/**
 * Comparison and Unification of Terms
 *
 * [SWI Prolog Documentation](https://www.swi-prolog.org/pldoc/man?section=compare)
 */
package prolog.builtins

import prolog.Answers
import prolog.Substitutions
import prolog.ast.terms.Atom
import prolog.ast.terms.Operator
import prolog.ast.terms.Term
import prolog.logic.applySubstitution
import prolog.logic.equivalent
import prolog.logic.unifyLazy

/**
 * Unify Term1 with Term2. True if the unification succeeds.
 */
class Unify(private val term1: Term, private val term2: Term): Operator(Atom("="), term1, term2) {
    override fun satisfy(subs: Substitutions): Answers = sequence {
        val t1 = applySubstitution(term1, subs)
        val t2 = applySubstitution(term2, subs)

        yieldAll(unifyLazy(t1, t2, subs))
    }

    override fun applySubstitution(subs: Substitutions): Unify = Unify(
        applySubstitution(term1, subs),
        applySubstitution(term2, subs)
    )
}

class NotUnify(private val term1: Term, private val term2: Term) : Operator(Atom("\\="), term1, term2) {
    private val not = Not(Unify(term1, term2))
    override fun satisfy(subs: Substitutions): Answers = not.satisfy(subs)
    override fun applySubstitution(subs: Substitutions): NotUnify = NotUnify(
        applySubstitution(term1, subs),
        applySubstitution(term2, subs)
    )
}

class Equivalent(private val term1: Term, private val term2: Term) : Operator(Atom("=="), term1, term2) {
    override fun satisfy(subs: Substitutions): Answers = sequence {
        val t1 = applySubstitution(term1, subs)
        val t2 = applySubstitution(term2, subs)

        if (equivalent(t1, t2, subs)) {
            yield(Result.success(emptyMap()))
        }
    }

    override fun applySubstitution(subs: Substitutions): Equivalent = Equivalent(
        applySubstitution(term1, subs),
        applySubstitution(term2, subs)
    )
}

class NotEquivalent(private val term1: Term, private val term2: Term) : Operator(Atom("\\=="), term1, term2) {
    private val not = Not(Equivalent(term1, term2))
    override fun satisfy(subs: Substitutions): Answers = not.satisfy(subs)
    override fun applySubstitution(subs: Substitutions): NotEquivalent = NotEquivalent(
        applySubstitution(term1, subs),
        applySubstitution(term2, subs)
    )
}