1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130

/// Crate `eliminator` implements clause subsumption and var elimination.
use {
    super::{EliminateIF, Eliminator},
    crate::{assign::AssignIF, cdb::ClauseDBIF, types::*},
};

impl Eliminator {
    pub fn try_subsume<A, C>(
        &mut self,
        asg: &mut A,
        cdb: &mut C,
        cid: ClauseId,
        did: ClauseId,
    ) -> MaybeInconsistent
    where
        A: AssignIF,
        C: ClauseDBIF,
    {
        match subsume(cdb, cid, did) {
            Some(NULL_LIT) => {
                #[cfg(feature = "trace_elimination")]
                println!(
                    "BackSubsC    => {} {} subsumed completely by {} {:#}",
                    did, cdb[did], cid, cdb[cid],
                );
                cdb.detach(did);
                self.remove_cid_occur(asg, did, &mut cdb[did]);
                if !cdb[did].is(Flag::LEARNT) {
                    cdb[cid].turn_off(Flag::LEARNT);
                }
                self.num_subsumed += 1;
            }
            // To avoid making a big clause, we have to add a condition for combining them.
            Some(l) if cid.is_lifted_lit() => {
                #[cfg(feature = "trace_elimination")]
                println!("BackSubC subsumes {} from {} and {}", l, cid, did);
                strengthen_clause(asg, cdb, self, did, !l)?;
                self.enqueue_var(asg, l.vi(), true);
            }
            _ => {}
        }
        Ok(())
    }
}

/// returns a literal if these clauses can be merged by the literal.
fn subsume<C>(cdb: &mut C, cid: ClauseId, other: ClauseId) -> Option<Lit>
where
    C: ClauseDBIF,
{
    debug_assert!(!other.is_lifted_lit());
    if cid.is_lifted_lit() {
        let l = Lit::from(cid);
        let oh = &cdb[other];
        for lo in &oh.lits {
            if l == !*lo {
                return Some(l);
            }
        }
        return None;
    }
    let mut ret: Lit = NULL_LIT;
    let ch = &cdb[cid];
    let ob = &cdb[other];
    debug_assert!(ob.lits.contains(&ob[0]));
    debug_assert!(ob.lits.contains(&ob[1]));
    'next: for l in &ch.lits {
        for lo in &ob.lits {
            if *l == *lo {
                continue 'next;
            } else if ret == NULL_LIT && *l == !*lo {
                ret = *l;
                continue 'next;
            }
        }
        return None;
    }
    Some(ret)
}

/// removes `l` from clause `cid`
/// - calls `enqueue_clause`
/// - calls `enqueue_var`
fn strengthen_clause<A, C>(
    asg: &mut A,
    cdb: &mut C,
    elim: &mut Eliminator,
    cid: ClauseId,
    l: Lit,
) -> MaybeInconsistent
where
    A: AssignIF,
    C: ClauseDBIF,
{
    debug_assert!(!cdb[cid].is(Flag::DEAD));
    debug_assert!(1 < cdb[cid].len());
    cdb.touch_var(l.vi());
    debug_assert!(!cid.is_none());
    if cdb.strengthen(cid, l) {
        // Vaporize the binary clause
        debug_assert!(2 == cdb[cid].len());
        let c0 = cdb[cid][0];
        debug_assert_ne!(c0, l);

        #[cfg(feature = "trace_elimination")]
        println!(
            "{} {:?} is removed and its first literal {} is enqueued.",
            cid, cdb[cid], c0,
        );

        cdb.detach(cid);
        elim.remove_cid_occur(asg, cid, &mut cdb[cid]);
        cdb.certificate_add(&[c0]);
        asg.assign_at_root_level(c0)
    } else {
        #[cfg(feature = "trace_elimination")]
        println!("cid {} drops literal {}", cid, l);

        #[cfg(feature = "boundary_check")]
        assert!(1 < cdb[cid].len());

        elim.enqueue_clause(cid, &mut cdb[cid]);
        elim.remove_lit_occur(asg, l, cid);
        unsafe {
            let vec = &cdb[cid].lits[..] as *const [Lit];
            cdb.certificate_add(&*vec);
        }
        Ok(())
    }
}