php解释器模式( interpreter pattern)

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<?php
/*
The interpreter pattern specifies how to evaluate language grammar or expressions.
We define a representation for language grammar along with an interpreter.
Representation of language grammar uses composite class hierarchy, where rules
are mapped to classes. The interpreter then uses the representation to interpret
expressions in the language.
*/

interface MathExpression {
    public function interpret(array $values);
}

class Variable implements MathExpression {
    private $char;
    
    public function __construct($char) {
        $this->char = $char;
    }
    
    public function interpret(array $values) {
        return $values[$this->char];
    }
}

class Literal implements MathExpression {
    private $value;
    
    public function __construct($value) {
        $this->value = $value;
    }
    
    public function interpret(array $values) {
        return $this->value;
    }
}

class Sum implements MathExpression {
    private $x;
    private $y;
    
    public function __construct(MathExpression $x, 
        MathExpression $y) {
        $this->x = $x;
        $this->y = $y;
    }
    
    public function interpret(array $values) {
        return $this->x->interpret($values) + 
            $this->y->interpret($values);
    }
}

class Product implements MathExpression {
    private $x;
    private $y;
    
    public function __construct(MathExpression $x, 
        MathExpression $y) {
        $this->x = $x;
        $this->y = $y;
    }
    
    public function interpret(array $values) {
        return $this->x->interpret($values) *
            $this->y->interpret($values);
    }
}

$expression = new Product(
    new Literal(5),
    new Sum(
        new Variable('c'),
        new Literal(2)
    )
);

echo $expression->interpret(array('c' => 3));
?>