package prolog.ast.logic

import prolog.Answers
import prolog.ast.Database.Program
import prolog.Substitutions
import prolog.ast.terms.*
import prolog.builtins.True
import prolog.flags.AppliedCut
import prolog.logic.applySubstitution
import prolog.logic.numbervars
import prolog.logic.occurs
import prolog.logic.unifyLazy

/**
 * ‘Sentence’ of a Prolog program.
 *
 * A clause consists of a [Head] and body separated by the neck operator, or it is a [Fact].
 *
 * @see [Variable]
 * @see [Predicate]
 */
abstract class Clause(var head: Head, var body: Body) : Term, Resolvent {
    val functor: Functor = head.functor

    override fun solve(goal: Goal, subs: Substitutions): Answers = sequence {
        // If the clause is a rule, unify the goal with the head and then try to prove the body.
        // Only if the body can be proven, the substitutions should be returned.
        // Do this in a lazy way.

        // Since we are only interested in substitutions in the goal (as opposed to the head of this clause),
        //  we can use variable renaming and filter out the substitutions that are not in the goal.
        val (end, renamed: Substitutions) = numbervars(head, Program.variableRenamingStart, subs)

        val reverse = renamed.entries.associate { (a, b) -> b to a }
        Program.variableRenamingStart = end

        var newSubs: Substitutions = subs + renamed
        unifyLazy(applySubstitution(goal, subs), head, newSubs).forEach { headAnswer ->
            headAnswer.map { headSubs ->
                // If the body can be proven, yield the (combined) substitutions
                newSubs = subs + renamed + headSubs
                body.satisfy(newSubs).forEach { bodyAnswer ->
                    bodyAnswer.fold(
                        onSuccess = { bodySubs ->
                            var result = (headSubs + bodySubs)
                                .mapKeys { applySubstitution(it.key, reverse)}
                                .mapValues { applySubstitution(it.value, reverse) }
                            result = result.map { it.key to applySubstitution(it.value, result) }
                                .toMap()
                                .filterNot { it.key in renamed.keys && !occurs(it.key as Variable, goal, emptyMap())}
                            yield(Result.success(result))
                        },
                        onFailure = { error ->
                            if (error is AppliedCut) {
                                // Find single solution and return immediately
                                if (error.subs != null) {
                                    yield(Result.failure(AppliedCut(headSubs + error.subs)))
                                } else {
                                    yield(Result.failure(AppliedCut()))
                                }
                                return@sequence
                            } else {
                                yield(Result.failure(error))
                            }
                        }
                    )
                }
            }
        }
    }

    override fun toString(): String = if (body is True) head.toString() else "$head :- $body"

    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (other !is Clause) return false

        if (head != other.head) return false
        if (body != other.body) return false

        return true
    }

    override fun hashCode(): Int {
        return super.hashCode()
    }
}