::Reason;
use smallvec::SmallVec;
/// Literal redundancy status for recursive minimization
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum LitStatus {
/// Literal is known to be redundant
Redundant,
/// Literal is known to be non-redundant (essential)
Essential,
/// Status is unknown (not yet checked)
Unknown,
}
/// Statistics for recursive minimization
#[derive(Debug, Default, Clone)]
pub struct RecursiveMinStats {
/// Number of clauses minimized
pub clauses_minimized: u64,
/// Number of literals removed
pub literals_removed: u64,
/// Number of recursive checks performed
pub recursive_checks: u64,
}
/// Recursive clause minimization manager
pub struct RecursiveMinimizer {
/// Status of each literal during minimization
status: Vec<LitStatus>,
/// Stack for depth-first traversal
stack: Vec<Lit>,
/// Statistics
stats: RecursiveMinStats,
}
impl RecursiveMinimizer {
/// Create a new recursive minimizer
pub fn new(num_vars: usize) -> Self {
Self {
status: vec![LitStatus::Unknown; num_vars * 2],
stack: Vec::new(),
stats: RecursiveMinStats::default(),
}
}
/// Resize the minimizer for a new number of variables
pub fn resize(&mut self, num_vars: usize) {
self.status.resize(num_vars * 2, LitStatus::Unknown);
}
/// Get statistics
#[must_use]
pub fn stats(&self) -> &RecursiveMinStats {
&self.stats
}
/// Minimize a learned clause using recursive minimization
///
/// Returns the minimized clause
pub fn minimize(
&mut self,
clause: &[Lit],
clauses: &ClauseDatabase,
reasons: &[Reason],
level: &[u32],
current_level: u32,
) -> SmallVec<[Lit; 8]> {
// Reset status for all literals
for status in &mut self.status {
*status = LitStatus::Unknown;
}
// Mark all literals in the clause that are at the current decision level
let mut abstract_level = 0u32;
for &lit in clause {
let var = lit.var();
if level[var.index()] == current_level {
self.status[lit.index()] = LitStatus::Essential;
abstract_level |= 1 << (level[var.index()] & 31);
}
}
// Try to remove each literal
let mut result = SmallVec::new();
let mut removed = 0;
for &lit in clause {
let var = lit.var();
// Skip if at decision level 0 (always keep)
if level[var.index()] == 0 {
result.push(lit);
continue;
}
// Skip if already marked as essential
if self.status[lit.index()] == LitStatus::Essential {
result.push(lit);
continue;
}
// Check if this literal is redundant
if self.is_redundant(lit, clauses, reasons, level, abstract_level, current_level) {
self.status[lit.index()] = LitStatus::Redundant;
removed += 1;
self.stats.literals_removed += 1;
} else {
self.status[lit.index()] = LitStatus::Essential;
result.push(lit);
}
}
if removed > 0 {
self.stats.clauses_minimized += 1;
}
result
}
/// Check if a literal is redundant using recursive analysis
fn is_redundant(
&mut self,
lit: Lit,
clauses: &ClauseDatabase,
reasons: &[Reason],
level: &[u32],
abstract_level: u32,
_current_level: u32,
) -> bool {
self.stats.recursive_checks += 1;
self.stack.clear();
self.stack.push(lit);
while let Some(current_lit) = self.stack.pop() {
let var = current_lit.var();
// Check cached status
match self.status[current_lit.index()] {
LitStatus::Redundant => continue,
LitStatus::Essential => return false,
LitStatus::Unknown => {}
}
// Get the reason for this literal
let reason = &reasons[var.index()];
match reason {
Reason::Decision => {
// This is a decision literal, not redundant
return false;
}
Reason::Theory => {
// Theory propagation - treat as non-redundant for now
return false;
}
Reason::Propagation(clause_id) => {
// Check all literals in the reason clause
let Some(clause) = clauses.get(*clause_id) else {
return false;
};
for &reason_lit in &clause.lits {
// Skip the implied literal itself
if reason_lit.var() == var {
continue;
}
let reason_var = reason_lit.var();
let reason_level = level[reason_var.index()];
if reason_level == 0 {
// Unit literal, continue
continue;
}
// Check if the level is compatible
if (abstract_level & (1 << (reason_level & 31))) == 0 {
return false;
}
// Add to stack for recursive check
if self.status[reason_lit.index()] == LitStatus::Unknown {
self.stack.push(reason_lit);
}
}
}
}
// Mark this literal as checked
self.status[current_lit.index()] = LitStatus::Redundant;
}
true
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::clause::ClauseDatabase;
use crate::literal::{Lit, Var};
use crate::trail::Reason;
#[test]
fn test_recursive_minimizer_creation() {
let minimizer = RecursiveMinimizer::new(10);
assert_eq!(minimizer.status.len(), 20); // 2 * num_vars
}
#[test]
fn test_recursive_minimizer_resize() {
let mut minimizer = RecursiveMinimizer::new(10);
minimizer.resize(20);
assert_eq!(minimizer.status.len(), 40);
}
#[test]
fn test_recursive_minimizer_stats() {
let minimizer = RecursiveMinimizer::new(10);
let stats = minimizer.stats();
assert_eq!(stats.clauses_minimized, 0);
assert_eq!(stats.literals_removed, 0);
assert_eq!(stats.recursive_checks, 0);
}
#[test]
fn test_minimize_empty_clause() {
let mut minimizer = RecursiveMinimizer::new(10);
let db = ClauseDatabase::new();
let reasons = vec![Reason::Decision; 10];
let level = vec![0; 10];
let clause: Vec<Lit> = vec![];
let result = minimizer.minimize(&clause, &db, &reasons, &level, 0);
assert_eq!(result.len(), 0);
}
#[test]
fn test_minimize_unit_clause() {
let mut minimizer = RecursiveMinimizer::new(10);
let db = ClauseDatabase::new();
let reasons = vec![Reason::Decision; 10];
let level = vec![0; 10];
let clause = vec![Lit::pos(Var::new(0))];
let result = minimizer.minimize(&clause, &db, &reasons, &level, 0);
assert_eq!(result.len(), 1);
}
}