pubgrub/src/commonMain/kotlin/PartialSolution.kt

package net.cijber.pubgrub

import net.cijber.pubgrub.packages.PackageSelection
import net.cijber.pubgrub.stubs.PackageId
import net.cijber.pubgrub.stubs.Version

class PartialSolution<P : PackageId, V : Version<V>> {
    private var decisionLevel = 0
    private val assignments = mutableListOf<Assignment<P, V>>()
    private var backtracking = false

    private val decisions = mutableListOf<Assignment<P, V>>()

    private val positive = mutableMapOf<P, Term<P, V>>()
    private val negative = mutableMapOf<P, Term<P, V>>()

    infix fun relation(term: Term<P, V>): SetRelation {
        positive.get(term.pkg)?.let {
            return it relation term
        }

        negative.get(term.pkg)?.let {
            return it relation term
        }

        return SetRelation.Overlaps
    }

    fun derive(pkg: PackageSelection<P, V>, exclusive: Boolean, incompatibility: Incompatibility<P, V>) {
        assign(Assignment(pkg, !exclusive, incompatibility, decisionLevel, assignments.count()))
    }

    private fun assign(assignment: Assignment<P, V>) {
        assignments.add(assignment)
        register(assignment)
    }

    private fun register(assignment: Assignment<P, V>) {
        val pkg = assignment.pkg
        val oldPositive = positive[pkg]
        if (oldPositive != null) {
            positive.remove(pkg)
            oldPositive.intersect(assignment)?.let {
                positive[pkg] = it
            }
        }

        val oldNegative = negative[pkg]
        val term = if (oldNegative == null) {
            assignment
        } else {
            assignment.intersect(oldNegative)!!
        }

        if (!term.exclusive) {
            negative.remove(pkg)
            positive[pkg] = term
        } else {
            negative[pkg] = term
        }
    }

    fun satisfier(term: Term<P, V>): Assignment<P, V> {
        var assignedTerm: Term<P, V>? = null;
        for (assignment in assignments) {
            if (assignment.pkg != term.pkg) continue

            assignedTerm = if (assignedTerm == null) {
                assignment
            } else {
                assignedTerm.intersect(assignment)
            }

            if (assignedTerm?.satisfies(term) == true) {
                return assignment
            }
        }

        error("[BUG] $term is not satisfied")
    }

    fun backtrack(decisionLevel: Int) {
        backtracking = true
        val packages = mutableSetOf<P>()
        while (assignments.lastOrNull()?.decisionLevel?.let { it > decisionLevel } == true) {
            val removed = assignments.removeAt(assignments.lastIndex)
            packages.add(removed.pkg)

            if (removed.isDecision) {
                decisions.remove(removed)
            }
        }

        for (pkg in packages) {
            positive.remove(pkg)
            negative.remove(pkg)
        }

        for (assignment in assignments) {
            if (packages.contains(assignment.pkg)) {
                register(assignment)
            }
        }
    }
}