3.3. Interpreter

3.3.1. Scopo

Per un dato linguaggio, definisce la rappresentazione della sua grammatica come «Espressioni non terminali» ed «Espressioni Terminali» «così come un interprete per le frasi di quel linguaggio.

3.3.2. Esempi

• Un esempio di interprete binario, dove ogni termine è definito dalla propria classe.

3.3.3. Diagramma UML

3.3.4. Codice

Potete trovare questo codice anche su GitHub

AbstractExp.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Interpreter;

abstract class AbstractExp
{
    abstract public function interpret(Context $context): bool;
}

Context.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Interpreter;

use Exception;

class Context
{
    private array $poolVariable;

    public function lookUp(string $name): bool
    {
        if (!key_exists($name, $this->poolVariable)) {
            throw new Exception("no exist variable: $name");
        }

        return $this->poolVariable[$name];
    }

    public function assign(VariableExp $variable, bool $val)
    {
        $this->poolVariable[$variable->getName()] = $val;
    }
}

VariableExp.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Interpreter;

/**
 * This TerminalExpression
 */
class VariableExp extends AbstractExp
{
    public function __construct(private string $name)
    {
    }

    public function interpret(Context $context): bool
    {
        return $context->lookUp($this->name);
    }

    public function getName(): string
    {
        return $this->name;
    }
}

AndExp.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Interpreter;

/**
 * This NoTerminalExpression
 */
class AndExp extends AbstractExp
{
    public function __construct(private AbstractExp $first, private AbstractExp $second)
    {
    }

    public function interpret(Context $context): bool
    {
        return $this->first->interpret($context) && $this->second->interpret($context);
    }
}

OrExp.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Interpreter;

/**
 * This NoTerminalExpression
 */
class OrExp extends AbstractExp
{
    public function __construct(private AbstractExp $first, private AbstractExp $second)
    {
    }

    public function interpret(Context $context): bool
    {
        return $this->first->interpret($context) || $this->second->interpret($context);
    }
}

3.3.5. Test

Tests/InterpreterTest.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Interpreter\Tests;

use DesignPatterns\Behavioral\Interpreter\AndExp;
use DesignPatterns\Behavioral\Interpreter\Context;
use DesignPatterns\Behavioral\Interpreter\OrExp;
use DesignPatterns\Behavioral\Interpreter\VariableExp;
use PHPUnit\Framework\TestCase;

class InterpreterTest extends TestCase
{
    private Context $context;
    private VariableExp $a;
    private VariableExp $b;
    private VariableExp $c;

    public function setUp(): void
    {
        $this->context = new Context();
        $this->a = new VariableExp('A');
        $this->b = new VariableExp('B');
        $this->c = new VariableExp('C');
    }

    public function testOr()
    {
        $this->context->assign($this->a, false);
        $this->context->assign($this->b, false);
        $this->context->assign($this->c, true);

        // A ∨ B
        $exp1 = new OrExp($this->a, $this->b);
        $result1 = $exp1->interpret($this->context);

        $this->assertFalse($result1, 'A ∨ B must false');

        // $exp1 ∨ C
        $exp2 = new OrExp($exp1, $this->c);
        $result2 = $exp2->interpret($this->context);

        $this->assertTrue($result2, '(A ∨ B) ∨ C must true');
    }

    public function testAnd()
    {
        $this->context->assign($this->a, true);
        $this->context->assign($this->b, true);
        $this->context->assign($this->c, false);

        // A ∧ B
        $exp1 = new AndExp($this->a, $this->b);
        $result1 = $exp1->interpret($this->context);

        $this->assertTrue($result1, 'A ∧ B must true');

        // $exp1 ∧ C
        $exp2 = new AndExp($exp1, $this->c);
        $result2 = $exp2->interpret($this->context);

        $this->assertFalse($result2, '(A ∧ B) ∧ C must false');
    }
}