87 lines
3.5 KiB
Kotlin
87 lines
3.5 KiB
Kotlin
package prolog.ast.logic
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import prolog.Answers
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import prolog.ast.Database.Program
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import prolog.Substitutions
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import prolog.ast.terms.*
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import prolog.builtins.True
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import prolog.flags.AppliedCut
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import prolog.logic.applySubstitution
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import prolog.logic.numbervars
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import prolog.logic.occurs
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import prolog.logic.unifyLazy
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/**
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* ‘Sentence’ of a Prolog program.
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*
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* A clause consists of a [Head] and body separated by the neck operator, or it is a [Fact].
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*
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* @see [Variable]
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* @see [Predicate]
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*/
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abstract class Clause(var head: Head, var body: Body) : Term, Resolvent {
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val functor: Functor = head.functor
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override fun solve(goal: Goal, subs: Substitutions): Answers = sequence {
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// If the clause is a rule, unify the goal with the head and then try to prove the body.
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// Only if the body can be proven, the substitutions should be returned.
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// Do this in a lazy way.
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// Since we are only interested in substitutions in the goal (as opposed to the head of this clause),
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// we can use variable renaming and filter out the substitutions that are not in the goal.
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val (end, renamed: Substitutions) = numbervars(head, Program.variableRenamingStart, subs)
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val reverse = renamed.entries.associate { (a, b) -> b to a }
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Program.variableRenamingStart = end
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var newSubs: Substitutions = subs + renamed
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unifyLazy(applySubstitution(goal, subs), head, newSubs).forEach { headAnswer ->
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headAnswer.map { headSubs ->
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// If the body can be proven, yield the (combined) substitutions
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newSubs = subs + renamed + headSubs
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body.satisfy(newSubs).forEach { bodyAnswer ->
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bodyAnswer.fold(
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onSuccess = { bodySubs ->
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var result = (headSubs + bodySubs)
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.mapKeys { applySubstitution(it.key, reverse)}
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.mapValues { applySubstitution(it.value, reverse) }
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result = result.map { it.key to applySubstitution(it.value, result) }
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.toMap()
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.filterNot { it.key in renamed.keys && !occurs(it.key as Variable, goal, emptyMap())}
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yield(Result.success(result))
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},
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onFailure = { error ->
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if (error is AppliedCut) {
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// Find single solution and return immediately
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if (error.subs != null) {
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yield(Result.failure(AppliedCut(headSubs + error.subs)))
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} else {
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yield(Result.failure(AppliedCut()))
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}
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return@sequence
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} else {
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yield(Result.failure(error))
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}
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}
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)
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}
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}
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}
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}
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override fun toString(): String = if (body is True) head.toString() else "$head :- $body"
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override fun equals(other: Any?): Boolean {
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if (this === other) return true
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if (other !is Clause) return false
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if (head != other.head) return false
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if (body != other.body) return false
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return true
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}
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override fun hashCode(): Int {
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return super.hashCode()
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}
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}
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