GroundWitnesses.h

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/*
 * Souffle - A Datalog Compiler
 * Copyright (c) 2020, The Souffle Developers. All rights reserved
 * Licensed under the Universal Permissive License v 1.0 as shown at:
 * - https://opensource.org/licenses/UPL
 * - <souffle root>/licenses/SOUFFLE-UPL.txt
 */
 
/************************************************************************
 *
 * @file GroundWitnesses.h
 *
 * Transformation pass to ground witnesses of an aggregate so that
 * they can be transferred to the head.
 *
 ***********************************************************************/
 
#pragma once
 
#include "ast/TranslationUnit.h"
#include "ast/transform/Transformer.h"
 
#include <string>
 
namespace souffle::ast::transform {
 
/**
 * Apply a grounding so that the witness of a selection aggregate (min/max)
 * can be transferred to the outer scope. Here is an example:
 *
 * Tallest(student) :- _ = max height : { Student(student, height) }.
 *
 * student occurs ungrounded in the outer scope, but we can fix this by using the
 * aggregate result to figure out which students satisfy this aggregate.
 *
 * Tallest(student) :- n = max height : { Student(student0, height) },
 *                      Student(student, n).
 *
 * This transformation is really just syntactic sugar.
 *
 */
class GroundWitnessesTransformer : public Transformer {
public:
    std::string getName() const override {
        return "GroundWitnessesTransformer";
    }
 
    GroundWitnessesTransformer* clone() const override {
        return new GroundWitnessesTransformer();
    }
 
private:
    bool transform(TranslationUnit& translationUnit) override;
};
 
}  // namespace souffle::ast::transform